temp gauge question

[Layne02]

So my temp gauge stopped working. I have tried another old gauge , didn't work. Changed sender, didn't work, so finally used my head and used voltmeter. I have 12v at the sender, so ruled that out, took gauge off and can't get 12v at any of the terminals that gauge plugs into. So, should I be? Am I losing it somewhere?

 

[Honky Kong jr]

So my temp gauge stopped working. I have tried another old gauge , didn't work. Changed sender, didn't work, so finally used my head and used voltmeter. I have 12v at the sender, so ruled that out, took gauge off and can't get 12v at any of the terminals that gauge plugs into. So, should I be? Am I losing it somewhere?

The signal from the engine to the gauge is a ground signal not power. Ground your signal wire it should take your gauge all the way to one side. I wanna say hot side of gauge.

 

[Layne02]

Ok, so if I ground it and it takes it all way over , what does that get me? Gauge is good?

 

[Honky Kong jr]

Ok, so if I ground it and it takes it all way over , what does that get me? Gauge is good?

Yup and good wiring

 

[Layne02]

OK, so, I took sender wire off, grounded it, nothing, then tried a ground jumper wire inside at dash. Grounded the left pole on back of gauge and gauge pegged out on hot side. So, assume gauge is ok, I know sender is brand new, so wiring problem somewhere.

 

[RustyPile]

You haven't said what model you're working on. Pre 1980 temp sender is different from 1980 - 87.. Same applies to gauge.. Early or late gauge will work, but you have to match sender to gauge. With the gauge removed, you can test the cluster wiring for the gauge. With the ignition turned on, the terminal on the right will have 12 volts on it. No voltage on the top --- it's the ground. However the continuity can be tested.. Connect a test light between the right terminal and the top terminal.. The light should glow. If it does, the cluster circuit is good.. The left terminal is the sender circuit.. Ground the wire at the sender.. Connect the test light between the right terminal and left terminal and the test light should glow.. One caveat must be noted here.. If the gauge's built-in resistor is bad, the gauge will still respond to grounding the sender wire, but it's readings will be inaccurate..

Locate the temp gauge thread I started about 2 months ago.. Troubleshooting a faulty temp gauge circuit is all spelled out in it...

 

[RustyPile]

Here ya go...

http://www.gmsquarebody.com/threads/temp-gauge-issues.19919/

 

[Layne02]

OK, so I haven't had a chance to get into this for a while, but, we have snow and ice today in Ga so I am home and decided to test this out. So, I did the terminal tests like Rustypile suggested and all tests passed. So, if that means wiring is all good, then my guess is sender or gauge is bad.

So to recap, temp gauge was working fine, then suddenly stopped. I changed sender with new one. No change. Changed gauge with an Old gauge. No change. Changed gauge with a NOS gauge, no change.

My mind doesn't quite follow all the resistance stuff , so not sure how to check all that , I am stuck.

Oh, this is an 87 GMC. The senders and gauges I tried were supposedly for an 87. Any thoughts?

 

[Layne02]

Oh, i did try grounding sender wire and gauge does peg to hot side. ( First time I evidently didn't have it grounded good enough, hard to do this stuff by yourself sometimes)

 

[75gmck25]

If you look at a temp gauge troubleshooting guide, the explanation of how they work is fairly simple.
- 12 volts runs to one terminal of the gauge, then through the gauge and through the wire to the sender in the block.
- The sender is just a big variable resistor, and it varies bases on temperature.
- When the engine is cold, the sender resistance is high, and the gauge reads cold
- When the engine is hot, sender resistance is low, and the gauge reads hot

First test - Key on, sending unit wire grounded (zero resistance), gauge goes to max hot
2nd test - turn key on, put high resistance between sending unit and ground (typical would be 1500 ohms), gauge will read near zero

If these basic tests work, then you need to know the manufacturer's table of resistance vs. gauge reading. For example, this post says that these are the typical values for a GM temp gauge http://www.gafiero.org/bbs/index.php?topic=641.0 Based on the table, a 185 ohm resistance between the wire and ground should equate to 210 degrees on the gauge, while 1600 ohms is 100 degrees.

You can buy resistors from Radio Shack or any electronic supply place, and just string them together to get the resistance you want. Resistance is additive, so wiring a 1000 ohm resistor in series with a 500 ohm resistor would give you 1500 ohms. Do not put them in parallel, since resistance no longer adds up.

My plan is to check the calibration on my gauge; however, mine does not have numbers. Since I would like 200 degrees to read a little under the center line, I will just calibrate it by popping off the pointer and making it read there with 225 ohms resistance. Then I'm going to figure out what tick mark it is at with 120 ohms (230 degrees), and consider that as the max I ever want to see in normal driving.

Bruce

 

[RustyPile]

My plan is to check the calibration on my gauge; however, mine does not have numbers. Since I would like 200 degrees to read a little under the center line, I will just calibrate it by popping off the pointer and making it read there with 225 ohms resistance. Then I'm going to figure out what tick mark it is at with 120 ohms (230 degrees), and consider that as the max I ever want to see in normal driving.

Bruce

Bad idea.. The gauge response to resistance change is not linear.. If you reset the needle, the only place in regards to actual coolant temperature the gauge reading will be accurate is when the coolant temperature is at 200 degrees, plus or minus a few degrees.. All other readings will be a
shot in the dark", indicating a much lower or higher temperature than is actually present.. Compare this approach to "resetting" the needle of your speedometer because you don't like where it "sets" at a certain speed.. The resistor mounted to the back of the gauge is what is used to calibrate the gauge.. Small changes of a very few ohms of that resistor would be a better approach to what you want to accomplish.. However, even that is not a recommended approach.. If you want a more accurate reading indicated in degrees, A late model gauge with the accompanying sending unit will give a better solution..

 

[RustyPile]

OK, so I haven't had a chance to get into this for a while, but, we have snow and ice today in Ga so I am home and decided to test this out. So, I did the terminal tests like Rustypile suggested and all tests passed. So, if that means wiring is all good, then my guess is sender or gauge is bad.

So to recap, temp gauge was working fine, then suddenly stopped. I changed sender with new one. No change. Changed gauge with an Old gauge. No change. Changed gauge with a NOS gauge, no change.

My mind doesn't quite follow all the resistance stuff , so not sure how to check all that , I am stuck.

Oh, this is an 87 GMC. The senders and gauges I tried were supposedly for an 87. Any thoughts?

Remove the gauge from the cluster for this test.. Remove the resistor from the back of the gauge.. Resistors don't have "polarity" so no need to make note of which end goes where, Only which two terminals where the resistor connects.. Using a reasonably accurate multimeter, check the resistor value.. It varies from gauge to gauge but yours should be in the area of 85 - 90 ohms.. A bad resistor results in bad gauge readings...

Never use Teflon tape to seal the sender unit threads.. Bare metal to bare metal is the proper method.. Sealant tends to introduce resistance and/or prevents a good electrical contact..

 

[Layne02]

I did resistor test on 2 gauges, one read 85, one read 90.

Funny you mention the teflon tape, I had originally put tape on it, but later remembered that was a no no and removed it.

When I test the sender , I get nothing, but to make sure , tell me if I am doing it right. I set meter to ohms, unhook sender wire, put red lead to sender terminal, and black lead to a ground. Nothing happens.

 

[Layne02]

Ok, got it working, evidently there was some teflon left on threads or something, took it out and put it back in. Tightened as much as I dared , checked meter and it was alive again. Guess it wasn't grounding good enough . So scratch that off the list. Thanks everyone.