Temp Dummy Light - Wiring Help

[Yamariv]

My Temp dummy light is always on as a default and is getting annoying driving around like that. Can someone explain to me how the dummy light system works on these trucks and maybe how to test the sensor to see if it's faulty or maybe test the wiring?

I'm pretty sure I've found the sensor, it's the one that goes into the head. Looks like I have a gauge sensor near the Thermostat housing. The dummy light sensor I have, only has 1 wire going to it and clipping on.

Why would it be constantly on?

Thanks for any help you can provide!

[dmjlambert]

The temp warning light sensor connects the wire to ground for an overheat condition. The bulb is already connected to 12V+ in the instrument cluster, so the sensor completing the connection to ground means the bulb glows. To tell if the problem is the sensor, unplug the wire from the sensor. If the light goes out, and the engine is not actually overheating, then that would mean the sensor has gone bad. The more likely problem is that wire is shorted to ground somewhere along the wire run. So if you unplug the wire at the sensor and the light continues to stay on, there is probably nothing wrong with the sensor and you need to track down your wire short. Another possible culprit is the wire goes to a terminal on your start switch that shorts it to ground and makes the temp warning light come on when the key is in the start position. This is a bulb check. If you can't find a short in your wiring, you can disengage that wire from the connector that goes to your start switch and see if the problem goes away. You could have a start switch going bad. I think the most likely scenario is a pinched wire that is shorted to ground. Have you done any maintenance, such as separated the transmission from the engine and put it back, and maybe pinched a wire?

[Yamariv]

Originally Posted by dmjlambert

The temp warning light sensor connects the wire to ground for an overheat condition. The bulb is already connected to 12V+ in the instrument cluster, so the sensor completing the connection to ground means the bulb glows. To tell if the problem is the sensor, unplug the wire from the sensor. If the light goes out, and the engine is not actually overheating, then that would mean the sensor has gone bad. The more likely problem is that wire is shorted to ground somewhere along the wire run. So if you unplug the wire at the sensor and the light continues to stay on, there is probably nothing wrong with the sensor and you need to track down your wire short. Another possible culprit is the wire goes to a terminal on your start switch that shorts it to ground and makes the temp warning light come on when the key is in the start position. This is a bulb check. If you can't find a short in your wiring, you can disengage that wire from the connector that goes to your start switch and see if the problem goes away. You could have a start switch going bad. I think the most likely scenario is a pinched wire that is shorted to ground. Have you done any maintenance, such as separated the transmission from the engine and put it back, and maybe pinched a wire?

Awesome info and explanation dmjlambert, thank you for the help! Ok, I'll have to play around in the garage and see what I can find in the next few days, your write up gives me a good start!

Not sure if this helps or adds to the mystery but before the light came on permanently, I noticed the plug was very loose on the sensor lead at the engine and tightened it up by crimping the crimp a little tighter so I makes good contact on the lead. I'm guessing that it wasn't making any contact originally hence the no dash light before? I can't remember if the temp light lit up for the few seconds when I turned the key on before I messed with the crimping. I think it did come on an go out. Not sure if my memory is correct on that though, can't say for sure that it lit up and went out.

Also, there seems to be two temp sensor replacements for the dummy light. One has a single lead (like the one I have now) and there's another one with two leads. The two lead one must be wrong and I should stick with the single lead for my truck?

Cheers

[dmjlambert]

Temp sensors are sometimes used for things other than the warning light. It could be the one with 2 leads is for the TCS emissions control system that was on the 71 and 72 trucks, or it could be something a previous owner installed to control a radiator fan. I think the one in the head probably has the single lead and is for the warning light. You will want it to switch on at the correct temperature. Sensors for things like emissions controls and fans turn on at normal operating temperature, so you don't want the light coming on when the truck gets warmed up.

If you have a hole in the head that is 1/2 NPT size as on the stock engine, you would use a Wells/Autozone TU51 sensor, and if you have an engine that is a couple years newer you may have 3/8 NPT hole in the head and would use a Wells/Autozone TU78 sensor. Those 2 sensors both switch at overheating temperatures.

TU51 switches on at 251 +-10 degrees F, and TU78 switches on at 240 +-10 degrees F, so either one is suitable for an overheat light. Just use the one that has the correct thread size for your hole in the head. Contrary to popular belief it is OK to use one or two wraps of telfon tape on the threads when screwing it in.

I believe the temp warning light wire is dark green. You can probably trace where that goes and test with a test light or ohm meter to make sure it is in good shape and doesn't have any shorts. But the first test is probably just unplug the wire from the sensor and see what your warning light does.

[Yamariv]

Originally Posted by dmjlambert

Temp sensors are sometimes used for things other than the warning light. It could be the one with 2 leads is for the TCS emissions control system that was on the 71 and 72 trucks, or it could be something a previous owner installed to control a radiator fan. I think the one in the head probably has the single lead and is for the warning light. You will want it to switch on at the correct temperature. Sensors for things like emissions controls and fans turn on at normal operating temperature, so you don't want the light coming on when the truck gets warmed up.

If you have a hole in the head that is 1/2 NPT size as on the stock engine, you would use a Wells/Autozone TU51 sensor, and if you have an engine that is a couple years newer you may have 3/8 NPT hole in the head and would use a Wells/Autozone TU78 sensor. Those 2 sensors both switch at overheating temperatures.

TU51 switches on at 251 +-10 degrees F, and TU78 switches on at 240 +-10 degrees F, so either one is suitable for an overheat light. Just use the one that has the correct thread size for your hole in the head. Contrary to popular belief it is OK to use one or two wraps of telfon tape on the threads when screwing it in.

I believe the temp warning light wire is dark green. You can probably trace where that goes and test with a test light or ohm meter to make sure it is in good shape and doesn't have any shorts. But the first test is probably just unplug the wire from the sensor and see what your warning light does.

Awesome info , thanks! I was was able to unplug the connection at the sensor and voila, dummy light went off!! That was too easy thanks to your help

Ok, so now I have to figure out if it's the 1/2 inch sensor or the 3/8th's. Does the measurement refer to the threads (I assume this) or the head of the sensor bolt? Thing is, the seller of the truck blew to the 71 motor and replaced it with a "later" I6 250ci. I think he said it was from the mid 70's but who knows..

Is there any way to tell what sensor I need by a pic? I'll attach a few to see what you think

[Yamariv]

On a side note, I eventually want to change out my gauge cluster for the fancier one with the temp, oil pressure and DC gauge. In the pic below, it looks like there is a second temp sensor in the Thermostat housing. Would that the the sensor for the gauge? I'm guessing the donor truck the motor came from had a temp gauge.

[dmjlambert]

Yes the measurement is the thread size. I tend to take the part with me to the parts store to compare, so that's what I would recommend. Have the parts store get both sensors on hand for you and you can buy the one you need when you look at them and compare to the bad one.

1/2" NPT is .81 inches if you can see some of the threads on the sensor, and measure. 3/8" NPT is .65 inches. I don't have experience in measuring NPT correctly, so you have to take my advice with a grain of salt. I found the measurements on this web site: https://blog.sisupply.com/2020/07/02...he-first-time/

[PbFut]

The normal location for the temp sensor is in the head on driver side. Green wire with a grey/black asbestos shield over the wire as it runs over the engine at the intersection of intake manifold and valve cover. If the truck is equipped with gauge, the sensor is a thermistor adjusting resistance to ground. If you have an idiot light cluster the sensor is a thermal switch that closes contact to ground at a given temp. Any sensor in the intake manifold by the thermostat or in the passenger side head have been added for a non oem purpose. There are many combos of adapters and sensors to mate various heads and sensors as needed. Temp sensors and switches are very simple devices. A simple harbor freight multi meter is a cheep very useful tool for trouble shooting our very simple electrical systems.

[dmjlambert]

On your side note about changing to a gauge instrument cluster, the sensor you show in the thermostat housing is not compatible with the stock gauge instrument cluster temperature gauge. There is only a very specific sensor type that will give the correct reading on the stock temperature gauge.
Here is the one I have and what the correct sender looks like is in the listing here https://www.amazon.com/gp/product/B000C9SM22/
And I understand from reading other threads here on the forum that also a company called Lectric Limited sells one that works well.

The gauge sender only comes in 1/2" NPT, so if you need one for 3/8 hole you must get the 1/2" NPT sender and have a machinist turn it down and re-thread it for 3/8". There are a couple of forum members here who offer that service, A1971Blazer and brian mac.

On the gauge cluster there is also a temp warning light, and in the stock wiring for that instrument cluster it is left unused. It would be possible to continue using the temp warning light by simply connecting the switch type sender that is your engine's head to that temp warning light, and wire the temperature gauge separately using the gauge type sender in the other head (if you get an 8-cylinder engine) or thermostat housing. I plan to wire mine up that way, so I have both methods of showing me an overheat condition.

The pinout of the gauge instrument cluster and the warning light instrument cluster connecting plug are completely different from each other. You would need to re-pin the connector and add wiring. It may not be very valuable to add wiring for the ammeter if you don't already have that wiring in the engine compartment, because that battery gauge gives you very little useful info. A lot of people upgrade there gauge type instrument clusters to have a volt gauge instead.

[Yamariv]

Originally Posted by dmjlambert

On your side note about changing to a gauge instrument cluster, the sensor you show in the thermostat housing is not compatible with the stock gauge instrument cluster temperature gauge. There is only a very specific sensor type that will give the correct reading on the stock temperature gauge.
Here is the one I have and what the correct sender looks like is in the listing here https://www.amazon.com/gp/product/B000C9SM22/
And I understand from reading other threads here on the forum that also a company called Lectric Limited sells one that works well.

The gauge sender only comes in 1/2" NPT, so if you need one for 3/8 hole you must get the 1/2" NPT sender and have a machinist turn it down and re-thread it for 3/8". There are a couple of forum members here who offer that service, A1971Blazer and brian mac.

On the gauge cluster there is also a temp warning light, and in the stock wiring for that instrument cluster it is left unused. It would be possible to continue using the temp warning light by simply connecting the switch type sender that is your engine's head to that temp warning light, and wire the temperature gauge separately using the gauge type sender in the other head (if you get an 8-cylinder engine) or thermostat housing. I plan to wire mine up that way, so I have both methods of showing me an overheat condition.

The pinout of the gauge instrument cluster and the warning light instrument cluster connecting plug are completely different from each other. You would need to re-pin the connector and add wiring. It may not be very valuable to add wiring for the ammeter if you don't already have that wiring in the engine compartment, because that battery gauge gives you very little useful info. A lot of people upgrade there gauge type instrument clusters to have a volt gauge instead.

Ok so dilemmas on what to do now. So you think the sender spot in the thermostat housing would be an ok spot for the temp gauge once I decide to try and switch everything over to the fancier gauge set? If that's the case, I can get the correct dummy light sensor and leave it in the head and use it for now.

I have a stock fancier used gauge cluster from another truck currently but was thinking of tackling that install in a few months. I like your idea of retaining the dummy light AND having the temp gauge! My preliminary research was saying that the plugs are completely different as well. Have you heard of anyone successfully doing the conversion to the fancier stock gauges from a dummy light dash? I'm assuming re-pinning the wires to the correct location is doable? I don't mind adding a few wires for the other gauges power etc.

Yeah, the amp meter will be a bit of a pain. I found a video where I guy takes a volt meter out of a square body and converts the volt gauge to fit in the amp meter spot. Might have to do that cause I heard there's a special resistor and wiring needed for the amp meter.

[Yamariv]

Originally Posted by dmjlambert

Temp sensors are sometimes used for things other than the warning light. It could be the one with 2 leads is for the TCS emissions control system that was on the 71 and 72 trucks, or it could be something a previous owner installed to control a radiator fan. I think the one in the head probably has the single lead and is for the warning light. You will want it to switch on at the correct temperature. Sensors for things like emissions controls and fans turn on at normal operating temperature, so you don't want the light coming on when the truck gets warmed up.

If you have a hole in the head that is 1/2 NPT size as on the stock engine, you would use a Wells/Autozone TU51 sensor, and if you have an engine that is a couple years newer you may have 3/8 NPT hole in the head and would use a Wells/Autozone TU78 sensor. Those 2 sensors both switch at overheating temperatures.

TU51 switches on at 251 +-10 degrees F, and TU78 switches on at 240 +-10 degrees F, so either one is suitable for an overheat light. Just use the one that has the correct thread size for your hole in the head. Contrary to popular belief it is OK to use one or two wraps of telfon tape on the threads when screwing it in.

I believe the temp warning light wire is dark green. You can probably trace where that goes and test with a test light or ohm meter to make sure it is in good shape and doesn't have any shorts. But the first test is probably just unplug the wire from the sensor and see what your warning light does.

Any idea on the part number if I went AC Delco for the temp light sensor? We don't seem to have auto zone in my area in Canada and I'm having a tough time finding anything from our local auto parts store. I did find the temp gauge sensor on Amazon though, thank you for that!

[LNP]

I would say your sensor is a 3/8 as I see its screwed into an adapter which most likely is 1/2. The pipe thread size is a reference to the inside diameter of the pipe not the outside diameter of the thread. Plugs sensors and such that use pipe thread will still go by the pipe ID even though they dont have a hole in them. If you use open end wrenches to measure the thread on your sensor 1/2 would be like a 13/16 wrench and 3/8 is closer to 17mm.

[dmjlambert]

I don't have any experience with these guys, but here is a page that sells a standard motor version and they have a cross reference to different brands here;
https://partshawk.com/standard-motor...re-switch.html
It was the 4th hit when googling "wells TU78 switch". The thing I don't like when shopping for these is they don't tell you the important technical spec that matters, the temperature at which it switches. I would be tempted to connect it to a multimeter and dunk it in a coffee can of antifreeze on the stove and use the multimeter and a candy thermometer to see at what temperature it switches on.

[dmjlambert]

On the sender that is installed on your engine, take a wire brush and brake, carburetor, or engine cleaner to that thing and clean it up, and see if it fixes it. Maybe there is too much gunk on it and it is causing a short. Just an idea.

[Yamariv]

Originally Posted by dmjlambert

I don't have any experience with these guys, but here is a page that sells a standard motor version and they have a cross reference to different brands here;
https://partshawk.com/standard-motor...re-switch.html
It was the 4th hit when googling "wells TU78 switch". The thing I don't like when shopping for these is they don't tell you the important technical spec that matters, the temperature at which it switches. I would be tempted to connect it to a multimeter and dunk it in a coffee can of antifreeze on the stove and use the multimeter and a candy thermometer to see at what temperature it switches on.

Originally Posted by dmjlambert

On the sender that is installed on your engine, take a wire brush and brake, carburetor, or engine cleaner to that thing and clean it up, and see if it fixes it. Maybe there is too much gunk on it and it is causing a short. Just an idea.

That's a good idea, maybe i'll pull it out and clean it up, you never know! Dumb question but do you think there would be rad fluid that high in the head or should I be able to pop that bad boy out w/o any coolant leaking?

No biggie either way, I'm going to do a rad flush in the next week or so so I'll have to coolant drained

Thanks for the link, never heard of those guys either for switches. It says factory tested to meet specs but then they don't say the specs! lol

[Yamariv]

Has anyone seen any threads in the past where someone has upgraded their gauge cluster from idiot lights to the fancier stock cluster with all the gauges. All I find is threads of people upgrading to aftermarket not upgrading to the stock gauge set

[LNP]

Here is a link to a google search with a bunch of links for this forum on switching.

https://www.google.com/search?domain...D%3A1%3B&hl=en

[Yamariv]

Originally Posted by LNP

Here is a link to a google search with a bunch of links for this forum on switching.

https://www.google.com/search?domain...D%3A1%3B&hl=en

Thank you for the link I went down the rabbit hole and found a bunch of threads explaining exactly what I need