AudiMT

Picture 1 is from the 3-pin coolant sensor that drives the temperature gauge in the car. The brown wire is ground. The blue/brown wire controls the temperature gauge in the dash and that's working fine. The blue-white wire, along with the blue-yellow wire from the coolant expansion tank's level-sensor, should cause the coolant warning light on the dash to flash if the coolant temperature is too high and/or the coolant level is too low, respectively. I traced the blue-white wire to the 5-pin red connector shown in picture 2. There is nothing for this to plug into. You can also see a couple of blue-yellow wires in there, connected to one pin. These blue-yellow wires connect to the correct pin on the instrument panel that causes the coolant warning light to flash. From all the wiring diagrams that I've seen (I don't have the wiring diagrams for my exact model of car), the blue-white and the blue-yellow eventually get tied together somewhere in the chain so either, or both, can trigger the coolant warning light on the dash. This discovery came about because I've been trying to fix the instrument cluster on my Audi 80. In order to test the warning lights, I've been triggering them by connecting the relevant wires to ground which then simulates the fault. The cooling warning light should have come on when the blue-white wire was shorted to ground at the sensor's wiring plug but it did nothing and so started the investigation... The way things stand at the moment, the coolant warning light will only flash if the coolant level is too low. It seems to be that way by design but it seems a bit strange to leave one out. If I jump the blue-yellow and the blue-white wires together in the connector (easily done), I should get both warning functions. Any thoughts on this?

I finally got to the bottom of this. I didn't notice the small indicator lights on the two front wings! I was so focused on the main indicator lights that I completely forgot about these. And these turned out to be important... The wiring that goes to the front indicators also goes to the indicators on the wings. When I was doing continuity tests, because the wing indicator bulbs were still in there (and working), the meter saw it as the black and brown wires for the bulb connected together. Had I removed this bulb, the circuit would have been broken and it would have metered as expected.

Thanks Gareth for getting back. I do have a Haynes manual for the car but it's up to 1990 only - got that from eBay. There doesn't seem to be anything for my particular model or year. I've tried the official Haynes online site too. All the wiring diagrams that I've found so far, even those in German, don't show that extra cable that disappears into the wing. That arrangement exists for both sides of the front indicators. Those diagrams have been really useful for diagnostics as quite a lot stays the same, but I can't find any reference to that cable (4). Even those diagrams that are supposedly for an Audi B4 1992, don't show it. I've even used a borescope to see close to the bulkhead if I can see any cables coming in but it's just too difficult. 🙂

I'm interested in where cable (4) goes. It seems to disappear into the wing and I can't find it in any of the wiring diagrams. (1) Orange indicator bulb goes here. (2) Connects to (3). (3) The black-white wire comes from yellow connector 3 in the Aux panel. It's for the left-hand-side turn signal. (4) Has (1) and(2) coming out of it and it disappears into the wing. If I do a continuity test on (1) and (2), it seems to show that the black and brown are connected together? Does that make sense? Any help is most appreciated. Thanks!

Thanks for the reply, Steve. Apologies for the extremely late reply! I managed to get to the bottom of this. It turned out to be the coolant temperature sensor - blue one, 2-pin (025 906 041A) - the one that goes to the ECU. There is another one next to it - white-one, 3-pin (053 919 501A) - which goes to the gauge in the instrument panel. Basically, the blue sensor was telling the ECU the engine was still cold and so added more fuel, raised the revs in order to try to heat it up, if my understanding is correct. The old one was reading around 250k ohms at around 12 degrees C and the new one was reading around 3.5k ohms at the same temperature, according to my notes! After the new sensor went in, the revs dropped and have stayed fine ever since. I'm not sure what the idle revs are supposed to be but there are few other issues with the car, so the revs might be a tad too high. They are currently around 900 rpm.

When stationary, if I rev the engine to say 3k to 4k rpm, the revs stay high. In fact, above 4k rpm, they can start to increase! It's the same problem when driving except, when applying the brakes, the revs will eventually drop but it takes a while. The car is idling about 1200 rpm and the idling isn't erratic. If I disconnect the electrical connector to the idle control valve, the revs drop to a more normal 800 to 900 rpm. Put the connector back on and they jump to around 1200 rpm. I tried cleaning the ICV and I think it's made the situation worse. I also tried another ICV from an old Golf GTI - exact same part number. It didn't really make a difference except this time when I removed the electrical connector to the ICV, the engine stalled. I looked for vacuum leaks as there is a vacuum pipe that goes into the main inlet manifold. If I pump the brake peddle a few times, the revs will rise a little - not sure if that's normal. It's an anti-stall feature! 🙂 It's annoying as the car is always screaming and not great when trying to change gears. Is this an ICV problem or is there something else going on? Thanks!