71 New Gauge Cluster Issues - Grounding?

Michael-71SS

New Member
Feb 4, 2021
12
I found a few similar issues throughout the forum, but don't think I found one that touched my exact issue.

After recovering my seats but before reinstalling them, I decided that it was high time to replace the entire dash in my 71 SS. As it normal for these, about every tab was busted on the gauge cluster holding it in, so I have replaced pretty much every piece of plastic and rubber under the dash. The brand new gauge cluster is assembled, with a few new gauges including a new ammeter (needle was busted years ago) and fuel gauge (used to bounce a lot so decided to replace it while I had it apart). The Temp gauge was still original and assembled with the original tach and speedo into the cluster.

Installed a new printed circuit across the back of the gauge cluster (seemed to match old one correctly), and even applied dielectric to each connection including the master cable.

While waiting for a new front speaker to come back in (first one was blown), I decided to check on the bulbs in the cluster to make sure they were functional, so plugged everything in to test. Naturally, I looked at that connector, no major corrosion, but decided to clean it up with a brush to scuff it up and reapply dielectric grease. Got ahead of myself and shorted it out causing a brief spark and blowing the 3A fuse *(Yes, I definitely know better than to work on it without disconnecting the battery again and kicked myself instantly for being lazy or forgetful at my age I forgot which it was). However, after replacement of the fuse, I still had a few bulbs out, so I worked with the metal tabs on the bulb holders so they would make better contact.

Got all bulbs working after a few tweaks, but noticed that Temp and Fuel gauges were pegged past Full and past overheat respectively. I know from the great diagnostics that I saw posted by @grzewnicki on this thread: https://nastyz28.com/threads/1970-camaro-fuel-gauge-wiring.272285/ (BTW thanks for the refresher, it has been a long time since I had to diagnose these, so it was a great Jumpstart) - that it is likely a grounding issue to the cluster. Since I don't see any separate ground wires (except the ones for the headlight and wiper switches), I have to assume that the ground is coming through the main connector that contacts the printed circuit, unless I am missing one.

I dont think that assembling it back into the dash is going to affect the grounding of a plastic cluster, so am trying to troubleshoot prior to reassembly.

Any ideas? Did I just lose a separate ground somewhere?
 

djorgensen3

Veteran Member
Oct 15, 2009
903
Peoria, Az
Be careful with dielectric grease. It does not conduct electricity but is meant to keep moisture out of a connection. That could be a problem with poor connections if grease got in between.
 

Michael-71SS

New Member
Feb 4, 2021
12
Thanks! Yes, we use it all the time in our security camera installations. The metal makes the contact, the dielectric helps seal out the moisture to keep the contact from corroding over time. Have never run into a time where it prevented contact, but did run into a few bulb holders where the contacts weren't touching the printed circuit anymore and had to be re-bent.
 

Twisted_Metal

Administrator
Staff member
Lifetime Gold Member
Feb 26, 2004
30,999
Bloomington, MN
A fuel gauge reading past "Full" is an open circuit.

The gauge is an Ohm meter with the resistance varying by the position of the float at the sending unit.
Your gauge is getting power and ground but the sending unit is not sending a signal.
(Infinite resistance)
It's either the signal wire at the cluster, at the tank or the sending unit ground by the tank.

I believe the temp gauge works the same way and would show the same behavior if there was an open circuit for the temp sending unit.

Since it's the cluster you've worked on recently... I suspect a poor connection for those two wires at the cluster connector or the circuit panel at the back of the cluster.

Hopefully, the fuse did it's job and you didn't mess up anything else.
 

Michael-71SS

New Member
Feb 4, 2021
12
Oh my, I read that chart 3 times and misread it each time, you are definitely right. That is a disconnected signal wire for each of those gauges. I think my brain kept wanting to go back to a ground wire because it would have been common between the 2. Now to dig out the wiring diagram to check the colors. Thank you!

I should be able to ohm out each of those lines disconnected to see what they are reading and should be able to find specs for expected resistance.
 

Michael-71SS

New Member
Feb 4, 2021
12
Well, the Temperature gauge was an interesting one. Apparently, the old gauge had a nut on a terminal where there was no connection to the printed circuit board. Upon closer review, there is a contact that does not touch the stud on the back of the gauge which the nut contacts completing the circuit. Interesting design, but that fixed the temp gauge. Still have not found my break in the circuit with the fuel gauge, I am beginning to suspect that the new gauge is bad, so will have to test with the old gauge tonight (replaced only because it was bouncing). Including picture just in case anyone else runs into this. I am a firm believer that every stud needs a nut, and every screw goes back in, but I missed this one because it didn't seem to have a purpose
Temp-Gauage-71-Camaro-Back.jpg
 

Michael-71SS

New Member
Feb 4, 2021
12
Dug around a little while longer, and even pulled out the gauge to make sure that the new gauge wasnt just bad.

  • 58 ohms on the signal wire when the key is off
  • Both gauges operated normally when externally powered and signal shorted to ground
  • 12 V on + at gauge when all hooked up key on
  • Ground is good for gauge terminal
Still needle moves pretty far past full key on.

Ohmmeter goes nuts if I turn key on while checking resistance at the back of the gauge terminal. So I checked for power at the signal terminal with key on...8VDC.

Next I will have to check and see what happens with gauge cluster unplugged with key on, and see if it is getting voltage somewhere else, otherwise likely a problem with something partially shorting in the gauge cluster I suppose.

Also will have to try powering one of the gauges while not installed and connecting to the #4 Tan/blk wire (fuel sending unit wire) with gauge cluster unplugged to see what happens after I check voltage.

Talking through this myself as I write. Open to other suggestions or ideas.
 

Jim Streib

https://www.flickr.com/photos/[email protected]/albums
Apr 6, 2004
558
Saint Louis, MO USA
  • Both gauges operated normally when externally powered and signal shorted to ground
  • 12 V on + at gauge when all hooked up key on
  • Ground is good for gauge terminal
Still needle moves pretty far past full key on.

Open to other suggestions or ideas.

If I had fuel gauges operating normally when externally powered but then not in the dash but with a test meter things looked fine then I might be looking at the testing meter you have as maybe it is giving a false incorrect reading ?.

IF that was not the case, then I would go old school and have the fuel gauge mounted in the dash and ready to go with everything it needs as for as voltage, ground, and signal and then take a jumper from a known good ground behind the dash and connect it to signal terminal of the gauge that goes back to the sending unit in the gas tank.

If the gauge needle did not swing to empty or below I would then leave this wire intact and temporarily wired up and then with another wire connect from a known good ground point behind the dash and connect it to the gauges housing or it's ground wire.

If the gauges needle still does not swing down to empty or below, then take a third wire and connect off of a fused terminal in the fuse block and connect it to the power terminal of the gauge.

If after all three of these temporary wire connections are made then this would be the same as bench checking the gauge and the needle should have swung to empty or below with these test connections paralleled with the original connections.

If not, something is being overlooked.

Jim
 




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