"Engine Temp" light/sending unit question

[Dave 001]

The sending unit that turns the "Engine Temp" light on....... is it a one shot deal (as in once the light comes on, it stays on, and the sending unit must be replaced)? Or once the engine cools off, the light goes out? I would expect the later but (long story short) after just working on one, I'm thinking the first scenario might be correct. Thanks

 

[franklin2]

It goes on and off. It's just a temperature switch. Above a certain temp it closes and grounds the wire, below the temp and it opens and ungrounds the wire. When it grounds the wire it turns the light on, and it also pegs the temp gauge in the cluster.

Before I bolted my fan solid I saw my light go on and the gauge peg several times. I could actually smell some of the oil spots on the engine burning as well. No permanent damage done though, the system worked, I pulled over and let it cool off when it happened.

 

[Dave 001]

It goes on and off.

Thanks, that's what I needed to know.

 

[icanfixall]

This engine temp switch cloese at 242 degrees coolnat temp. Then it will peg the dash coolant gauge. To test this remove the wire from the drivers side head large sender. Ground it. Watch the dash gauge peg over to the "L" and even past that. No matter if the engine is cold or hot. Grounding this sender wire makes the happening. As the engine cools this sender will open and the gauge comes off the pegged mark.

 

[Dave 001]

This engine temp switch cloese at 242 degrees coolnat temp. Then it will peg the dash coolant gauge. To test this remove the wire from the drivers side head large sender. Ground it. Watch the dash gauge peg over to the "L" and even past that. No matter if the engine is cold or hot. Grounding this sender wire makes the happening. As the engine cools this sender will open and the gauge comes off the pegged mark.

There is no connection between the "Engine Temp" sender and the temperature gauge (unless an F series is wired differently than an E series and they are usually the same). I have the factory wiring manual (for '88 and '93) and the two systems are completely seperate. If the engine is so hot that the "Engine Temp" light is on, then the temperature gauge is already buried from its own sender.

 

[icanfixall]

They are electrically connected on the f sreies rigs. Do the test I posted about and see... I had to do it to know it was true... I'm the kind that has to touch the paint when I read the WET PAINT sign..

 

[international]

They are electrically connected on the f sreies rigs. Do the test I posted about and see... I had to do it to know it was true... I'm the kind that has to touch the paint when I read the WET PAINT sign..

Im going to have to agree with Dave...at least on my 94. My gauge stopped working while replacing my injectors and glow plugs a while ago. After searching for the three coolant sensors and figuring out what they each do I have concluded the one by the IP controls the IP stuff, the large one on the drivers side (outside) of the head controls the over temp light and the small sensor on the top front of the head buried behind the A/C compressor controls the gauge. Now, is the gauge sensor available at any of the "big 3" parts store? I can find the large one that controls the over temp light but that's it. Thanks!

 

[typ4]

on the back of the dash is a circuit board that "looks"at the overheat switch an buries the gauge as well as turning the light on.It also turns on the engine light. the gauge sender is a commom item, sw925 iirc. uhaul has a bunch, the overtemp used to be available but not sure anymore.

 

[Dave 001]

on the back of the dash is a circuit board that "looks"at the overheat switch an buries the gauge as well as turning the light on.It also turns on the engine light. the gauge sender is a commom item, sw925 iirc. uhaul has a bunch, the overtemp used to be available but not sure anymore.

I don't doubt what you are saying but that's not how mine is. I grounded the "Engine Temp" sender wire, the light came on and the gauge did nothing (engine about 30 deg. F when I did this). The gauge works just fine otherwise. No such circuit board on an '88 or a '93.....and your signature reads you have a '91.......that's weird. But why would the temperature gauge need to "look" at the "Engine Temp" sender also? If the engine was hot enough for the "Engine Temp" light to come on, then the gauge sender would already have the gauge buried?

 

[icanfixall]

Well guys... On my completely stock 89 crewcab I can remove the sender wire thats in the drivers side head and ground it. The dash gauge will pass the "L" and pegg no matter what temp the coolant is at.

 

[Dave 001]

Well guys... On my completely stock 89 crewcab I can remove the sender wire thats in the drivers side head and ground it. The dash gauge will pass the "L" and pegg no matter what temp the coolant is at.

Thanks for the input. It might come in handy. If I was working on a truck wired like yours, it would be a real head-scratcher when I did not get the results I would get from working on a truck wired like mine........(did that make any sence??).

 

[icanfixall]

Yes it does... I found this out about my 89 a few years ago. A member here explained how this works. So I tested it. Sometimes the large sender in the drivers side front of the head goes bad and this is how one can test the function. I replaced mine when I melted the engine after a freeze plug poped out with 5200.00 miles the first major rebuild and all that nice machine work. Note to self... The total loss of coolant will not show up on the dash or after market gauges. No coolant seeing the sender... No temp alarm.. When the engine saw 330+ degrees of oil the lifters sounded off. The engine slowed down.. Finally quit all together. Coasted over to the emergancy parking on the grade. Tried to crank it... Nope.. Not happening scooter... Pistons stuck to cylinder walls. May that engine RIP... Surpeisingly the piston scuffing was not a bad as I felt it could have been. Even the rods and mains looked great but while I'm in that deep.. Replace them and the oil pump... The oil cooler got another new set of o rings too. That oil sure smelt horrible.. Almost as bad as differancial oil thats been in for years...