Coolant Temperature Sensor/Sender Wiring for 1986 E30 with S52

[MaxBell]

Please help diagnose this stupid issue.

Facts:

1. The temp gauge in the cluster reads blue (cold) all the time, even after running the car for a while.
2. When I connect pin 4 of my C101 to ground, the temp gauge reads full red.
   1. This confirms that the wiring from the cluster to the engine bay is sound.
   2. This confirms that the temp gauge in the cluster is sound.
3. When I connect the S52 harness to the car and connect the temp sensor connector to ground, the temp gauge reads full red.
   1. This confirms that the wiring for the engine harness is sound.
   2. This confirms the temp sensor is properly connected to the cluster.

I have changed the temp sensor (brown top) for another, it doesn't work. This temp sensor was also off an old engine.
Is there anything else I should check, or just drain the coolant and swap the sensor?

[82eye]

Please help diagnose this stupid issue.

Facts:

1. The temp gauge in the cluster reads blue (cold) all the time, even after running the car for a while.
2. When I connect pin 4 of my C101 to ground, the temp gauge reads full red.
   1. This confirms that the wiring from the cluster to the engine bay is sound.
   2. This confirms that the temp gauge in the cluster is sound.
3. When I connect the S52 harness to the car and connect the temp sensor connector to ground, the temp gauge reads full red.
   1. This confirms that the wiring for the engine harness is sound.
   2. This confirms the temp sensor is properly connected to the cluster.

I have changed the temp sensor (brown top) for another, it doesn't work. This temp sensor was also off an old engine.
Is there anything else I should check, or just drain the coolant and swap the sensor?

have the sensors been re-threaded or the housing re-tapped ? the m20 sensor does not fit into an s52. thinking the issue may be there.

[MaxBell]

It was rethreaded, I didn't tap the head. But now that I think about it, how exactly does a sensor ground through an aluminum head? Isn't aluminum garbage at grounding and usually corrodes at the contact point?

[82eye]

It was rethreaded, I didn't tap the head. But now that I think about it, how exactly does a sensor ground through an aluminum head? Isn't aluminum garbage at grounding and usually corrodes at the contact point?

it probably grounds back through the harness.

[e30davie]

Aluminum is fine at "grounding".

Try running a temporary "Ground" from a good ground to the body of the temperature gauge sensor. I assume you are talking about the original e30 single wire temp sensor.

You can just strip the wire back like 50mm and wrap it around the sensor body itself.

If this makes it work then you do have a grounding issue to the metal that the sensor is screwed into. maybe you can run a more permanent ground to a bolt somewhere on the same bit of metal to a bolt somewhere close on the engine that has a proper ground.

Do you have good main engine grounds hooked up? i assume so if your starter motor works.

[MaxBell]

Yeh the car has been running and driving for a while now with the S52. Brilliant idea about grounding it with a wire, will test that. That should give the final solution for my German car.

Update: nothing in the cluster when I grounded the sensor. I am going to go with "it's bad".

[e30davie]

I re-read your opening post and missed that it reads full read when its plugged in. reading full suggests that the sensor wire is grounded directly. if when you are in the reading full state, and you remove the wire from the top of the sensor does it go back to zero? if it does that proves the wiring is correct.

one test you can do in situe is get a multimeter and put it between the sensor connector tab and the body of the sensor on resistance. if its buggered it will ready zero resistance, or open circuit (full resistance)

I cant recall what the values should be but if its not buggered it will read something other than zero or open circuit.