Temp light always on - 1983 D-150

[loweyj]

My temperature warning light decided today that it will remain lit.

This morning I took the truck down to the local car wash and gave the engine compartment a good cleaning. On the way home I noticed the temp. idiot light shining bright red. I got home and checked the upper rad hose and rad and both were not hot to the touch. Ok I thought... Thermostat. So, I went out and bought one, installed it, fired up the truck and let her idiol for a few minutes, then took her for a quick drive... The light remained on the whole trip. When I arrived back home, I checkd the upper hose and rad and both were hot. I let the truck sit for two hours ( during which I boiled the old thermostat and of course... nothing wrong with it ). I then went out and fired up the truck after it had cooled right down and as soon as I started it, the idiot light came on, faintly at first, but with-in a few seconds it was glowing bright red. This all took place in under a minute, so it obvisously is not a overheating issue.

So, am I right to assume that the sending unit is at fault, or should I be thinking of other possibility's also?

[Chump]

Sending unit or the wiring to it may be shorting out. The sending unit is just a on/off switch that works at a set temperature. Could be during your wash that you disturbed the wiring to the sender. find the sender wiring and trace it to see if there are any bare spots grounding out. The sender you can get at any good auto parts store if you need one.

http://www.partsamerica.com/product_images/img/nie/ts26561-1.jpg

[acton mike]

the key here is the fact that the idiot light, if i read yo properly, was glowing faintly and then came on bright red

The light is either full on or off - nothing in between

and so methinks you have an electrical problem as opposed to an overheating problem

Buy a new sender - they are cheap
but also run the wiring to see if the insulation has chaffed somewhere along the line

you also may have soaked the wiring harness causing problems there

a warning to all - be careful when power washing the engine compartment either at home or at the carwash

[Chump]

When tracking down electrical problems you can have a a ground that just barely makes(high resistance) or makes fully(low/no resistance). If you take the wire off the sender and ground it out properly, the light will come on full bright. If you take the wire off and try to ground the wire out in a spot with engine grease/crud the light may not come on all the way do to poor conductivity to ground. ie: Headlights with a poor ground still work but not as bright. :):) The sender is supposed to provide a fully conductive ground at a set temperature(approx 250*-275*) so if the internals of the sender are marginal, so is the ground. Now, if the wire to the sender brushes up against the engine due to poor/burnt insulation from wire routing issues, you will also experience a poor conductive ground(short) at that point, the light will barely come on, flicker or fully turn on and the sender may be perfectly good and the engine is not overheated.

This basically applies to the idiot light sensor circuit. A dash temp "gauge" uses a different sender which has variable resistance.

When looking for these kinds of problems remember that the problem involves the "whole" circuit and not just the easily replaceable parts. This same diagnostic theory also applies to the newer computer controlled vehicles. I have seen many a person replace a part because the code set in the computer said the (example...EGR malfunctioned) which most people replace the EGR valve and still have the same error code appear only to find out they have a vacuum or wiring problem.

Sorry to ramble on but I have been doing this for many years and try to help everyone help themselves the best way possible. Hope this helps some. :D

PS...it could be just a bad sender also! :D :rck:

Regards-Stevo

[loweyj]

Thanks very much for the replies guys !

Well, this morning my wife used the truck to do some running around, while I went to the city to pick up the sensor. She told me the temp light was on all day while she had the truck. When I got home tonite, I went out and was about to install the new sensor, but needed to back the truck into the driveway first. I fired her up and no temp light. I took it out for a drive and never once did the light come on.

So, do you figure it could have been something resulting from the engine compartment wash I gave it, that caused this to happen? Possibly water or chemical getting into connector? This seems weird to me though, as I had the connector off a few times when checking for a short, but that was before the wife had the truck today and it was still lighting the temp light.

lol, oh well, I guess I would have needed a new thermostat and sensor eventually !!! :)

[77Utiline]

More than likely the issue is with the sender connector. Check and make sure it isn't wallowed out

[Speed Dragon]

Sounds like a faulty sending unit or bad connection at it.