MMI display and buttons are not working

[c spoke]

Hi! I'll do my very best to keep this pointed:

Vehicle: '14 S4 6MT - Prestige Plus w/ B&O sound system

For at least a couple of months, MMI display would occasionally not power on upon vehicle start. The problem would always resolve itself, usually by simply turning the vehicle back off and on. For the last ~4 weeks, however, MMI display has been completely dead. Nothing ever comes on the display anymore, not even a splash screen.

Symptoms include:

• Black MMI Screen - no image ever displays and it never powers on
• MMI-related buttons in the center console near the gearshift do not work or illuminate. Push start and e-brake buttons illuminate and work fine.
• My phone can't see the vehicle's Bluetooth broadcast anymore. I've double checked with multiple phones/computers to confirm it's not the phone's Bluetooth as the cause.
• Pushing the Mode button on the left side of the steering wheel does nothing anymore. It used to toggle through some different UIs in the instrument cluster screen.
• The CD area of the front center dashboard appears to work fine. I can load a CD and it grabs a hold of it. Pressing the eject button successfully spits out the disc. There is red light illuminating this area as well.
• "3 finger reset" does not cause any changes to the above, but if I put my ear very closely I can hear a little "electronic squeel" (for lack of a better word) upon releasing the buttons during the reset sequence. It also sounds like something inside the MMI (behind the display unit?) powers on and starts spinning.

What I have done so far:

• Visual check of relevant fuses.* Relevant meaning any fuses noted in documentation for MMI modules (amp, radio, nav, bluetooth etc...)
• Optical loop bypass check of the two modules located in the left trunk compartment. I believe these were the radio and amp units - they were super easy to access. I observed the optical red light flashing in the fiber connectors on both modules when I went to swap in the bypass loop.
• Partial removal of center console MMI button trim around gear shifter. A brief visual inspection inside did not reveal any liquid damage in or around the buttons/controls.
• VCDS scan & diagnostics:

I have the full VCDS scan data available but thought it would be more relevant to show the MOST loop optical diagnostic test here first. Please let me know what you all think and what other data might be helpful. Thank you in advance for any bright ideas! Shouldn't there be more modules on the MOST loop? I know for a fact that this car has nav and bluetooth..

[Smac770]

Post the complete scan log.
Bus looks complete to me. The MMI 3G J794 integrated most of the separate components that existed in a MMI 2G config, like the media changer, the AMI, the Bluetooth, the Nav data source, etc.

[c spoke]

Chassis Type: FL-AU48 (8R0)
Scan: 01 03 05 08 09 15 16 17 19 22 36 3C 42 44 46 47 52 53 56 5F
62 6C 72 A9

Mileage: 68210km-42383miles

01-Engine -- Status: Malfunction 0010
03-ABS Brakes -- Status: OK 0000
04-Steering Angle -- Status: OK 0000
05-Acc/Start Auth. -- Status: OK 0000
08-Auto HVAC -- Status: OK 0000
09-Cent. Elect. -- Status: OK 0000
15-Airbags -- Status: OK 0000
16-Steering wheel -- Status: OK 0000
17-Instruments -- Status: OK 0000
19-CAN Gateway -- Status: Malfunction 0010
22-AWD -- Status: OK 0000
36-Seat Mem. Drvr -- Status: OK 0000
3C-Lane Change -- Status: OK 0000
42-Door Elect, Driver -- Status: OK 0000
44-Steering Assist -- Status: OK 0000
46-Central Conv. -- Status: Malfunction 0010
47-Sound System -- Status: Cannot be reached 1100
52-Door Elect, Pass. -- Status: OK 0000
53-Parking Brake -- Status: OK 0000
56-Radio -- Status: Cannot be reached 1100
5F-Information Electr. -- Status: Cannot be reached 1100
62-Door, Rear Left -- Status: OK 0000
6C-Back-up Cam. -- Status: OK 0000
72-Door, Rear Right -- Status: OK 0000
A9-Struct. Borne Sound -- Status: OK 0000
-------------------------------------------------------------------------------
Address 01: Engine (J623-CTUB) Labels:. 06E-907-551-CGW.clb
Part No SW: 8K5 907 551 G HW: 4G0 907 551 A
Component: 3.0l V6 TFSIH07 0007
Revision: --H07---
Coding: 0A190034052600060000
Shop #: WSC 06208 000 00000
ASAM Dataset: EV_ECM30TFS0218K5907551G 001007
ROD: EV_ECM30TFS021_AU48.rod
VCID: 3A2D3955ECD4E86B817-806E

1 Fault Found:
6262 - Idle Control System RPM
P0507 00 [104] - Higher than Expected.
Intermittent - Confirmed - Tested Since Memory Clear
Freeze Frame:
Fault Status: 00000001
Fault Priority: 2
Fault Frequency: 9
Mileage: 67718 km
Date: 2021.01.24
Time: 12:23:16

Engine RPM: 788.00 /min
Normed load value: 4.3 %
Vehicle speed: 0 km/h
Coolant temperature: 108 °C
Intake air temperature: 62 °C
Ambient air pressure: 970 mbar
Voltage terminal 30: 14.624 V
Unlearning counter according OBD: 37
Engine speed: actual: 768 /min
MAF_ENVD: 119.83529 mg/stroke
Coolant temperature (unfiltered): 96.75 °C
Ignition angle: actual: -9.375 °
MAP_1_ENVD: 366.15820 hPa
TPS_ENVD: 0.0 %

Readiness: 0000 0000

-------------------------------------------------------------------------------
Address 03: ABS Brakes (J104) Labels:. 8K0-907-379-V2.clb
Part No SW: 8K0 907 379 CP HW: 8K0 907 379 CP
Component: ESP8 quattro H09 0020
Revision: -------- Serial number: --------------
Coding: 281D1A01
Shop #: WSC 06325 000 00000
VCID: 094F9699F93233F3D8D-805C

No fault code found.

-------------------------------------------------------------------------------
Address 05: Acc/Start Auth. (J393) Labels:. 8K0-907-064-05.clb
Part No SW: 8K0 907 064 JM HW: 8K0 907 064 JM
Component: BCM2 2.0 H14 0650
Revision: 00002001
Coding: 0F814040000000000000
Shop #: WSC 06325 000 00000
VCID: 045187ADDE106E9BABB-8050

Subsystem 1 - Part No SW: 8K0 905 852 E HW: 8K0 905 852 E
Component: J764 ELV MLB H40 0046
Serial number: 0214000070944009

No fault code found.

-------------------------------------------------------------------------------
Address 08: Auto HVAC (J255) Labels:. 8Kx-820-043-3Z.clb
Part No SW: 8K1 820 043 AQ HW: 8K1 820 043 AQ
Component: KLIMA 3 ZONEN H04 0122
Revision: C1000000 Serial number: 01110420142088
Coding: 6600228000
Shop #: WSC 06325 000 00000
VCID: 77ABE0619FC63503BE9-8022

No fault code found.

-------------------------------------------------------------------------------
Address 09: Cent. Elect. (J519) Labels:. 8K0-907-063.clb
Part No SW: 8K0 907 063 DJ HW: 8K0 907 063 DJ
Component: BCM1 1.0 H16 0103
Revision: 09016001 Serial number: 00000786677402
Coding: 26000091863CAFE0240048C300003D0D0F9C9308A209022100 0008004000010200040030000000
Shop #: WSC 06325 000 00000
VCID: 7AADF955ACD4286BC17-802E

Subsystem 1 - Part No SW: 8K1 955 119 B HW: 8K1 955 119 B Labels: 1KX-955-119.CLB
Component: MITSUBA250314 H18 0461
Serial number: 1403200703551
Coding: 004577

Subsystem 2 - Part No SW: 8U0 955 559 B HW: 8U0 955 559 B Labels: 8U0-955-559.CLB
Component: G397_RLFS H06 0002
Serial number: 22496454
Coding: 02005D

Subsystem 3 - Part No SW: 8K0 941 531 AS HW: 8K0 941 531 AS
Component: E1 - LDS H03 0031
Serial number: 00000000000204141633

Subsystem 4 - Part No SW: 4G0 907 410 A HW: 4H0 907 410 A
Component: USHL4.2 H30 0030
Serial number: 0000002274555RB51103

Subsystem 5 - Part No SW: 8K5 941 329 HW: 8K5 941 329
H01 0031

Subsystem 6 - Part No SW: 8K5 941 329 HW: 8K5 941 329
H01 0031

Subsystem 7 - Part No SW: 4H0 907 658 HW: 4H0 907 658
Component: AQ_Hum_Sensor H03 0003
Serial number: 06B88227000000000001

No fault code found.

-------------------------------------------------------------------------------
Address 15: Airbags (J234) Labels:| 8K0-959-655.clb
Part No SW: 8K0 959 655 P HW: 8K0 959 655 P
Component: AirbagVW-AU10 H48 0610
Serial number: 003KJDC076LF
Coding: 303844354730303030304A55304C384B3053
Shop #: WSC 47996 001 1048576
ASAM Dataset: EV_AirbaECUVWAUDI010 003010
ROD: EV_AirbaECUVWAUDI010_003_AU48.rod
VCID: 4ACD6995BC3478EB117-801E

Seat occupied recognition:
Subsystem 1 - Part No SW: 8K0 959 339 C HW: 8K0 959 339 C
Component: BF-Gewichtss. H02 0040
Serial number: 5810000R140000JP7Q

Crash sensor for side airbag: driver side:
Serial number: 6334MSME443C51046

Front Passenger's Side Airbag Crash Sensor:
Serial number: 6344MSME4438207EG

Rear Side Airbag Crash Sensor (on driver's side):
Serial number: 6352QSME2440434CA

Passenger side rear thorax airbag crash sensor:
Serial number: 6362QSME54453B40E

Crash sensor for front airbag: driver side:
Serial number: 63732SME4C402150R

Crash sensor for front airbag: front passenger side:
Serial number: 63832SME564A2D4AB

No fault code found.

-------------------------------------------------------------------------------
Address 16: Steering wheel (J527) Labels:. 8K0-953-568.clb
Part No SW: 8K0 953 568 M HW: 8K0 953 568 M
Component: Lenks.Modul H04 0005
Revision: -------- Serial number: 90372NDA000PL
Coding: 000016
Shop #: WSC 06325 000 00000
VCID: 44D147AD9E90AE9BEBB-8010

Component: E221 - MFL H03 0004

No fault code found.

-------------------------------------------------------------------------------
Address 17: Instruments (J285) Labels:. 8T0-920-xxx-17.clb
Part No SW: 8K0 920 983 HW: 8K0 920 983
Component: KOMBIINSTR. H35 0564
Revision: 00100000 Serial number: ZAI58494F
Coding: 8BB509008B01D705020000
Shop #: WSC 06325 000 00000
VCID: 72BDD175F4A4502B897-8026

No fault code found.

-------------------------------------------------------------------------------
Address 19: CAN Gateway (J533) Labels:. 8T0-907-468.clb
Part No SW: 8R0 907 468 P HW: 8R0 907 468 D
Component: GW-BEM 5CAN-M H11 0060
Revision: 1Z120030 Serial number: 3041Z144040C34
Coding: 80C303FF00
Shop #: WSC 06325 000 00000
VCID: 4CC16F8DB6C066DB23B-8018

Subsystem 1 - Part No SW: 8T0 915 181 HW: 8T0 915 181
Component: J367-BDMHella H03 8041
Serial number: 8933663003

3 Faults Found:
00463 - Control Module for Digital Sound Package (J525)
004 - No Signal/Communication
Freeze Frame:
Fault Status: 01100100
Fault Priority: 2
Fault Frequency: 1
Reset counter: 150
Mileage: 67713 km
Time Indication: 0
Date: 2021.01.24
Time: 11:54:00

Freeze Frame:
Bin. Bits: 11111
Voltage: 13.60 V
Count: 6
Bin. Bits: 00000

01304 - Radio
004 - No Signal/Communication
Freeze Frame:
Fault Status: 01100100
Fault Priority: 2
Fault Frequency: 1
Reset counter: 150
Mileage: 67713 km
Time Indication: 0
Date: 2021.01.24
Time: 11:54:00

Freeze Frame:
Bin. Bits: 11111
Voltage: 13.60 V
Count: 6
Bin. Bits: 00000

03158 - Control Module for Information Electronics 1 (J794)
004 - No Signal/Communication
Freeze Frame:
Fault Status: 01100100
Fault Priority: 2
Fault Frequency: 1
Reset counter: 150
Mileage: 67713 km
Time Indication: 0
Date: 2021.01.24
Time: 11:54:00

Freeze Frame:
Bin. Bits: 11111
Voltage: 13.60 V
Count: 6
Bin. Bits: 00000


-------------------------------------------------------------------------------
Address 22: AWD (J492) Labels: 8K0-907-163.clb
Part No SW: 8K0 907 163 B HW: 8K0 907 163 A
Component: 4 Wheel Drive H14 0100
Serial number: 000104140001413
ASAM Dataset: EV_AllWheelDiffeQSPCAN 003019
ROD: EV_AllWheelDiffeQSPCAN.rod
VCID: 2F3B180127963DC3269-807A

No fault code found.

-------------------------------------------------------------------------------
Address 36: Seat Mem. Drvr (J136) Labels:. 8T0-959-760.clb
Part No SW: 8K0 959 760 D HW: 8K0 959 760 D
Component: MEM-FS H06 0077
Revision: AA000000 Serial number: 00000097775738
Coding: 00581B00000000000000
Shop #: WSC 06325 000 00000
VCID: 3B233C51E36ED1639A1-806E

No fault code found.

-------------------------------------------------------------------------------
Address 3C: Lane Change (J769) Labels:| 5Q0-907-566.clb
Part No SW: 8T0 907 566 B HW: 8T0 907 566 B
Component: SWA Master H05 0071
Serial number: 0330308617
Coding: 010211
Shop #: WSC 06324 000 00000
ASAM Dataset: EV_LaneChangAssis 001020
ROD: EV_LaneChangAssis_AU21.rod
VCID: 3FDB4841F7F6CD43B69-806A

Lane change assistance control module 2:
Subsystem 1 - Part No SW: 8T0 907 568 B HW: 8T0 907 568 B
Component: SWA Slave H05 0071
Serial number: 0332105917
Coding: 010211

No fault code found.

-------------------------------------------------------------------------------
Address 42: Door Elect, Driver (J386) Labels:. 8K0-959-793.clb
Part No SW: 8T0 959 793 P HW: 8T0 959 793 G
Component: TSG FA H09 0305
Revision: -------- Serial number: --------------
Coding: 011C13204E00410001
Shop #: WSC 06325 000 00000
VCID: 56F58DE5780C2C0B85F-8002

No fault code found.

-------------------------------------------------------------------------------
Address 44: Steering Assist (J500) Labels:| 8K0-909-144.clb
Part No SW: 8K0 909 144 G HW: 8K0 909 144 B
Component: RCEPS H40 0508
Revision: 00001000 Serial number: 0003455273
Coding: 100000010001
Shop #: WSC 06324 000 00000
ASAM Dataset: EV_RCEPSAU48X 008015
ROD: EV_RCEPSAU48X_008.rod
VCID: 35372A69C5CA07136C5-8060

No fault code found.

-------------------------------------------------------------------------------
Address 46: Central Conv. (J393) Labels:. 8K0-907-064-46.clb
Part No SW: 8K0 907 064 JM HW: 8K0 907 064 JM
Component: BCM2 2.0 H14 0650
Revision: 00002001 Serial number: 0331989177
Coding: 0106063A00FA404580B0013200000000
Shop #: WSC 12345 123 61029
VCID: 045187ADDE106E9BABB-8050

Subsystem 1 - Part No SW: 8K0 959 719 B HW: 8K0 959 719
Component: GTO BF H01 0080
Serial number: 00000000000000825021

Subsystem 2 - Part No: 1K0 951 605 C
Component: LIN BACKUP HO H05 1501

Subsystem 3 - Part No SW: 8K0 959 591 B HW: 8K0 959 591
Component: J245 PS050 H08 0043

3 Faults Found:
01503 - Bulb for 3rd Brake Light (M25)
012 - Electrical Fault in Circuit - MIL ON
Freeze Frame:
Fault Status: 11101100
Fault Priority: 2
Fault Frequency: 1
Reset counter: 150
Mileage: 67713 km
Time Indication: 0
Date: 2021.01.24
Time: 11:53:59

02616 - Fuel Tank Cap; Unlock
009 - Open or Short to Ground
Freeze Frame:
Fault Status: 01101001
Fault Priority: 2
Fault Frequency: 1
Reset counter: 151
Mileage: 67715 km
Time Indication: 0
Date: 2021.01.24
Time: 12:16:39

02615 - Fuel Tank Cap; Lock
009 - Open or Short to Ground
Freeze Frame:
Fault Status: 01101001
Fault Priority: 2
Fault Frequency: 1
Reset counter: 152
Mileage: 67730 km
Time Indication: 0
Date: 2021.01.24
Time: 17:56:05


-------------------------------------------------------------------------------
Address 47: Sound System
Cannot be reached

-------------------------------------------------------------------------------
Address 52: Door Elect, Pass. (J387) Labels:. 8K0-959-792.clb
Part No SW: 8T0 959 792 P HW: 8T0 959 792 G
Component: TSG BF H09 0305
Revision: -------- Serial number: --------------
Coding: 011C13204E00410001
Shop #: WSC 06325 000 00000
VCID: 55F78AE9650A27138C5-8000

No fault code found.

-------------------------------------------------------------------------------
Address 53: Parking Brake (----) Labels: 8K0-907-801-V2.clb
Part No SW: 8K0 907 801 N HW: 8K0 907 801 J
Component: EPB-4 H01 0005
Revision: -------- Serial number: 10000000360961
ASAM Dataset: EV_ParkiBrake 001028
ROD: EV_ParkiBrake_AU48.rod
VCID: 3A2D3955ECD4E86B817-806E

No fault code found.

-------------------------------------------------------------------------------
Address 56: Radio
Cannot be reached

-------------------------------------------------------------------------------
Address 5F: Information Electr.
Cannot be reached

-------------------------------------------------------------------------------
Address 62: Door, Rear Left (J388) Labels:. 8K0-959-795.clb
Part No SW: 8T0 959 795 Q HW: 8T0 959 795 H
Component: TSG HL H07 0305
Revision: -------- Serial number: --------------
Coding: 010C13204000210001
Shop #: WSC 06325 000 00000
VCID: 59EF86D909120373A8D-800C

No fault code found.

-------------------------------------------------------------------------------
Address 6C: Back-up Cam. Labels: 8T0-907-441.clb
Part No SW: 8T0 907 441 C HW: 4L0 907 441 B
Component: J772__Rearview 0040
Revision: --H09--- Serial number: 525PA8-J4290SQ
Coding: 1000006
Shop #: WSC 06325 000 00000
VCID: 3829235DDE58FA7B773-806C

No fault code found.

-------------------------------------------------------------------------------
Address 72: Door, Rear Right (J389) Labels:. 8K0-959-794.clb
Part No SW: 8T0 959 795 Q HW: 8T0 959 795 H
Component: TSG HR H07 0305
Revision: -------- Serial number: --------------
Coding: 010C13204000210001
Shop #: WSC 06325 000 00000
VCID: 59EF86D909120373A8D-800C

No fault code found.

-------------------------------------------------------------------------------
Address A9: Struct. Borne Sound (J869) Labels: 4H0-907-159.clb
Part No SW: 8K0 907 159 HW: 4H0 907 159 A
Component: SAS-GEN 2 H06 0005
Serial number: 140325201137
ASAM Dataset: EV_SoundActuaGen2AU64X 002005
ROD: EV_SoundActuaGen2AU64X_002.rod
VCID: 72BDD175F424502B897-8026

No fault code found.

End----------(Elapsed Time: 01:44, VBatt start/end: 14.4V/14.6V)-----------
Thursday,04,March,2021,14:34:32:46875
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 10 x64
VCDS Version: 21.3.0.0 (x64) HEX-V2 CB: 0.4543.4
Data version: 20210226 DS325.0
www.Ross-Tech.com

[Smac770]

All three of those components, MMI 3G J794, B&O amp J525, and Radio R, are the same - one pin for 12v, one pin for ground, one pin for ring-break diagnosis (purple/green wire), and the optical connection. All the other connections are local devices like the J685 display, the speakers, the antenna, etc. So communication from the J533 to those devices is only over the MOST bus. It's a ring; any device on the ring has an issue, the whole ring goes down.

You have a loop test that says things are fine. You have a scan that says the loop is down.

You have a bypass plug. Yes, the radio R and the amp J525 are in the rear left. The MMI J794 is the thing in the dash you put the CD into. It should also be easy to access, if you have the MMI removal keys. In VCDS, can you directly go to 47 or 56? If you bypass the J794, can you go to them now?

Sounds like something is starting to flake out. With the bus down, the J794 is not going to initialize, and that means no J685 FID (front information display) or Bluetooth or AMI operations. You never mention if radio playback is working or not, or if the volume knob on the console is effective (it should not, but the one on the wheel should still work maybe).

Try the loop test with 3dB attenuation. Maybe that'll yield a different result.

[c spoke]

I appreciate the insight very much - thank you. I've ordered the MMI removal keys and they get here on Friday, so I'll be able to pull out the J794and try bypassing it. I'll report back on whether I can reach 47 or 56 in VCDS then (I cannot reach them currently). Radio playback does not work. The power/volume knob does not appear to do anything. I've tried different sequences and combinations of pressing, holding and turning the knob.

The only sounds I can recall hearing in the car currently:

• Door ajar ring
• Seatbelt reminder
• Beep when shifting into R
• Sensor alert beep when approaching objects in reverse

Results, in order, from four consecutive optical tests I ran yesterday:

1. Regular test first. This test revealed a couple of ERRORs (SS below)
2. 3dB test. All modules came back OK. (SS below)
3. Regular test again. This time all modules came back OK. (WTF!)
4. Regular test once more. All modules came back OK again.

[Smac770]

Something certainly seems up with the MOST bus. Whether it's a cable or a device is hard to say, considering the only means to communicate with the devices is via the bus.

When you do the optical test, you're telling the J533 to send out a signal on the ring-break diagnosis line. All devices who receive the signal then do an electrical and optical self-diagnosis and send the report via the MOST bus. Apparently, the ring goes:
J533 (0x19) -> J794 (0x5F) -> R (0x56) -> J525 (0x47) -> back to J533
so a problem at the J794 should only see it offline, while a problem at the Radio would cut off the J794 and Radio.

With the J794 out of the bus, none of the center console controls (the E380) will be functional. But the steering wheel controls should still work, as they are read by the J527 and passed via CAN to the J533 to send over the MOST. Assuming it sends them directly to the J525 and R and not to the J794 to be reviewed and resent.

The bypass loop, this was a loop for the cable plug or the device plug? It needs to be for the cable plug; both exist out there.

If once the optical test is showing all good, can you not then go to address 0x5F and 0x56 and look at the DTCs?

[c spoke]

I pulled the J794 out of the dash, bypassed it with the loop and then ran the optical test about 15-20 times, toggling between 3dB and standard tests. The results each time were different than the last. Electricals were always OK, but opticals were giving ERRORS, usually just two components at random, sometimes three. I can't recall ever seeing the entire bus show ERRORs, though at least once the test showed everything OK. I never noticed the display, MMI buttons, or steering wheel buttons become enabled during this testing. After running the tests with the J794 bypassed, I reconnected and put it back in the dash and then ran the scan a few more times, getting 4 consecutive OKs across the board, again toggling between 3dB/Standard.

During all of the above, I tried several times but was unsuccessful in going directly to modules 5F, 56 and 47 in VCDS. The connection just times out and says the module can't be reached. Even when the optical test shows everything OK, the modules are unreachable.

Questions:

• When you say the bluetooth and nav and other modules are all integrated into the J974, does that mean that the actual components themselves are physically located within the same box in the dash? If so, then there are only three physical components with optical connectors on the bus (not counting the Gateway)?
• Is there any specific sequence I should follow to give the system the right opportunity to come online after bypassing a component? (i.e three-finger reset, power vehicle OFF/ON after waiting 10 seconds etc..)

I ordered another bypass loop after reading about situations where multiple modules could be flaking out. It's arriving today so I'll be back out there running optical tests again with various combinations of the modules bypassed. I suppose it's also possible that the bypass loop itself is faulty ..ugh Here are the items I have:

• https://www.amazon.com/gp/product/B076BFVP8B
• https://www.amazon.com/gp/product/B07PLBK6QH

[Smac770]

Could be a cable issue or a device issue. But a lack of comms certainly puts the issue on the optical bus. But for all you know, it could be the optical port on the J533 itself that's starting to flake out. A diagnostic that produces such random results is not particularly helpful. Do note it is not a VCDS program, just VCDS invoking the program in the J533.

Here's the layout you're dealing with:

B8 MMI 3G MOST bus.jpg

There's four optical cables and four plugs into four devices. Each plug has one end of two different cable. Two cables are short, the J533 and J794 are practically neighbors in the dash, and the J525 and Radio are neighbors in the cargo hold. Two cables are long, the runs between the dash and cargo.

If bypass loops on the plugs at the J525 and R do not bring the J794 back online, at least to the point of being able to communicate with the J533, you have to consider either the J533 or the J794 is broken or the cable loop (ie, one of the four cables) has an issue. To separate the loop, it would be useful to run a cable directly between the J533 and a particular device, a point to point loop. 5 meter cables are on amazon in the $30 range, and that would let you run the J533 directly to each device one by one to see which operates cleanly when isolated from the existing cables in the car and the other devices. Might be informative.

[c spoke]

This is super helpful info - thank you. Do you happen to know where the optical runs that connect the in-dash <--> trunk modules physically reside in this setup? I've ran more optical tests in VCDS and am still getting similar results: everything is OK when the modules are all connected but, when I bypass J794, I get random modules ERROR on the optical side (electrical is always OK) and they are different modules failing each time. I've ordered cabling and will try some direct runs as the next troubleshooting step. Thanks again for the help. I am excited to (eventually) find out what the root cause is here.

PS: LMAO - well, here's that second bypass loop I got via Amazon:

How could this even have happened anyway... It's like somebody intentionally cut it with scissors and then packaged it up. This, boys, is sometimes what you get when trying to save money on the generally overpriced genuine Audi items.

[Smac770]

No, unfortunately one thing the wiring diagram document does not document is the physical routing of the wiring. I imagine it either runs under the center console or down the left edge of the floor board. Maybe something like in the pic of this MMI retrofit - https://www.audi-sport.net/xf/thread...t-done.274667/

As for the Amazon product, that sucks. The statistics game of mail order.

[c spoke]

Alright so I used a male 2 male 5m optical cable to connect the J533 individually to each module one at a time (J974, J525, R). Running the optical diagnostic test in each case produced expected and consistent results (the gateway and the one connected module both passed both optical and electrical, while the others failed on the optical test). Even when I had these direct runs setup, however, I still could not get to the connected module directly in VCDS.

A couple things I'm trying to wrap my head around at this point:

1. When trying to access the J794 via VCDS directly, I understand that the data flow is from my computer(VCDS) --> OBD port --> J533 --> J794. For that last hop from J533 --> J794, is it over the same MOST optical line that the optical diagnostic test uses, or is it over a different line (one of the pins on the multi connector)?
2. From other posts I read about similar MMI issues, the common behavior seems to be that the J685 displays a splash screen for a few moments and then shuts off after an open MOST loop is determined by the system. Considering that the J685 never powers on at all in my case, does that point to the problem being with either the J685/J974/E380 specifically?

I've been thinking more about this, and there are two events that come to mind within the last ~6 months:

• APR Ultracharger install + ECU flash to the appropriate file (no downpipes)
• Small soda spill in cup holders, perhaps ~3oz. The cup was in the rear of the two main cupholders and spilled mostly toward the back, into the cupholder itself and some into the center armrest storage just behind it. I don't believe it spilled forward at all towards the E380. I soaked a lot of it up with a microfiber I had in the car at that moment, but there could have easily been some that continued on downward. I wonder if that could have caused anything related here? This is why I was asking where the runs are between the MMI modules in the trunk and dash. The 12V charging outlet in the armrest storage bin illuminates and charges a device.

[Smac770]

I went looking more into the MOST bus stuff and I think part of the problem here is going to be that the MMI central controller (J523 for MMI 2G or J794 for MMI 3G; the thing in the dash) is pretty much mandatory for the bus to operate.

Audi refers to the terms system manager for the J794 and diagnostic manager for the J533. I would have expected the ring to be able to function with at least one of the controlling modules online as the master. But those are Audi terms. "diagnostic manager" just means it looks at the roll call when the ring starts up to make sure all the installed modules that should have responded did respond. And to initiate and tally the ring-break diagnosis operation. But in MOST bus terms, the key functions are the timing master / network master / connection master. These are all in the J794 only, it would seem. Without that device online to basically administer the ring communications, there is no ring communications. This is why putting it seems people talk about putting the bypass loop on the plug to the J794 being a pointless exercise; removing the J794 means nothing is going to happen.

Unfortunately, Audi in their endless cost cutting as a VAG company, removed CAN bus communication from the J794. So if the J794 is having ring interaction issues, or is just outright not functioning, the ring is down and there's no way for the J533 to interact with the J794 to convey any idea of what's going on.

You did try J533 directly to J794 with that new cable. If things were good with the J794, communication should have been possible in that instance. I assume the cable inverted the cables on the plugs, ie, the left port on one plug of the new cable went to the right port on the other plug, etc. Ie, the in and out ports on each device are on the same side of the plug, and the in port on one device needs to connect to the out port on the other device, ....

Could be an issue with the E380 (buttons and dials on the center console to control the MMI) or the J794 itself (hard drive problems is not uncommon apparently). But without swapping in an alternative to see if any changed behavior, be it a known good into your car or yours into a known good car, there's no means to problem isolate. Apparently one can solder in a connection to allow a telnet to the J794, maybe the boot messages would be informative. But that's outside my scope.

I've spilled some soda on the buttons of my E380, and other than a very sticky return button that I'd have to press hard on to break loose once in a while, I didn't have any electrical operation issues. But pulling out the E380 is not that hard; you just have to get the shifter boot trim pulled up and then the ashtray and climate controls pulled out. Then you can pull it out and disassemble it a bit; see if there's anything you can clean up in case there's any stuck buttons or any observable issues with the wiring or circuit board.

Unfortunately, that's all that Audi's allowed for diagnostic analysis due to their business decisions. Swap parts one by one with a working car until you find the one part that when swapped, things start working. I think the focus needs to be on the J794 and E380. That everything is AWOL points to the ring master.

[RPMtech147]

NM, you said you looped out all the modules.

[Jnaut]

This is all good info as I’m having the exact same issue as the op. I’ve done loop bypass on 3 major modules ( the 2 in the truck and 1 in the dash ) . Same issues no communication. It’s true the J794 is the brain and it’s dead you can not communicate with any of the modules on the MOST . It’s 4K $ new from the dealer , but I’ve found some second hand , do you know if it’s PnP or programming/coding is needed ?

[c spoke]

Ok ok ok so after a few more rounds of testing, I brought the vehicle into my local shop. First of all, shout out to Smac who was right on point. They swapped in a working J794 right away and all was good.
Options from this point forward are simple:

1. Brand new J794 from Audi dealership + programming/install: ~$4K
2. Used J794 from ebay + programming/install: ~$800
3. Open up the J794 and attempt some sorcery: ~$100?

I went with option 2 for an immediate solution. Also, because I can hear the J794 power on, it grabs a CD, the fan runs etc... I think there's a chance that the root cause is software related and fixable. I'll probably make an attempt at salvaging it when I have time next, per the steps in this thread I found on ross-tech forums.

Smac, I really appreciate your input here - thank you. Isolating the issue to the J794 was absolutely right and probably saved me some labor charges at the shop on low-value testing to get to that same conclusion.

For others who have this issue:
One significant difference I noticed in this scenario than most other forums/threads I came across during my research is that the display unit J685 never powers on. Not even during initial vehicle start. This is different from the more common MOST loop break situation where the system starts up with a splash screen on J685 but then goes black after a few moments. In this case, J685 never output anything. If you get the splash screen powering on initially, the standard MOST loop testing method of bypassing/direct runs should apply. Absent of the splash screen, you have to figure it's something directly with J685, J794 or E380. These three are on a separate private CAN that connects to the MOST loop via the J794.