Coolant temperature sensor: resistance?

I have a '92 XJ 4.0 I-6 Auto tranny. Just put in a new stroker from Titan engines. Starts and runs! But after enough time for the engine to warm, and heater fan blows hot air - about 5 to 10 minutes operation, my temperature gauge hasn't moved, still indicating below 100 deg F.

I've read other posts and it seems the sensor is the one at the back of the head and not the one in the thermostat housing. Correct?

Does anyone know the electrical resistance this gauge should have at cold and 210 deg F? I can measure.

I did put in with teflon tape and note other posts say this might be the problem.

No reason to suspect the gauge or the sensor. Both worked fine with the old motor not more than a couple of weeks ago just prior to the swap.

Thanks!
Mike

I think the teflon tape is insulating it, that connector is a one wire so the sensor needs a path to ground in order to work. Quick enough to back it out, remove the tape and put it back in.

Self-grounding sensors are funny that way. I've had good luck using either a very thin layer of PTFE pipe dope (not tape - it's more of a paste) or RTV copper to seal those sensors. Don't use anything else - you remove the ground path (as Rich mentioned as well, and you already seem to suspect.)

If I even think it's a self-grounding sensor, I'll likely use RTV copper on it to save myself the headache of wondering.

The 95 FSM shows 100*F should be around 1365 ohms, and 210 should be 115 ohms. The quick check shown in the text is to ground the sensor wire and make sure the gauge pegs.

-edit-

Agreed that the tape may be insulating the sensor. Might be easier to check the body to engine block resistance before pulling the sensor.

Just found the 93 FSM which agrees with these numbers
TEMPERATURE GAUGE RESISTANCE TABLE
100F (38C) ...................................... 1365.0
220F (104C) ....................................... 93.5
260F (127C) ....................................... 55.1

Thanks for comments!

Measured sensor case to engine and see almost 0 Ohms. Not the teflon tape. See 12.3 VDC to engine at harness connector so the electrical system is trying. Measure open circuit sensor pin to engine. Pulled the sensor. Dang, it is visually apparent that the plastic connector is broken. I put it in with the motor in the vehicle and I must have broken it.

Purchased a new one. Installed. Now get readings!

But now my problems get worse: new engine heats right away to midway between 210 and 260 and it's a cold day here in San Diego. I've no reason to suspect the new sensor is wrong, have installed the old thermostat with which the old engine ran at about 190 deg F. Same radiator, and its only a couple of years old.

I have 11 quarts of coolant in the system. Books say system holds 12 and 12 is about the amount I drained out into my bucket.

New motor is a 4.6 liter but hard to imagine it puts out so much heat as to drive the radiator all the way to full heat transfer capacity on a cold day just driving up/down the street at 30 - 40 mph.

I'm at a loss...

Make sure you did not get the sensor for the idiot light dash. Thats a possibility.

Thanks, Rich

Possibly, but the indicated temperature seems right with the engine condition. New engine seems to heat up at about the same rate as old engine. It's just new engine keeps going beyond the 190 deg F that the old engine would have run at under today's conditions.

I'm wondering that maybe new engine is simply tight and spending more fuel simply pushing itself around. I notice that the starter motor doesn't seem as lively as with the old engine. It's to be expected that the new engine will be tight, but again hard to imagine it being so tight as to push the radiator to full capacity on a cold, low speed, day....

lawsoncl said:

The 95 FSM shows 100*F should be around 1365 ohms, and 210 should be 115 ohms. The quick check shown in the text is to ground the sensor wire and make sure the gauge pegs.

-edit-

Agreed that the tape may be insulating the sensor. Might be easier to check the body to engine block resistance before pulling the sensor.

Just found the 93 FSM which agrees with these numbers
TEMPERATURE GAUGE RESISTANCE TABLE
100F (38C) ...................................... 1365.0
220F (104C) ....................................... 93.5
260F (127C) ....................................... 55.1

Click to expand...

Removed and calibrated the sensor today...

at 37 C (98.6 F) measure 1397 Ohms
at 98.3 C (209 F) measure 152 Ohms

These measurements don't make sense compared to the tables above since I'm measuring *more* resistance at 209 than either 115 or 93 ohms. So my sensor should give indicated temperatures that are less than actual engine temperature if the tables above are correct, making my overheat problem even worse. I suspect however that the indicated temperature is greater than actual.

Guess the next thing I'll do is find a 152 Ohm resistor, connect it into the harness instead of the sensor, and see what the gauge reads. Stay tuned...

I get the same overrated temp readings with an aftermarket temp sender/switch. But your engine is going to run hotter with the tight rings an bearing clearances. An engine oil cooler might help the break-in process.

I've got a cheapo Standard Motor Products temp sender in my rebuilt 92, it indicates over the actual temperature. It stays solidly at 215° idling and in traffic, and backs down to about 211° on the highway. In other words, the range is shifted forward on the scale. No other signs of overtemp, besides a guage reading.

Been thinking of getting the OEM temp switch/sender 56027012, to see if I'm correct.