Weird temp gauge problem.

[Clint72]

Alright so I have a 72 GMC 350/400.
Bought the truck about two weeks ago and going through fixing everything to make a daily driver / weekend warrior.

So the issue I'm having is,
The temp gauge wasn't working, so I found the wire and the sender and bought a new sender. When going to replace, the connector basically broke into pieces.
So I put in the new sending unit, and put a new connector on, now the temp dummy light comes on and the gauge still doesn't work.
Any ideas?
Thanks,
Clint.

[ray_mcavoy]

Welcome Clint!

I'm not sure if you will be able to easily see it by looking up from under the dash or not, but try to get a look at the instrument cluster plug to see if there is a dark green wire going to pin #5 or pin #6.

The reason I ask is because pin #6 leads to the temp gauge while pin #5 leads to the temp warning light.

Normally, in trucks with the factory gauge cluster, the pin #5 connection to the warning light was left unused and the dark green wire went to pin #6. But if a previous owner converted your truck from a warning light cluster to factory gauges they might have missed re-pinning that wire to the temp gauge.

Another thing you can do is look for a dark green wire (or two) connected to the ignition switch connector (that might be easier to see). A green wire or two there would indicate a truck (or at least wiring harness) that was originally configured for a warning light cluster.

[Clint72]

Thanks for the reply, and the welcome!

Earlier I did look at that and if I remember, it was in pin 5. I guess it could have been converted to a gauge cluster, but I'm not sure why. I am the second owner, and previous owner was an older gentleman that used the rig strictly as a farm truck.

When I get back home tonight I'll check the ignition switch.
If it had been swapped and it's not original, what would I need to do in order to get the gauge to work?

[Clint72]

Alright, so I went ahead and pulled the instrument panel.
It is in fact a factory gauge instrument panel truck, but there are a couple cut and spliced wires.
Tomorrow I'm gonna check all the wiring out and get it corrected.
I'll post some pictures too!

Hopefully I can figure out why my battery gauge isn't working too!

[Boog]

Clint it sounds like you have a couple of wiring issues to sort out. Worth mentioning here is the temp sending units are different between the temp gauges and temp lights. I found out the hard way. Welcome aboard.

[VetteVet]

.................................................................................................... ........

Originally Posted by Clint72

Alright, so I went ahead and pulled the instrument panel.
It is in fact a factory gauge instrument panel truck, but there are a couple cut and spliced wires.
Tomorrow I'm gonna check all the wiring out and get it corrected.
I'll post some pictures too!

Hopefully I can figure out why my battery gauge isn't working too!

To start with, if you have a gauge cluster then there won't be a light to see, unless someone has tried to install one. There is a spot in the gauge cluster
for the light but no wiring was run to it. The wire was on the pin 5 spot on the light cluster plug and it was moved to the pin 6 on the gauge cluster. Just like Ray said.

Here are the wiring differences between the two.

light:



Gauge:



The pins are 1 through 6, bottom left to top left, and 7 through 12, top right to bottom right


Gauge layout:




The dotted red line is the missing wire for the temp light



Light cluster on ignition switch- green wires one from sender and one to the no.5 cluster plug pin 5.

[Clint72]

Alright, here are a couple pictures of what I found on the harness.
Everything looks to be correct except for the fact that there is a wire coming out of terminal 5 and it's been spliced to a green wire, then two come out of 6 but ones spliced and the other is cut.
I really appreciate y'all help!

and boog, I figured that out when buying the new sending unit, the one i put in is for gauges, thanks!

[ray_mcavoy]

Originally Posted by Clint72

Alright, here are a couple pictures of what I found on the harness.
Everything looks to be correct except for the fact that there is a wire coming out of terminal 5 and it's been spliced to a green wire, then two come out of 6 but ones spliced and the other is cut.

Yeah, definitely looks like someone has been in there monkeying around with the temp gauge wiring.

Seeing as how there are green wires going to both #5 and #6, it is possible that the original owner had troubles with the temperature gauge at some point. And rather than fix the gauge, they took advantage of the fact that the gauge cluster had the provision for a temp light and made some changes to make that light functional.

Also, judging by the color of the wires leading to pins #1 and #12, it does look like the wiring has been converted from warning lights to gauges. The factory gauge cluster wiring used a black wire with a white stripe to pin #1 and a black wire to pin #12 for the ammeter (battery gauge). Yours appears to have brown and blue wires going to those pins (which were likely re-purposed from the old GEN light (brown) and OIL light (blue) wires).

Now, that doesn't necessarily mean that the truck was converted from warning lights to gauges. The other possibility is that the original wiring got damaged and someone swapped in a cab harness from a donor truck that had warning lights (and modified it to work with the gauge cluster).

In any case, getting the temp gauge wiring back to the stock configuration (with the dark green wire from the sending unit connected to pin #6) would be the first step to getting that gauge functional again.

[VetteVet]

.............................

Originally Posted by Clint72

Alright, here are a couple pictures of what I found on the harness.
Everything looks to be correct except for the fact that there is a wire coming out of terminal 5 and it's been spliced to a green wire, then two come out of 6 but ones spliced and the other is cut.

Well yes but not really. Your plug does not show the two ammeter wires on 1 and 12, instead I see a blue wire on 12 and nothing on 1 that I can see. These two wires are not needed if the PO didn't want an ammeter and used a voltmeter instead.I think the harness on this truck was originally set up to be an idiot light harness because the black wires are missing, and had the sender wire for the temp gauge run to the cluster plug first on pin 5 and then to the key switch, instead of what the diagram shows.
The factory could have put the double green wire on the cluster plug pin 5.

When the previous owner converted to the gauge cluster, he repinned the cluster plug and cut one of the two green wires that went to pin five, and one on pin 6 and just left the extra wire loose on pin 6. The other green wire on six would go to the block sender.

The green wire on 5 must run back to the key switch or else it is grounded somewhere, because it has to be grounded to turn on the light in the cluster, if there is one.You haven't said yet.

The second green wire on Pin 5 would have to go to a second sender in the block or head to have the light work properly. It may be the cause of the light coming on when the key is on. Remember these are the ground wires for the light.

The blue wire in pin 12 was probably in pin 1 originally and he just stuck it in 12 because it wouldn't be needed for the oil pressure gauge. and the gauge cluster doesn't have an oil light.

I really appreciate y'all help!

and boog, I figured that out when buying the new sending unit, the one i put in is for gauges, thanks!

[VetteVet]

Darn Ray you beat me. LOL

[Clint72]

Man I'm glad I found this forum, y'all are awesome!
So I tested the wire to the sender, and I have it hooked to pin 6 now.
The second wire coming from pin 6 I can just snip and leave disconnected correct?
What about the second green wire coming from the harness?
It was connected to pin 5.
It appears to be running to the ignition switch.

As far as the ammeter, I'm not concerned about getting it fixed because it appears most done work anyhow, I'll likely switch to a voltmeter sometime down the road.
I did check all the wiring under the hood searching for the wires with fuses that it's supposed to have and I don't see them. So it's fine. I've got some other electrical questions, but I reckon I'll start a new thread on those.

Once again, thanks for the help!

[Clint72]

And so it continues, I just realized that my passenger head ALSO has a sending unit in it.
It appears to be a light switch rather than gauge.
It has a green with white stripe wire running to it.
From all of my looking at wiring diagrams, I'm 90% sure these trucks didn't have both it was one of the other.

Edit:

Ah I've gotten closer!!
I looked up a 73-87 engine wiring diagram and it matches some of the wires I had being unused in my engine bay!

[ray_mcavoy]

Originally Posted by VetteVet

Darn Ray you beat me. LOL

LOL ... looks like we both noticed the same thing with the ammeter & temp gauge wires giving evidence of a light to gauge swap at some point.

Originally Posted by Clint72

So I tested the wire to the sender, and I have it hooked to pin 6 now.
The second wire coming from pin 6 I can just snip and leave disconnected correct?
What about the second green wire coming from the harness?
It was connected to pin 5.
It appears to be running to the ignition switch.

Yes, you can clip off the extra wire from pin 6 (leaving only the one going to the sending unit). And you can completely eliminate any wires going to pin 5. You can also remove the green wire from the ignition switch. That was only used with the temp light to provide a "bulb check" feature. It grounded the circuit while cranking the engine to turn on the temp light so the driver would know the bulb was okay.

Originally Posted by Clint72

And so it continues, I just realized that my passenger head ALSO has a sending unit in it.
It appears to be a light switch rather than gauge.
It has a green with white stripe wire running to it.
From all of my looking at wiring diagrams, I'm 90% sure these trucks didn't have both it was one of the other.

That sending unit in the passenger side head with the green/white wire is most likely for the TCS (Transmission Controlled Spark) system. TCS was an early emissions system (introduced sometime around 1970 or 1971) that only allowed full vacuum to the distributor advance when the engine was cold and/or the transmission was in high gear. The TCS system also included a vacuum solenoid valve on the engine and a couple of relays (mounted on the outside of the firewall for 71 and inside the cab on the firewall for 72). Wiring for this doesn't show up on many of the 67-72 diagrams so there have been lots of questions on this site asking about "mystery parts" associated with this system.

[Clint72]

just finished putting everything back together, letting the truck warm up now , it works!!! I do believe part 1 of this wiring mess is complete!

I left the ground to the ignition there in case I decide to connect the dummy light for extra assurance, but snipped the others and we're good to go!

I know who to tag in my posts if they are about electrical!
Thanks a bunch fellas!

Edit : new problem.. I now see that the truck is running hot. Well damn.

[ray_mcavoy]

Glad to hear you got it working!

The running hot could simply be due to the new sending unit not matching the gauge. There have been numerous reports of the aftermarket replacement sending units not having the correct temperature vs. resistance curve to give an accurate reading on the stock gauge.

Lectric Limited offers accurate replacement sending units. But before investing in another sending unit, you might want to try testing the accuracy of the one you have now as shown here --> http://www.madelectrical.com/worksho...mp-gauge.shtml

[Clint72]

I think I got it figured out, lost some coolant when changing the sending unit and forgot to top it off, so I did that.
I think the thermostat may be going out though because I left the radiator cap off and started the truck and watched the temp gauge and the water level and once the gauge hit H the thermostat finally opened and I added more. Was probably low before also. But once it did open, it instantly dropped below half on the gauge, then slowly went up to h again, did that a few time she until the radiator no longer dropped water level. Ended up adding about a half gallon of water.
Once that was done it stayed steady about 3/4 way.
Possibly air lock?

I'll change the thermostat and flush the cooling system just because.
Being that I just bought the truck I'm slowly doing basic maintenance since it appears the PO negleglected it.

Truly appreciate y'alls help! Hopefully ill be able to be helpful and informative on subjects that I do know about.