Wiring Schematic for early 3-wire Coolant Fan Switch

[DPDISXR4Ti]

I'm planning to use the 22mm boss coolant fan switch from an early (circa 1999) B5 in an alternate application. I haven't been able to find a schematic so I was hoping someone might be able to confirm the wiring. I believe that the brown wire on one end is Gnd and the red wire on the other end is the high-speed fan circuit - yes, no, maybe?

[walky_talky20]

You are correct. The fan switch is a "dual thermal" switch, so there are 2 thresholds. This fan switch was used in a LOT of cars dating back to the 80's, so colors can vary. B5 colors are:

Brown - Ground input
Brown/Something - Low Speed Ground trigger output
Red/Something - High Speed Ground trigger output

Here's a diagram with the 4-pin switch, which works the same, but just has 2 separate ground wires.

[DPDISXR4Ti]

Thanks. I just found some more supporting data, which I'll add here.



Where #3 is the common circuit and #1 is the high speed fan (higher temp trigger).

And here's one that shows the three different quick-connectors used by VAG in this era. For the type 2 and 3 connectors, I find this information:

Red +12V from fuse 19, 30A under dash
Red/White (low speed trigger)
Red/Yellow (high speed trigger)

What's interesting is that the type 2 and 3 connectors use heavier gauge wiring than the type one. I have all three in front of me, clipped off from junkyard donors, as I type this. And the numbering doesn't quite sync up. Need to study some more....

[walky_talky20]

I would not run the fan current through this switch. It it is meant to trigger relays.

It is also not a match to the "Type 1" connector in your photograph. That's showing the modern "3.5 series" connector type from the mid-90's and up. The fan switch connector is a much older design. It dates back to at least 1980 or earlier.

[DPDISXR4Ti]

I would not run the fan current through this switch. It it is meant to trigger relays.

It is also not a match to the "Type 1" connector in your photograph. That's showing the modern "3.5 series" connector type from the mid-90's and up. The fan switch connector is a much older design. It dates back to at least 1980 or earlier.

I'll be using relays for sure. Thanks for the clarification on the type 1 connector. Indeed the clip being at the end of the connector (near terminal 1) doesn't match what's on the B5. The B5 connector is simply a rubber boot which doesn't have a clip - it just relies on friction to hold it in place. I actually prefer that as it's simple and seals just fine.

Just out of curiosity, did they actually not use a relay on the applications that use the type 2 and type 3 connectors? I think I pulled them all off 1998-2005 Mk4 Jetta/Beetle/GTI applications.

[walky_talky20]

Kind of. Mk4 Jetta/Golf/Beetle used a Fan Control Module, so the fan switch is just providing data to the module. Does not carry the current of the fan.

[DPDISXR4Ti]

Kind of. Mk4 Jetta/Golf/Beetle used a Fan Control Module, so the fan switch is just providing data to the module. Does not carry the current of the fan.

I wonder why they bothered with the heavier gauge wire then (as compared to the B5). I suppose it could be as simple as being handled by a different design team that just didn't like using a small gauge wire.

For reference, the B5 switch can have a part# of 321959481C or 191959481A
Low-speed on temp of 91 C, off 84
High-speed on temp of 102 C, off 95

The switches used on the Mk4 (Jetta,etc) have a part# of 110 959 481A. Even though two different connector types are used (type II and type III in the pic above), the same part# is used. They are marked "102 / 95" which equates to:

Low-speed on temp of 95 C, off 84
High-speed on temp of 102 C, off 92

I'm planning to use the B5 switch so that it will get the fan spinning a little sooner (91 instead of 95).

[walky_talky20]

Notably, all of these VW/Audi switches are setup as Normally Open, "fan on make".

90's Toyota Camry's (the reliability benchmark of the world, in my opinion) had their switches setup Normally Closed (fan on BREAK). So if there was ever a wiring fault, cut wire, loose connector popped out, etc - the fan would then run constantly and you would soon realize "hey, my fan runneth all the time - this causes me basically no problems, but why is this?". However with the VAG switches the situation is "hey, I guess a wire broke and my fan never came on like it should have and I totally melted my engine. What do?".

Also makes for super easy testing of the Camry electric fan system: pop hood, unplug fan switch, fan runs instantly, everything works. Makes for a super easy workaround if ever you desire the fan to run to constantly. Unplug switch, fan runs.

And this is why I drive Audi's. I mean...wait.

[DPDISXR4Ti]

Notably, all of these VW/Audi switches are setup as Normally Open, "fan on make".

90's Toyota Camry's (the reliability benchmark of the world, in my opinion) had their switches setup Normally Closed (fan on BREAK). So if there was ever a wiring fault, cut wire, loose connector popped out, etc - the fan would then run constantly and you would soon realize "hey, my fan runneth all the time - this causes me basically no problems, but why is this?". However with the VAG switches the situation is "hey, I guess a wire broke and my fan never came on like it should have and I totally melted my engine. What do?".

Also makes for super easy testing of the Camry electric fan system: pop hood, unplug fan switch, fan runs instantly, everything works. Makes for a super easy workaround if ever you desire the fan to run to constantly. Unplug switch, fan runs.

And this is why I drive Audi's. I mean...wait.

I've thought about precisely the scenario you describe and agree that a "normally closed" switch makes sense. You could do that with a second relay in the circuit, but I try to keep my stuff as brain-dead simple as possible.

Looks like '92 - '01 Camry (and others) uses the same switch. As used on most Japanese applications, looks like a metric thread with an O-ring. I wonder if you can buy stand-alone in-line fittings to accept one of these? I think on the native Toyo application they are built in to the intake manifold or some such (i.e. nothing junkyard harvest-able for generic use).