-93 Audi 80 Quattro 1.8L, Coolant temperature sensor/circuit, with scope pics

Hi,

I was working on this Audi today, it has an issue with the engine temp circuit for the gauge. Complaint: car running, temp gauge stays at zero all the time.

This car has three temp sensors in the thermostat casing, one for the ECU, one for the gauge and one for the overheat warning light(I believe). I didn't find any solid wiring diagrams.

My procedures:

- KOEO unplug the sensor, voltage measurements, one wire 0V and the other 10,3V. 10,3V feed seemed a little odd to me but I continued with keeping that on my mind.
- KOEO jump the wires, gauge jumped to 90 degree celcius which is the normal operating temperature.
- KOER sensor still unplugged, voltage measurements, both wires had around 10V and lots of hash.
- KOER jump the wires, gauge does not work.
- Again KOEO sensor unplugged, jump the wires, gauge works and voltage measurements 0V and 10,3V.
- KOEO engine hot sensor plugged, gauge wont work = bad sensor as far as I'm concerned.

I still have this issue KOER: both wires about 10V with lots of hash. See the pics.

I'm thinking some problem with the harness. Could it be that the temp sensor wiring is shorted to some sensors wiring that creates a signal only when engine is cranking or running? Like for example crank sensor?

KOER hash


KOEO ground, which also had this weird hash going on


KOER hash (other wire)


KOEO feed

Substituted values can be a pain in the neck, I don't do much work on VAG cars but on some of the cars I work on KOER if the cts circuit is out of range the fan runs slow speed and the temperature gauge on the dash goes to "N" and the PCM reports 75c coolant temperature , no engine check light.

I don't think this is because of substituted values. There is separate sensor for engine management plus the cooling fan works like it should. This sensor and circuit is for temp gauge. With sensor unplugged jumping the wires KOEO, gauge works. It tells me that the circuit and the gauge works, right? But what happens when the engine is running throws me off. Why both wires have 10V KOER (sensor still unplugged)? And what is that rhythmic and steady hash that appears KOER?

Could it be shorted to a circuit that is only on when the engine runs, try to pull fuses and watch the temp gauge if it comes alive I guess you don't have that many fuses. 10 volts on both wires unplugged try a test lamp connected to + and probe the ground wire and check if it is a good ground

Temp gauge on older Audis normally is 10-volt feed but the ground is through body ground but the sensor is a two wire sensor and the other wire could be a switch but then you should not have a reading on the temp gauge when jumping those two wire together because this wire is not ground it should be power engine running, but it should explain 10 volts on the wire when running but not the ground when key on.