Coolant temp issues...

[Savageftr]

Anyone have issues with their temp gauge reading lower than it should?

I replaced the sending unit but it still reads less than half what it should be at operating temp. I have confirmed it's warming up properly.


It's like there is high resistance at one of the connections between the gauge and the sensor or ....the gauge is no good.


Wondering if others have had this issue and how they fixed it.

Truck is an 86 C15.

 

[Bextreme04]

there are different gauges and different sending units for various years. There should also be a resistor on the back of the gauge that you can change to match the sending unit. What is the resistance you are seeing in the sending unit? Unplug the connector from the sending unit and then test resistance from the sending unit terminal to battery ground. Then plug it back into the sender and test resistance from the gauge to ground. They should be very close. If they are not close, you have a corroded or loose connection somewhere. If they are close, you probably have a bad gauge resistor, or the wrong gauge/sender matching.

 

[75gmck25]

It can be difficult to match sending unit to gauge, since GM changed them over the years. However, I thought that by ‘86 they were using an easy to find sending unit.

The sending unit for my ‘75 has a different resistance range than newer parts and I finally found the right one to replace the discontinued Delco item. I did multiple google searches and read many articles before I finally discovered which was the right one. Mine is a Standard Motor Products TS6 with a single button connection, but I doubt that is the right one for your truck.

 

[Savageftr]

there are different gauges and different sending units for various years. There should also be a resistor on the back of the gauge that you can change to match the sending unit. What is the resistance you are seeing in the sending unit? Unplug the connector from the sending unit and then test resistance from the sending unit terminal to battery ground. Then plug it back into the sender and test resistance from the gauge to ground. They should be very close. If they are not close, you have a corroded or loose connection somewhere. If they are close, you probably have a bad gauge resistor, or the wrong gauge/sender matching.

I got 2 different sensors from Rockauto. The first one had a resistance of about 600ohms and the second about 5k ohms. The 5k ohm one matches what I had in there from the factory. The sensor I changed out actually wasn't the issue at all.

The 600 ohm sensor reads too hot and then 5k ohms sensor reads too cold. Obviously the resistance changes with temp too. They are about the same amount "out" but in different directions.

I think I need to check for a connector or wire with high resistance somewhere along the way. I'll take the cluster out I guess and start there. Maybe the I'll find a corroded connection.


Is there some where I can find what the different resistors were on the gauge or is it just trial and error. I need a higher resistance resistor for one sender and a lower one for the other...

 

[Snoots]

See if any of this helps . . .

If not, there's always beer!


!

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[C10MixMaster]

I got 2 different sensors from Rockauto. The first one had a resistance of about 600ohms and the second about 5k ohms. The 5k ohm one matches what I had in there from the factory. The sensor I changed out actually wasn't the issue at all.

The 600 ohm sensor reads too hot and then 5k ohms sensor reads too cold. Obviously the resistance changes with temp too. They are about the same amount "out" but in different directions.

I think I need to check for a connector or wire with high resistance somewhere along the way. I'll take the cluster out I guess and start there. Maybe the I'll find a corroded connection.


Is there some where I can find what the different resistors were on the gauge or is it just trial and error. I need a higher resistance resistor for one sender and a lower one for the other...

I believe there are 2 coolant sensors on a CCC computer controlled carbureted engine , one for the gauge and one for the computer. 5K is way high for a gauge sensor. The 600 ohm is probably correct for your truck . Put the sender in the frig , grab a beer then your done with your beer retest the sender its probably closer to 1365 ohms when cold . might be a bad gauge or someone might have changed gauges and miss matched them at some point

COOLANT TEMPERATURE GAUGE PARAMETERS

For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω

For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω

 

[Snoots]

I believe there are 2 coolant sensors on a CCC computer controlled carbureted engine , one for the gauge and one for the computer.

You are right sir! I hadn't considered that his engine was 'one of those'.

 

[Savageftr]

My engine is not a computer controlled carb. I never had cats or the throttle position sensor. These carbs do have a dual capacity accelerator pump but if you unplug it, it just defaults to normal operation.

This is a Canadian truck and we had a lot less emission controls than you guys had down south. I do have the version with 1 knock sensor and the 5 pin ignition control module though. I might bypass the knock sensor system if I think it's retarding the timing when it shouldn't. It seems to be working though.

So the factory sensor looked like this. The top of the second sensor and the bottom of the 4th. The replacement sensor I bought to "try" looks like the 4th one pictured and they both read exactly the same. So I think the sensor is fine. When I measured the resistance is was 45ish degrees so that fits right into the chart at 5k ohms.

I need to get a chance to check the circuit from the gauge to the sensor to eliminate that before I chase my tail in circles.

thanks for the info and chart. Helps a lot!

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[MikecLA]

I went thru all this on my 74. First off the factory gauge is not linear, meaning 1/4, 1/2, 3/4 mean nothing. 195 on mine is 1/4 up the gauge. I run a manual gauge as well to prove this. I have a new gauge ,proper sender and all. Even when i was a GM tech thru the 90s they purposely run gauges cold to avoid people panicking" my car is over heating its at 3/4!!!" seems that wa the case in the 70s too.

 

[Savageftr]

I went thru all this on my 74. First off the factory gauge is not linear, meaning 1/4, 1/2, 3/4 mean nothing. 195 on mine is 1/4 up the gauge. I run a manual gauge as well to prove this. I have a new gauge ,proper sender and all. Even when i was a GM tech thru the 90s they purposely run gauges cold to avoid people panicking" my car is over heating its at 3/4!!!" seems that wa the case in the 70s too.

I hear what your saying but when your gauge has temperatures on it you would think they should be close.

Mine is metric so it's got 40(cold) 100(12 oclock) and 125(hot). It usually runs just over the 40 mark which is just wrong. operating temp should be around 80C to 90C.

under heavy trailer load it goes to the line between the 40 and 100 (around 10 oclock). I think when it warms up like that im pushing 220F

I should get a temp gun to confirm I guess. Or just keep running it like it is.