Coolant temperature switch and coolant overheat switch fail at the same time?

[headhunter38]

A few days ago I lightly pressure washed my engine bay. Held the wand at least a foot away except right by the fuel filter and number 1 injector to clean out some fuel/oil that accumulated there. Since then I've had some electrical issues. Both the coolant overheat switch and the coolant temperature switch seemed to have failed on me simultaneously. The temperature guage wigs out and says the engine is overheating and the check engine light is on. The truck fast idles all the time even when fully warmed up. When both of these switches are disconnected the truck operates as normal. My question is, is there something else that could have failed causing these issues instead of the actual switches themselves failing? Ive looked at the wiring diagrams and have read several threads about both of these switches and I'm wondering if I'm missing something.

 

[Clb]

My guess is you created something by disturbing the status quo.

Look at the eng. Bay harness section.
Bad grounds
Shorted harness
Verify the throttle is free

Disconnecting the idle up solenoid and Verify it isn't the issue first.

You got a wet short I bet
Dry it out either with aairleaf blower first, then eng heat for at least a half hr running .

 

[IDIBRONCO]

Since the check engine light is supposed to light up if the temperature gauge pegs all the way hot, then I would assume that it's working right if it only comes on when the gauge reads all the way hot.
I would concentrate on the gauge sender part. When I worked on these for a living, we got in a hot water parts washer that just used soap for a degreaser. At first, we'd throw all of the parts that we changed from engine to engine (like the senders) in the washer. After a while, I started noticing that the lights and gauges sometimes had problems, and we'd have to put in a different sender to fix the problem. I went back to the old way of using brake cleaner, compressed air, and a wire wheel on a bench grinder to clean them up and the issues stopped happening. I told the shop foreman, but he didn't believe me. He said that soap and hot water couldn't affect the senders like that, but I know what I know. These were senders that didn't have issues when the trucks came in. Even the fast idle switches would stop working sometimes. I would try another temperature sender and see if that fixes the problem.

 

[ihc1470]

You would not by chance be low on coolant and the engine is actually overheating a bit? Since it is two different senders and both are saying hot it would make me wonder.

 

[KansasIDI]

So, my 91 just has one coolant temperature sender (Canadian market truck). Is it possible that his truck just has one as well?

 

[ihc1470]

Are we using the same terminoligy? The unit that controls the gauge is called a sender. The unit that controls the light is a switch. First is a variable resister the second open close switch. The only thing that I recall is different is a Murphy system where the switch is built into the gauge. So you are correct in that your truck would only have one sender, if it had the warning light also there should be a switch for it.

 

[Jim993]

The temperature gauge went way high on my '85 F250 4x4 couple of months ago. I checked actual engine temperature with my infrared thermometer - all fine.

Replacing the instrument cluster voltage regulator solved the problem. And Dennis-Carpenter.com now has a solid state unit available for older Ford trucks.

You must be registered for see images attach

 

[franklin2]

What you guys do not realize, the coolant overtemp switch IS tied into the coolant temperature gauge, along with the coolant temperature sending unit. Both are wired together. Everything works like a conventional temp sending setup until the coolant temp switch activates. When it activates the gauge is pegged on purpose by the wiring from the temp switch.

Here's a diagram showing how the temp sending unit and the temp overheat switch are wired together. Look in the lower middle part of this diagram.

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[ihc1470]

Still two sending units one a switch and one a resistor. Doing the same thing a Murphy gauge would do with the exception that the switch is inside the gauge on a Murphy and its purpose is to shut down the engine instead of turning on a light.

You are also correct that I had not looked a the wiring diagram and did not know that Ford had wired it that way.

 

[IDIBRONCO]

What you guys do not realize, the coolant overtemp switch IS tied into the coolant temperature gauge, along with the coolant temperature sending unit.

Since the check engine light is supposed to light up if the temperature gauge pegs all the way hot, then I would assume that it's working right if it only comes on when the gauge reads all the way hot.

Seems like I've seen something like this before.

 

[headhunter38]

Ok so I was specifically talking about the coolant overheat switch and the coolant temperature switch for the cold fast idle solenoid and cold timing advance. The one on the front driver side cylinder head right above the exhaust manifold for the overheat switch. And the one in front of the number 1 injector behind the thermostat.

You got a wet short I bet
Dry it out either with aairleaf blower first, then eng heat for at least a half hr running.

Seems that you were right. I disconnected the overheat switch for a few days and drove it around. The fast idle problem resolved itself. I reconnected the overheat switch today and it hasn't been acting up again. Seems like took several days of driving to really dry it out and get it back to normal. So to anyone reading this who has the same problem, try disconnecting the switch for a few days and then reconnect it and see if the problem goes away. I would suggest to disconnect it so that the guage is not maxed out telling you that its overheating when it isnt. The temperature guage will function normally with the switch disconnected.

 

[Clb]

I posted this years ago, it may have been in response to a new to him truck....


Load it up heavy, take it to the hills and get it hot and sweaty for at least "2 hours STRAIGHT "
It takes around 75 miles of constant running to boil out the condensation in the oil.
You did the same only slower.
Glad it was easy.
Tip
Nolox or dielectric grease is your friend.

 

[IDIBRONCO]

Both the coolant overheat switch and the coolant temperature switch seemed to have failed on me simultaneously. The temperature guage wigs out and says the engine is overheating and the check engine light is on.

This is where some of the confusion came in. "Coolant temperature switch" made me think of the one that runs the gauge. Mentioning that the temperature gauge and the check engine light both act up only enforced that idea. Those are connected. The fast idle isn't connected to those and can act up independently.

 

[headhunter38]

This is where some of the confusion came in. "Coolant temperature switch" made me think of the one that runs the gauge. Mentioning that the temperature gauge and the check engine light both act up only enforced that idea. Those are connected. The fast idle isn't connected to those and can act up independently.

Yeah looking back i should have made it clear that I was also talking about the temperature switch for the fast idle system. I often find myself thinking faster than I can type and not realizing I didn't type what I thought I did.