As knowing the coolant temperature is rather important, I'm a bit concerned as my dash temperature gauge only goes a little past the warm mark (the one just to the right of the C mark) after a 30 minute compute in traffic.

The only specification I could find on the coolant sensor was in the 1979-1985 Haynes book. Cold it should be 105-233 ohms. Warm to normal operating temp 20-50 ohms.

Cold, mine read 1105 ohms.
I have not tested warmed up yet but I wanted to check the gauge. I grounded the sensor wire which would put it at 0 ohms to the gauge. Upon turning the key on, the gauge went all the way to the right.

From the test so far, am I correct in assuming the gauge is functioning correctly?
Can anyone confirm the correct ohm reading of a known working cold sensor? I don't want to buy a new one if mine is correct and the low reading is correct.

I recently rebuilt the engine. In the process I found the overflow tank was gunked and the radiator was falling apart and leaked. I now have a cleaned, tight, system with a slightly larger aluminum radiator. So prior to the rebuild the temp gauge was a little to the left of the mid point. Is this the more normal position?

Suggestions and advise are always welcomed.

 

Yes it is.

 

Time to replace the sensor then. Too bad I found out now, I had it in my hand not more than 3 weeks ago.
I suspect repeated overheating without knowing it eventually did in the engine.
Lessons learned.

Thanks.

 

if you were bored (i might do this, you might have something better to do), the needle positions and resistances are in the FSM, so you COULD put the appropriate resistor in there, turn the key on and see where the needle goes.

it will be in the right place, as its a Mazda.

i think a lot of 12A cars died from a mild long term overheating, IE it just ran a little hot for a long time.

i've had to replace, or rod out the radiator in i think all of the 12A's ive owned?

manual is here http://foxed.ca/index.php?page=rx7manual#firstgen

 

I was thinking of finding some screw or nail with the 20-50 ohm resistance and see where the gauge hits to further test the gauge.

I've replaced the original radiator with the rebuild as it was shot.

I'm investigating the best method to remove the sensor, now that the engine is fully assembled and in the car. Its a tight fit from any angle. I remember putting the coolant sensor in before the oil pressure sensor, so I may just pull the oil sensor out first.

 

Duh...I didn't think that one through very well.
Hmmm. Radio Shack.

 

Tested with 47 and 22 ohm resistors.
Normal operating range is 20-50 ohms.
My gauge appears to be fine, but I need to replace the sensor.

Thanks for the suggestion.

 

hmm i was thinking about finding a potentiometer, but if its that simple... i have the resistors

 

Tonight I replaced the coolant sensor.
It was necessary to remove the oil pressure sensor as I needed to use a 13mm socket on the sensor and the oil pressure sensor was in the way.
I used a plastic plug to stop the flow of oil once the oil pressure sensor was out.
Removing the coolant sensor and installing the new one was easy, lost just a little coolant, less than an oz.

Lesson learned: Verify the sensors when the engine is in pieces.

 

I had the same problem with mine after my rebuild. Luckily I had a few kegs laying around to steal a sensor from.