It's a MYSTERY!!!

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Dec 5, 2009 | 07:42 PM

54Guy

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From: North Central Washington

It's a MYSTERY!!!

96' Explorer with the 4.0 engine. Water temp gauge is pegged at "Cold". It's my neighbor's rig. I replaced the Water temperature sending unit that is located on top of the engine next to the thermostat housing. Guess what?

The gauge STILL not working right! He said that it worked right this past summer when it was hot out. But not now, when it's in the low 30's high 20's outside and cold out. His heater works fine, the radiator is full of coolant.

Now what?

Oh - one other thing: the a/c compressor cycles on and off every so often, even though the a/c ins not on due to the cold weather. Is that normal?

Dec 5, 2009 | 08:22 PM

85e150

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Check fuses, wires to the stock gauge.

If the defrost is on, the A/C will cycle. Part of a Federal Motor Vehicle Standard for defroster performance. Originally developed by GM. Dries the air first, then heats it for fast, fast, fast defrost.

Dec 5, 2009 | 08:42 PM

54Guy

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From: North Central Washington

Hm OK, good to know. I wondered, as it seemed fairly constant in cycling on and off.

Dec 5, 2009 | 09:31 PM

shorod

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From: Cedar Rapids, IA

If there is an open in the wiring for the engine coolant temperature sender or the sender itself it will read cold. Anything over 275 ohms will cause a cold reading. Similarly if there is a large air pocket in the cooling system you might get a cold reading while the air pocket is present. I would expect an air pocket to also cause uneven heat once the engine is up to temperature.

As for the A/C when defrost, the compressor will be engaged as long as the ambient temperature is above freezing.

-Rod

Dec 5, 2009 | 11:12 PM

54Guy

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From: North Central Washington

When we replaced the temperature sending unit, we put white silicone tape on the threads to seal it. Since then, I've heard that maybe this was NOT a good idea, as the sending unit is possibly supposed to contact the metal it's threaded into to ground properly. Is this correct or not?

The sending unit has only one wire coming out of it, so that kind of makes sense. Thanks.

Dec 6, 2009 | 02:57 AM

87 XLT

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From: SoCal

Try this simple little test: remove the rubber boot from the sender, shove a nail in it & ground the nail to the engine. Turn the key on & if the gauge does NOT read full hot the wire is open or the gage is bad.

BTW, the wire should be red with a white stripe.

Dec 6, 2009 | 08:46 AM

shorod

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From: Cedar Rapids, IA

Yes, the sender does get its ground through the brass of the unit. I would expect that the thread sealer tore on the threads as it was installed so it is probably getting a pretty decent ground, but it is worth confirming.

-Rod