Ground locations

[LAMOKI FLARE]

I have a B8 2011 S5 with the 4.2L V8. I have been encountering an odd issue of the entire car's electronics shutting down at intermittent, random points while driving (MMI, gauges, ABS, power steering all go dark then will either come back on, or immediately act normal upon turning car off and back on). I cannot consistently replicate the issue. I suspect I have a loose ground as I have tested the battery which reads 12.6V off and 14V at idle (which suggests the alternator is okay).

I have checked the ground in the trunk and it is good. There appears to be a ground near the coolant expansion tank which looks good.

I cannot find any information on where the engine ground is (I suspected it is on the motor mount, but could not find it from under the car). Any info on where this ground is and where to check it from would be greatly appreciated. If anyone has other ideas of what may be causing this problem please let me know.

[Smac770]

There's 15 ground points around the chassis, including several in the engine bay. The ground strap from the engine block to the chassis is at the right motor mount to the right engine bay frame rail.

If you don't have a copy of the wiring diagrams document from erwin.audiusa.com (along with all the other workshop manuals for your VIN), you could try and use the interface at cardiagn.com. All the grounds are documented in section 801.

"power steering" has no electrical aspect at all (well, there's the activation of the Servotronic solenoid by the J519), so not sure what you mean by it "going dark". ABS control module is not on the engine, and runs through different power supply and ground path (the left ground point in the engine bay) than anything on the engine.

[LAMOKI FLARE]

There's 15 ground points around the chassis, including several in the engine bay. The ground strap from the engine block to the chassis is at the right motor mount to the right engine bay frame rail.

If you don't have a copy of the wiring diagrams document from erwin.audiusa.com (along with all the other workshop manuals for your VIN), you could try and use the interface at cardiagn.com. All the grounds are documented in section 801.

"power steering" has no electrical aspect at all (well, there's the activation of the Servotronic solenoid by the J519), so not sure what you mean by it "going dark". ABS control module is not on the engine, and runs through different power supply and ground path (the left ground point in the engine bay) than anything on the engine.

The steering gets heavier when the car goes through this odd electrical fault and when it turns back on i’ll get an ABS malfunction code for a second.

By "right", do you mean passenger side?

Thanks for the info

[LAMOKI FLARE]

Also, where specifically on the motor mount would it be? I’m looking on the passenger side motor mount from under the car but do not see a ground wire on the mount or retainer. Is it tucked on the top side of the motor mount?

[Smac770]

That's the ground strap for the V8:


It's short, I suspect it's pretty much like on the I4:


Maybe the wire hanging off the right side here:

[LAMOKI FLARE]

Thanks for the help. I did find it and the cable is tight as could be. With your experience and the symptoms I described, does anything else jump out that I could easily check? If the issue persists I will get the car scanned and diagnosed when I return to school in Atlanta

[Smac770]

No, nothing stands out offhand. MMI shutting down is more complicated than simply the MMI control module, which would be simpler to assess. If anything on the MOST ring goes offline, the ring goes down, MMI goes down.

Gauges going dark, that would be the instrument cluster. But it could be just as easily a power issues as a comms issue. The instrument cluster needs to get the data over comms. By dark, you mean gauges go to 0, or all things go lights out, including the DIS (center screen)?

MMI and instrument cluster won't care if even the alternator failed; the battery would still be supplying them. The alternator and starter and jump start post are on the charging cable path from the battery. Then all the car electronics are on other cable paths from the battery. So the alternator runs the car while the engine is running, but it does so through the battery connections. And all these ground to the chassis through different points. The only common ground path point is the battery negative to chassis connection.

Yeah, it would be better to do a scan and see if DTCs point in some direction.

[LAMOKI FLARE]

No, nothing stands out offhand. MMI shutting down is more complicated than simply the MMI control module, which would be simpler to assess. If anything on the MOST ring goes offline, the ring goes down, MMI goes down.

Gauges going dark, that would be the instrument cluster. But it could be just as easily a power issues as a comms issue. The instrument cluster needs to get the data over comms. By dark, you mean gauges go to 0, or all things go lights out, including the DIS (center screen)?

MMI and instrument cluster won't care if even the alternator failed; the battery would still be supplying them. The alternator and starter and jump start post are on the charging cable path from the battery. Then all the car electronics are on other cable paths from the battery. So the alternator runs the car while the engine is running, but it does so through the battery connections. And all these ground to the chassis through different points. The only common ground path point is the battery negative to chassis connection.

Yeah, it would be better to do a scan and see if DTCs point in some direction.

Ty again. For the gauges it is both. The speedo goes to 0 and the entire cluster including center screen shut off

[Smac770]

The J285 instrument cluster unit has two pins connected to a fuse for power. I don't know why it needs two pins instead of just combining them internally. For an MY11, that would be fuse 4 in column brown on the passenger end of the dash. It goes to pins 31 and 32 of the 32-pin plug on the J285 (red/brown wires). Pins 29 and 30 (brown wires) are the ground lines, which also join together to run to a common ground point. There's actually a lot of things that connect to this ground line in the "dash panel wiring harness"

This dash panel wiring harness ground line grounds to the center tunnel ground point 687.

If we take the parallel example, the J794 MMI control unit in the dash above the climate controls (or is it just the J685 screen that's going out?), get power from a fuse in the right rear. But the screen gets power from the passenger end of the dash, fuse 3 in column brown. Right next to the J285 fuse. The ground point for the J794 and J685 is this same center tunnel ground point 687. Apparently, the ground wire is a second ground wire in the "dash panel wiring harness" again, but just for the screen. The J794 ground line runs straight to 687.

687 is the ground point on the left side of the center tunnel (688 is the right side). You might want to try and pull back the carpet and inspect the ground point for corrosion.

It could also be a problem with the supply line to the fuse column brown on the passenger end of the dash.

If the issue goes live and stays live such that you can measure some things, first up would be to test the voltage at the fuses to see if they are "12v". The best thing would be to test the pins 31/32 and 29/30 on the J285 plug to see if 31/32 tests 12v to a good chassis ground point, and if 29/30 tests 12v to a good 12v supply. One of those is going to fail, telling you if you have a voltage supply issue or a ground connection issue. But getting to that plug in the middle of a live situation would be a pita unless you prewired some backprobing breakout leads you can easily get to when the time comes.

But I'd focus on those two points to start with, the fuse column brown on the passenger end of the dash and the ground point on the left of the center tunnel at the base of the center stack. The evap drains run past that area; does that carpet ever feel damp?

[nhracecraft]

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