Coolant Temp Gauge Info

[Shawn]

Thought I should post this info since its valuable for others who are running into this issue. I recently replaced my cluster and my temp gauge was barely registering on the 1st tick mark. After doing some research I found the years can make a difference. I have a 1979 and I'm willing to bet my cluster was from 78 or older. This means the temp sensor and the gauge won't work together. The correct sensor for the older years should be AC Delco G1852. Thanks to @Wes Harden for that info. For the newer trucks, it will be a different sensor with higher ohm readings.

74-78 sensor:


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 - 1991

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

I would first check the sender's resistance under actual operating conditions. Remove the sensing lead from the sender and - with the engine cold - get an initial resistance reading. One probe on the sender's terminal and the other to a clean nearby spot on the cylinder head. I used a muli-meter on ohms to verify what it reads when cold and warm.

As the coolant warms up, keep checking to confirm that the resistance across the sender begins to fall. Since you are getting absolutely no reaction out of the indicator, don't be overly concerned with the actual ohmic values. Just confirm that resistance does decrease as a function of coolant temperature rise.

If you are interested, here is the temp/resistance matrix for 79-91 GM coolant senders:

If the resistance tracks downward with an increase in coolant temperature (even if it's a bit off), I would say you could eliminate the sender end of the circuit as the problem.

As you can see from the chart, the indicator would like to see a resistance of about 1600 ohms to bump the needle off "C" (100 F). And, when the engine reaches normal operating temperature (coolant @ 195 F) resistance across the sender should be about 240 ohms - which should drive the needle to just left of the midway mark. This is for 79 and newer models.

Notice the white ceramic part behind the gauge. Thats the resistor. You can replace it with a standard 90-100 ohm resistor if you find yours to go out and the gauge is pegged even with a good sensor.

Also- If you ever want to test/check the gauge out of the vehicle, here is how to hook it up.

 

[Wes Harden]

Great info.

 

[cheavyk10]

Really good write up. I ran into sort of the same issue, when I swapped in my crate motor I used the sending unit from a 76 in my 84 K10. Since then my temps have read pretty high. My fuel injection system has a temp sensor that I installed right behind the thermostat housing in the intake manifold, and I'm monitor that with the handheld controller. There is usually a 20 or 30 degree difference between the two. So if the fuel injections reading 180, I have 210 on the gauge. I kind of just got used to that, but maybe I'll dig into it more and get the proper sending unit.

 

[Wes Harden]

You need the second ver of sender, it is 3/8 pipe thread. Just need a bushing to to install

 

[Shawn]

Both of my holes in my BBC are 1/2" NPT. Side of head and Edelbrock intake manifold. Im using the manifold for this one. If you have a 3/8" NPT sensor you can get the 1/2 to 3/8 NPT adapter at most hardware stores. Should be getting my GM sensor today and will update.

 

[Shawn]

Temp gauge now reads accurate with the AC Delco G1852 1974-1978 sensor. This is normal operating temp mark. Before it would just stay on the first line next to "C" when it was 185 degrees.

 

[Big Ray]

thanks.
I would have been chasing my tail when I do my crate motor swap!

 

[6872xtc]

Thanks for the information!
I just swapped a sender into my '90 to get away from the 3/8" NPT. My engine is older, with Edelbrock heads, and needs a 1/2" NPT. The bushing made the sender tip flush with the shoulder of the bushing and also made it stick out of the head farther towards the header.
I used a Delco 213-4792, ordered it for a 1979 truck. It has a 1/4" spade connector, the '90 had a button top stud.
So far, it is closer than the original with a bushing.

The G1852 is what was listed for my '72, BTW.

 

[Shawn]

Thanks for the information!
I just swapped a sender into my '90 to get away from the 3/8" NPT. My engine is older, with Edelbrock heads, and needs a 1/2" NPT. The bushing made the sender tip flush with the shoulder of the bushing and also made it stick out of the head farther towards the header.
I used a Delco 213-4792, ordered it for a 1979 truck. It has a 1/4" spade connector, the '90 had a button top stud.
So far, it is closer than the original with a bushing.

The G1852 is what was listed for my '72, BTW.

Sounds like I can include this sensor for 69-72 then. I'll see if I can create a table for the older vehicle. Still easy to test with any multi-meter. Should be 68-350 ohms when warm.

Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

 

[Big Ray]

So, this has been percolating in my brain for some reason
So I measured my gauges.
The resistor in my 79 gauge read ~88 ohms on a cheap vatozone meter.
The resistor in my 84 reads ~43 ohms.
Guess GM used 78 style until they ran out then switched to the later style...?

The reason I did this is when I put the tach in I had some new gauges I had bought from someone here. They look like the first design without the icons explaining their function.
That was about the time my truck blew a radiator hose. So I put new hoses and a thermostat in it.
Gauge never read accurately after that.
So I stumbled into this thread much later. I bought a new temp sensor based on the info contained therein.
I will pull the gauge that's in it and measure it this evening.
I betting it's the higher resistance value, which would explain why the needle hardly moves at all.

Or they just replaced the temp sensor along the way with the wrong unit.

I will report test results afterwards.

 

[77snowwheeler]

What I found out is the year doesn’t matter at all. With all my after market gauges, water, fuel, the ohms, is the deciding factor. If that doesn’t match, nothing works together. All my gauges after market gauges work, great. I run high performance, so it was necessary, to run gauges that can be more specific.

 

[Big Ray]

The gist of my thread is the proper factory sensor has to match the proper factory gauge value.

 

[dyeager535]

It sounds like you are measuring the resistor, not the resistance of the sender, which is what the OP did. Am I incorrect?

From what I've been able to deduce from a fair number of 81+ cluster disassembles, the resistor is used to "tune" the gauge. The fact that the hand-written number on the gauge housing matches the resistor value indicates to me that the gauge was calibrated before it went into a cluster.

Without assuming the sender to be at a fixed resistance at a given temp, they would have had to tune every gauge to every sender.

 

[Joel Wilson]

It sounds like you are measuring the resistor, not the resistance of the sender, which is what the OP did. Am I incorrect?

From what I've been able to deduce from a fair number of 81+ cluster disassembles, the resistor is used to "tune" the gauge. The fact that the number on the gauge housing matches the resistor indicates to me that the gauge was calibrated before it went into a cluster.

Without assuming the sender to be at a fixed resistance at a given temp, they would have had to tune every gauge to every sender.

It never set well with me not knowing my actual temperature so I added a second temp gauge that measures coolant on the lower radiator hose. I still kept the original dash gauge as a backup. I didn't know about any of this when I put second gauge in. It just never set well.

 

[Big Ray]

It sounds like you are measuring the resistor, not the resistance of the sender, which is what the OP did. Am I incorrect?

From what I've been able to deduce from a fair number of 81+ cluster disassembles, the resistor is used to "tune" the gauge. The fact that the number on the gauge housing matches the resistor indicates to me that the gauge was calibrated before it went into a cluster.

Without assuming the sender to be at a fixed resistance at a given temp, they would have had to tune every gauge to every sender.

Yes, I measured the gauges.
I figured if GM changed the sensor they would have to change the gauge to work with it.

 

[dyeager535]

But you measured the resistor, correct?

The resistor, as far as I can tell, was less about making the gauge work with different senders, and more just making sure that at X ohms resistance, every gauge read Y degrees.

If the resistor was what allowed different gauges to work with the same sender, I would expect them all to be the same value. I believe the gauges internally are different if the resistance for a given temperature is different.

 

[Joel Wilson]

With the 2nd gauge and sender, they match for ohms but never measured them. Both sender and gauge are Bosch mechanical. Now you have me 2nd guessing. The wire for the gauges from senders would add to the measurement too.