Temperature gauge maxes out

[jerula]

Hello everybody,

I have just bought my first truck, a 1974 C10. I absolutely love it, but I have one thing that worries me a lot.

The seller told me that the water temperature sensor was defective and that the temperature gauge would slowly go to highest temperature within 1-2 minutes when starting the truck. He gave me a new sensor that I installed immediately when I came home. Unfortunately the problem remains ... when I start the truck, the water gauge will slowly rise and rise until it reaches its limit.
Having, maybe too much faith, in the seller on the day of buying it. I took the truck and drove it home - 300 miles. There was no problems at all. However, I am reluctant to drive more than absolutely necessary until I have fixed this problem.

Has any of you seen this problem before? Can you help me in a direction to verify if it is just the gauge or if there is something more serious wrong with the cooling system.

As said I am absolutely new at this, so please be gentle

br
Jesper

 

[Bextreme04]

If the truck was actually overheating within 1-2 minutes of startup, the motor would not have survived a 300 mile trip.

I would verify that the cooling system if functioning properly by:

1. Make sure fluid is topped off in radiator and to full cold line on reservoir.
2. Make sure there are no oil spots or residue floating on top of fluid.
3. Make sure engine oil is dark brown or black and does not look like weak hippie coffee.
4. Start truck and let idle for 10-15 minutes
5. Verify no fluid leakage
6. Feel top and bottom radiator hoses. The top hose should be hard and hot. Bottom hose should be slightly less hot.
7. Shut engine off and let cool. Open radiator cap and verify full.

Once you know the system is working correctly and not actually overheating, start troubleshooting the gauge and sensor. It is possible the sensor is for the wrong year truck, wrong engine, or that the wire is broken somewhere. A sort in the wire would cause the gauge to read 0. An open(broken wire) would cause the gauge to go all the way to max.

 

[jerula]

Thank you. I have done what you suggested. Sounds like the wire to the gauge is broken. Could a worn out thermostat be the cause too?

 

[Bextreme04]

If the thermostat wasn't opening and the gauge was maxing out for real... you'd be seeing some steam and it would not be driving well at all

 

[austinado16]

Welcome to the forums!

The fact that it drove 300mi means you don't have a mechanical issue with the engine or the cooling system (as has already been stated above).

Typically, coolant temp gauges are fed a little less than vehicle system voltage, so maybe 9-10 volts, instead of 13-14v. And then to make the temp gauge work, the coolant temp sensor provides a variable ground signal. So when the engine coolant is cold, the temp sensor provides a poor ground (high resistance), and that keeps the needle in the lower position. As the coolant warms up, the temp sensor improves the ground function, causing the gauge to read higher. So in the case where a gauge needle moves to full scale right away, you typically have a ground wire (the wire from the sender to the gauge) that is shorted (touching) ground somewhere. This is usually where the wire passes through the firewall, or through some tight area along the engine...like maybe it's gotten too close to the exhaust and has melted to the manifold, etc.

 

[45tt]

I’m in the same boat. I have a new thermostat, coolant flush and fill, temp sensor and my gauge still doesn’t read. Lmc truck or Classic industries has the new gauge for $40. Replacing the gauge is the next step. I guess after 45 years, it’s to be expected that they could fail.

 

[75gmck25]

Does your temp sending unit have a "nailhead" connector, where you slide the wire connector onto the terminal from one side? Your '74 should be similar to my '75, and its a challenge to find a sending unit that really matches the gauge. If you buy the one they suggest at the parts counter it might not have the nailhead connector, and your gauge will always read very low if it has the resistance profile for gauges in the later year trucks.

The original Delco sending unit is no longer available, and the replacement Delco 12334869 is hard to find at local parts stores. IIRC, the sending unit I finally found that seems to have the correct resistance is a Standard Motor Products TS6, and they had it in stock at Autozone.

The wire for the temp gauge is dark green and it runs along the driver's side valve cover and then connects to the sending unit in the driver's side head. Its supposed to have a heat-resistance sleeve on the last 8-10" to protect it from the exhaust manifold.

Bruce

 

[austinado16]

A couple tools that are handy for this type of diagnostic work:
- Infrared temp gun
- Digital Voltage/Ohm meter

You can use the temp gun to shoot the thermostat housing on the engine, and confirm engine coolant temp. You can shoot all over the radiator and confirm that coolant is uniformly being cooled towards the bottom, AND that there are or aren't any dead spots in the radiator due to clogged, or partially clogged core tubes. You can also shoot the hoses.

The digital voltage/ohm meter will allow you to test the back of the gauge for voltage and for the level of the ground signal during the changing coolant temp, as the engine warms up. Similarly, you can check the changing resistance to ground, right a the sender, as the engine warms up.

A quick test of a temp gauge is to simply disconnect the wire from the sender. With the key "on" the gauge should read nothing. If the gauge does read, you know that sender wire is touching ground somewhere along it's path. The 2nd test is to touch the sender wire to a metal part of the engine, thus giving the gauge "full ground." You should see the gauge peg to it's highest reading. If the gauge passes this test, you know it's receiving power, AND you know it probably works.....so your issue at that point, would solely be in the sender.

 

[jerula]

Thank you guys. You are just awesome!

Tonight I will take a look at the cable. Perform the 2 tests suggested by austinado16 and see if I can conclude anything from that.

Will keep you posted - and also so that others might benefit from my learnings

 

[jerula]

Hi guys,
So I did the following:

After having having had the engine running. Checked the upper tube from the cooler - very hot. The lower tube, not nearly as hot.

1. Removed the cable from the sensor
2. turned the key - gauge went to 0
3. moved the cable to have ground contact
4. turned the key - gauge went to max
So, gauge is working. Cable is working. Apparently flow is ok.

The sensor I got looks identical to the one that was already in the engine. The sensor is indeed one with a nailhead connector, a Standard Motor TS6 and so is the replacement. What are the odds that I have 2 broken sensors?
I'm a little puzzled here and not sure what my next step should be?

This is the old one I pulled out and the box from the new one that's in the engine now.

You must be registered for see images attach

 

[Blue Ox]

You have the expected resistance readings for the sensor. Put it in a pot of water with a thermometer, hook a meter up to it and cook it.

 

[Bextreme04]

The next steps I would use require a digital multimeter and possibly a temp gun like austinado suggested. If you dont have the temp gun, you can just use the multimeter. I would check the resistance to ground through the sensor with the engine cold. Start the engine and check again when fully hot. If it is within the range, you either have a really bad connection on the back of the temp gauge in the cluster that is causing the resistance readings to be off, or your gauge is bad. I would definitely pull the cluster and check/clean the connections on the back of the cluster and the grounds on the parking brake body

 

[jerula]

oh, so the gauge can still be bad even if the previous test was a success.
I will go out and buy a temp gun - sounds like a useful tool to have in my arsenal. Might a few days before I get around to it, but I'll be sure to report my findings back to you.

Thank you again

 

[jerula]

oh ... when I shoot the radiator and the hoses ... what readings should I expect here?

 

[Bextreme04]

Your upper hose will be the hot coolant directly from the thermostat. The lower hose is after it has been cooled through the radiator and entering into the water pump. I would think anywhere from 180-220 on the upper... no idea on the lower, but definitely should be lower. I would actually put it right on the temp sensor on the driver side head. It should be ambient temp when cold and probably 195ish when hot.

The previous tests verified that the signal wire is connected to the gauge and isn't a direct short to ground or a break to open. It did not verify that the gauge is properly reading within the resistance range that the sensor is supposed to provide. Now that you know it has a connection, I would check the connections to the back of the gauge for corrosion and check the grounds, they are common issues that can do strange things to electronics