RWD - Coolant Sensor Woes

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Coolant Sensor Woes

Ok folks, before I get to my issue, a little history. I had a head rebuilt by a local machine shop for my wife's 1994 940T. The 530 head was from an early car ('85 740T) so it had the wrong coolant sensor for the gauge. It had this one-wire sensor:

So I had the machine shop install the correct 2-wire sensor for the 940:

However, it turns out this sensor, while having the same threads, is deeper and the hole in the head was not tapped deep enough. This meant the 2-wire sensor only engaged 2 or so threads. The sensor started weeping and when I went to tighten it, the outer two threads tore out of the head. Bad news!

With some work wtih a tap, my friend and I were able to install the much shorter 1-wire sensor successfully without leaks! However, now the electrical gremlins have started!

We rewired the gauge so that is connected to the 1-wire sensor (which grounds through the head). The coolant gauge worked, but is extremely noisy as the needle moved with the blinker and changed wildly when things like the headlights were turned on! We tried grounding the 2nd wire (which is a ground wire) to the head, which helped a little bit, but we still see wild swings when the headlamps are turned on. In addition, the range of the sensor is different so the gauge is reading hot (above the middle point) even though the head is normal temps. It just reaches the red when the cooling fan kicks in.

In addition, the car has a new idle problem that did not exist before this issue with the coolant sensor. Now, as the car heats up and the gauge moves above the midpoint, the idle will slowly drop and drop until the engine almost dies. This behavior seems to go away when the coolant gauge is completely disconnected, although I only tested this once so I'm not 100% this is the case. I did a full smoke test to ensure we didn't introduce any new vaccuum leaks during the removal of the intake manifold to repair the sensor and found nothing.

Ok now come the questions...any ideas how we could properly rewire the gauge/cluster wiring to accomodate the 1-wire coolant sensor that we are now stuck with? Any ideas on how the wiring for this gauge could affect the idle? The separate ECU coolant sensor was unaffected and the engine runs normally otherwise.

Thanks in advance for any help and advice folks can give!


New Coolant Sensor Woes
posted by  MisterBLC  on Tue Jul 23 10:10 CST 2019 >

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