Temp gauge not reading

[Ikeece]

I’ve got an 82 C10 Scottsdale with the 305 in it and ever since i got it, i’ve never had any reading on the temperature gauge. I’ve replaced the temp sender in the block and i’ve tested about three different gauges and you guessed it, still no reading. I’ve tested the gauge in it now with power and it’s functional. I’ve also took a wire to the negative terminal of the battery and hooked it to the dark green wire that goes to the sender, and it makes the gauge go all the way to hot like i saw in another thread. One thing that may be causing an issue is the thermostat as I have no clue if it has one in it. Could that be the issue? And is there any way to check if it has a thermostat without pulling hoses?

 

[Ikeece]

UPDATE!!
I went and replaced the thermostat and i’m still not reading anything on the temperature gauge.

 

[75gmck25]

Temperature sending unit resistance and the matching gauge has changed over the years on a squarebody. For example, if you have an '82 gauge and a '75 sending unit, it will not read correctly because the resistance of the sending unit does not match the gauge. However, it will usually still display some type of reading.

Is it possible that you used teflon tape on the sending unit and it's not grounded to the block? It must have a good ground from the threads, and the engine must be grounded to the body, and then the instrument panel is also grounded to the body.

Grounding the green wire to the block or any other ground is the right process to verify the gauge works. If you want to the check the actual calibration you would need to buy a few resistors and simulate the resistance of the sending unit.

This used to be simple in the days of Radio Shack, but now you probably have to get the resistors by mail order. I don't think I could find the documentation right now, but I've seen a process that shows the resistance values to use for calibration. However, this chart should get you started. https://www.metroli.org/pdf/Temperature Sender Resistance Chart.pdf

 

[bucket]

I'm wondering if you haven't been buying temp senders, but rather temp switches? If a temp switch (for just an idiot light cluster) is used, the gauge won't read anything until the engine is overheating.

Parts store listings suck these days, so it wouldn't surprise me if the wrong part has been sold to you. I actually boycotted a certain AutoZone 25 years ago because I went there to get a temp sender for my '78 van, but they only had listings for a temp switch. Then the guy behind the counter argued with me and told me that a switch and a sender was the same thing.

@75gmck25 Do they not make variable resistors anymore? I used to buy them from a local electronics shop, to adjust improper temo gauge readings on early 90's Grand Prixs and also to bypass GM VATS systems that had gone wonky.

 

[Ikeece]

Temperature sending unit resistance and the matching gauge has changed over the years on a squarebody. For example, if you have an '82 gauge and a '75 sending unit, it will not read correctly because the resistance of the sending unit does not match the gauge. However, it will usually still display some type of reading.

Is it possible that you used teflon tape on the sending unit and it's not grounded to the block? It must have a good ground from the threads, and the engine must be grounded to the body, and then the instrument panel is also grounded to the body.

Grounding the green wire to the block or any other ground is the right process to verify the gauge works. If you want to the check the actual calibration you would need to buy a few resistors and simulate the resistance of the sending unit.

This used to be simple in the days of Radio Shack, but now you probably have to get the resistors by mail order. I don't think I could find the documentation right now, but I've seen a process that shows the resistance values to use for calibration. However, this chart should get you started. https://www.metroli.org/pdf/Temperature Sender Resistance Chart.pdf

last summer i replaced the gauge cluster and housing with the nicer silverado one and with that, im unsure of the year truck it came out of, the style is the same but it’s probably a few years newer than the original one. Is there any way i could figure out what year truck the cluster/gauge came from to pinpoint down what sending unit matches it? About the teflon tape thing, i replaced the sending unit when i first got the truck so i cant really recall if i put tape or not.

 

[Ikeece]

I'm wondering if you haven't been buying temp senders, but rather temp switches? If a temp switch (for just an idiot light cluster) is used, the gauge won't read anything until the engine is overheating.

Parts store listings suck these days, so it wouldn't surprise me if the wrong part has been sold to you. I actually boycotted a certain AutoZone 25 years ago because I went there to get a temp sender for my '78 van, but they only had listings for a temp switch. Then the guy behind the counter argued with me and told me that a switch and a sender was the same thing.

@75gmck25 Do they not make variable resistors anymore? I used to buy them from a local electronics shop, to adjust improper temo gauge readings on early 90's Grand Prixs and also to bypass GM VATS systems that had gone wonky.

I was looking around earlier and this was also brought to my attention. This definitely could be a possibility, As soon as i figure out exactly what i need im for sure going to buy a new sender and make sure its a sender

 

[Jgonick]

Grounding the green wire to the block or any other ground is the right process to verify the gauge works. If you want to the check the actual calibration you would need to buy a few resistors and simulate the resistance of the sending unit.

Just some info if you are curious
All you need is 150 ohm resistor & 220 ohm resistor wired in parallel ( I would use at least 1/2 watt resistors)- It will be roughly 90 ohms resistance- in the old service manuals, test on gauges use a J-24538-A tool- this is a homemade version of that tool.

 

[75gmck25]

Bottom line - based on Rockauto site, the TS76 sending unit or equivalent is probably the one you need, unless your cluster is '79 or older. With the older cluster, try the TS6 or TS6T.

One aspect of my '75 is that it only shows 1/4, 1/2, 3/4 scale in the gauge, so I have no idea what exact temp is being displayed. However, after installing an aftermarket gauge under the dash I am very happy to find that with a TS6T sending unit it shows 1/4 scale most of the time (which seems to be about 185-190), and up to 1/2 scale in very hot weather with A/C on (seems to be about 205-210). Its good enough.

-----------------------------------------------------
These were the notes I made when I was trying to get my gauge to be relatively accurate.

Specifications for sensors - the posting will totally screw up the columns, so I added a bunch of dashes to fill in

------------- 1974-1978----1967-1973------1979-1987 vehicles
Left line ------ 350------------350---------------1365
Middle --------- 68------------- 76--------------- 96
Right ---------- 46------------- 51--------------- 55

1513321 is correct sending unit for '75, but no longer produced by GM, so used TU-5 instead of recommended sending unit (which seems to be wrong)

Measured with sensors suspended in boiling water
-------------TU-5----------Recommended replacement for 1513321
Cold-------- 587---------------825
140--------- 241--------------360
165--------- 187--------------260
185--------- 149--------------204
203--------- 125--------------101

TS6T - nailhead connector (this is what I finally used)
100 degrees - 368 ohms
220 degrees - 80 ohms

TS6 - same resistance, different connector
------------------------------------------------

One aspect of my '75 is that it only shows 1/4, 1/2, 3/4 scale in the gauge, so I have no idea what exact temp is being displayed. However, after installing an aftermarket gauge under the dash I am very happy to find that with a TS6T sending unit it shows 1/4 scale most of the time (which seems to be about 185-190), and up to 1/2 scale in very hot weather with A/C on (seems to be about 205-210). Its good enough.