The car is S4 '88. The short block is a hybrid, using all S5 parts except the center iron. Rebuilt engine, new RAD, new mazda thermostat.

So... I have to use a S5 temperature sender. According to the FSM for both series, this shouldn't be a problem because both gauges have the same resistance range (154ohms at COLD to 13ohms at HOT) and both senders are supposed to have the same resistance at 80* centigrade (about 50ohms).

I have been through 3 senders, the last one being an expensive mazda sender. All have done the same thing: my gauge only goes to the COLD mark when fully warmed up.

Here's the thing that confuses me. I put a 13ohm resistor between the sender lead and ground and the gauge went to the HOT mark.

WTF am I missing?

 

Sounds like what you're missing is a good ground between the sending unit and the engine.
Are you using teflon tape maybe?

 

Nope, but the stock mazda sensor comes with a dry sealing material already on part of the threads.

I think pipe thread is such a force fit that it cuts through any tape or sealant and makes contact, but....

I will try bridging the ground between the sensor body and chassis grnd.

 

I agree about the interference fit but a supplemental ground couldn't hurt to try.

 

I tried a ground from the body of the sensor to the battery and it made absolutely no difference. The resistance between the sensor and intake manifold was 1 ohm. So, It's grounding with the iron well enough.

I'm beginning to think I need an S4 sensor...