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:
https://www.ipdusa.com/products/5907/105967-volvo-coolant-temp-sensor-for-dash-gauge-aftermarket-vr850-460191

So I had the machine shop install the correct 2-wire sensor for the 940:
https://www.ipdusa.com/products/7209/106490-volvo-engine-coolant-temperature-sensor-for-dash-gauge-swf-1362645

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!

Unfortunately as I mentioned,the correct sensor doesn't work due to the stripped out threads in the head. We could attempt a helicoil, but the hole is very shallow and we were worried about damaging the sealing conical surface with the drill/tap.

That diesel/6 cylinder sensor looks promising, but tasca says its NLA. Was this sensor ever used in US cars?

Using a nipple or some other extension to "remote mount" the sensor is interesting, but I have no idea how we would make the nipple seal against the head.

Yes its available in the US. I'm sorry the 1362643 was an old part number - this I got from old 740 940 pdf catalog. The new part number is 9186773. Same part.

See:
https://www.ebay.com/itm/Genuine-Real-Volvo-New-Engine-Temp-Sensor-9186773-NOS-NIB/272398163329?epid=1211105809&hash=item3f6c321d81:g:a1sAAOSwTA9X8F3G

https://www.ebay.com/itm/Volvo-Genuine-Temperature-Sensor-9186773/202719043494?hash=item2f32ff1fa6:g:~lQAAOSwG-NdE8CF

https://www.ebay.com/itm/Coolant-Temperature-Sender-Unit-Fits-VOLVO-960-Sedan-Wagon-1990-1994/401277743885?hash=item5d6e04730d:g:NRUAAOSw8IpcXLnY

Regards,
Amarin.

Looking at the 1994 940 temp gauge wiring diagram, the circuit is not compatible at all with one-wire sensor. Maybe you could try another 2-wire shorter probe sensor used in other Volvo models such as 740, 760, 960 and diesel. Part number is 1362643. Same thread with 2-wire plug. For comparison, your 2-wire long probe sensor has the part number 1362645. I can't tell where the gauge needle going to be but I'm guessing it won't mess with the wiring and idle.

See:
https://www.proxyparts.com/car-parts-stock/information/make/volvo/model/940/part/engine-temperature-sensor/partid/4610566/

https://www.ebay.ie/itm/Coolant-Temperature-Sensor-Volvo-740-760-940-960-S90-V90-/303191427758?hash=item46979e0aae

Amarin.

Just get the correct sensor. You Ohm value are incorrect for the gauge.

I know this sounds ugly, but.....
I wonder what would happen in you added a nipple and connector between the head and proper two wire sensor, such that you can tighten the sensor without it bottoming out. I’m guessing it would retard the reaction time but maybe not enough to matter?