Help me troubleshoot my temp gauge please!

[fishmunger]

So I've searched and read about every thread I could find on the subject. Here's where I'm at.

Gauge is pegged at hot.

I have replaced the half of the green wire that is in the engine bay and added the correct brand new connector that goes onto the temp gauge itself.

The previous owner had put a brand new sending unit in (at my request) right before I bought the truck (and it looks brand new).

At the advice of some threads, I added a little extra grounding wire on the back of the temp gauge itself. When I did that, I briefly got the gauge to 're-set' back to the cold side but then I hooked it back up it pegged hot again. I can no longer get it to re-set.

When I ground the end of the temp sender wire and turn the key on, I can see the gauge needle move a little bit (but still stays pegged on hot side) - this leads me to believe that my gauge is working?? Underneath the dash, I can't see anything abnormal with the wire that would lead me to believe its shorted. It's still in what appears to be the original harness/taping.

Any ideas, this is the last thing keeping me from having this baby back on the road! It's been down almost a year...

Thanks!

[pjmoreland]

Did your truck come originally with gauges or idiot lights?

[89AKurt]

I'm having the same issue with my '69. New sender. Mechanic said there needs to be some sort of resistor. I'm staying tuned.......

[jabborabbo]

Senders for idiot lights have a different resistance than those for gauges. A while back someone posted part numbers/sources for them....

[jsprengeler]

I have the same problem on my GMC - I'm all ears!!

[fishmunger]

Somebody help us please!

Mine has a factory gauge, no dummy light

[pjmoreland]

Which one of these does your sensor look like in terms of the style of connector?

[pjmoreland]

There is some good information in this post:

http://67-72chevytrucks.com/vboard/s...der+resistance

[tdangle]

In trouble shooting, the sender makes a lower resistance to ground for a higher (hotter) reading. if you disconnect the wire from the sending unit the gauge should read cold. If you short the wire to ground it will peg (with key on, engine does not need to be running). If the gauge does not respond correctly to these trouble shooting methods, something is either wrong with the gauge or wiring to the gauge. Check to ensure the wire is not being shorted to ground. If it is OK then you have a problem in the gauge or cluster. If you have an ohm meter, unplug the cluster under the dash and check the wire at the sensor (unhooked) to ground This will tell you if the wire is shorted to ground causing a pegged meter reading. If that is OK, then it is either the gauge or something in the cluster shorting, Good luck

[fishmunger]

pjmoreland- I have the 'gauge' style sender.

tdangle - if the problem is in my gauge/cluster, whats the next step? Purchase a new circuit board for the entire gauge cluster?

So to check the wire, I want to unplug both at the gauge and at the sender and then check its resistance, which should be super high (while disconnected) correct?

Thank you both!

[pjmoreland]

If you disconnect the printed circuit from the gauge terminal that the green wire connects to and your gauge still stays pegged, then it would seem your gauge is bad.
If the gauge pegs only if that terminal is connected, then there is likely a short somewhere on the green wire, which follows this path:

Circuit board to gauge cluster connector.
Cluster to firewall bulkhead connector
Firewall bulkhead connector to temp sender

You would need to track down which segment has the short in it and then physically locate the short. You could disconnect all of these connectors and then use an ohm meter to see which segment has continuity to chassis ground.

[dmjlambert]

I recommend post a picture of the back of the temp gauge.

[tdangle]

Originally Posted by fishmunger

pjmoreland- I have the 'gauge' style sender.

tdangle - if the problem is in my gauge/cluster, whats the next step? Purchase a new circuit board for the entire gauge cluster?

So to check the wire, I want to unplug both at the gauge and at the sender and then check its resistance, which should be super high (while disconnected) correct?

Thank you both!

Yes pretty much if you unplug the cluster and the wire to the sending unit and check resistance from that wire to ground it should be very high resistance, infinity actually. if that is OK, then yes you either have a short in your cluster printed circuit board or a bad gauge. Time to remove it and bench test it.

[custom10nut]

Originally Posted by 89AKurt

I'm having the same issue with my '69. New sender. Mechanic said there needs to be some sort of resistor. I'm staying tuned.......

To chime in on this.
I’m having the issues described, and have replaced everything to do with the cooling and temp monitoring system, except the gauge. I did order a new one, as it is the ONLY thing I haven’t replaced yet.
When I ordered the new gauge I also tried to order a resistor.
The place I ordered from informed me that the new gauges have the resistor built in, so there’s no need to add another.
I haven’t had the time to put my new gauge in, so I’m really hoping it’ll fix the issue.

[fishmunger]

Thank you guys so much. I'm going to give this another go and hopefully I will pinpoint the problem.

pjmoreland - I didn't realize the green wire was broken into 3 segments...I thought it was perhaps only broken at the firewall bulkhead connector. Do you think it would be likely that it is grounded at one of these connection points?

[pjmoreland]

Anything is possible in a 50 year old truck 😁

[89AKurt]

My trusty mechanic soldered some resistors on the wire between the gauge and sender, and now it sits in a normal position. He is old school, has a stash of WW2 era electronic parts.

[fishmunger]

OK, so today I was finally able to run some tests. After going thru this thread and the other one, I checked a few things.

1) On the sender itself, I had 790 ohms of resistance from the wiring connection post to the sender body. This sounds a little on the high side, but not the source of the problem, as this being high would keep my gauge pegged on cold, not hot? I have the opposite problem, pegged on hot.

2) The sender itself is grounded appropriately. There is a little, but not much resistance to ground from the sender body to the frame.

3) With the green wire disconnected from the sender, and disconnected from the gauge cluster, I get an 'open line' - no path to ground. If I reconnect the wire to the sender, I then get 790 ohms when I probe the terminal at the dash plug end of the wire. This leads me to believe that the wiring between the dash and sender is OK, and not shorted as I initially suspected.

4) Using an extra set of jumper wires from the battery terminals to the gauge posts, I am able to manipulate the gauge back and forth from pegged hot to pegged cold. I think this suggests the gauge itself is good?

5) From the top terminal of the gauge cluster to the bottom terminal, I get about ~190 ohms of resistance. I think others have gotten similar numbers, so I believe this means the resistor is good?

What should I check next?? Should I order a new gauge circuit board, or take mine off and clean it and put it back? Kinda of at a loss as to why this thing is not working.

[pjmoreland]

I wonder if there could be a short on the circuit board trace that carries the green wire signal to the gauge. With the wiring harness unplugged from the cluster, how much resistance is there between the metal case of the cluster and the gauge terminal that the sender is normally connected to?

[dmjlambert]

I don't know, it sounds like quite a puzzle. Can you post pictures of the back of the gauge and of the sender?

[fishmunger]

Which of the 4 terminals should i be checking for resistance to ground on the metal bezzle from?

[dmjlambert]

I get 38 ohms between top and bottom resistor posts. I suggest remove your resistor and test it separately. Here is the thread you need to read I think.
http://67-72chevytrucks.com/vboard/s...82#post8762882
We got forum member Seventy Two's gauge working in that thread.

[fishmunger]

OK, with dash cluster wiring harness disconnected, but circuit board in place, and one probe grounded to dash bezel case, I'm getting 208-210 ohms at the top terminal, 147 ohms at the side (12V hot terminal), 195 ohms at the other side (inboard) side terminal, and 0.7 ohms at the bottom terminal.

[fishmunger]

Also, stupid question, but should I be aware of anything before I disconnect my oil pressure sending unit from the back of the gauge? (It still works and I'd like to keep it that way!)

[pjmoreland]

Originally Posted by fishmunger

OK, with dash cluster wiring harness disconnected, but circuit board in place, and one probe grounded to dash bezel case, I'm getting 208-210 ohms at the top terminal, 147 ohms at the side (12V hot terminal), 195 ohms at the other side (inboard) side terminal, and 0.7 ohms at the bottom terminal.

You don't appear to have a short