Heat gauge pegs as soon as key turned

[Normanator]

1977 Cherokee 360 4v, TH400
Recently got my cluster re-done, solid job, and everything works well now with the exception of the heat and fuel gauges. Both sweep to max as soon as key is turned to first click. Fuel tank is drained, but sending unit hooked up, and temp sensor by thermostat housing is wired up as well. The PO had removed most of the wiring harness, and somehow miraculously didn't touch the plug for this side of the gauges (appears as such anyway), but I'm curious if it's possible there is a reversed positive/ground somewhere effecting both?
Have a new temp sensor inbound to pick up tomorrow even though this one was new last year (gauges didn't work at all to verify it was working), and I'm going to check wiring at the fuel sending unit just in case I reversed those there re-installing the tank, but does anyone have a thought as to anything else it might be?
Any input is greatly appreciated!

[dodgerammit]

The temp going to full hot immediately is a ground short. That is how you test for a bad sensor is to ground the gauge connector and verify the gauge moves to full hot. Probably the same on the fuel gauge. Don't let them stay long like that, or it'll cause gauge damage.

[Normanator]

Appreciate the tip. Since both are doing it I assume it is likely a short in that connector/harness (BRAKE light on dash is also stuck on, I figured it was my ebrake switch stuck on that). I will start there!

[tgreese]

The TSM has a page on testing and diagnosing the gauges. The '77 TSM is free to read and download on the Tom Collins site. Starts at page 3-54; all you have to do is read it.

You can't reverse the polarity, given the way the dash is built. The gauge sends current to the sender, which is a thermal resistor (a thermistor). The gauge is measuring the resistance to ground.

Above is correct that the sender shorted to ground will peg full hot. This suggests that you have a short somewhere between the dash and the engine. I would not assume that the brake light and the gauge are connected in any way.

[sonoraed]

Might try and disconnect wires at both senders and see if guages still peg would tell you if sender or wire, just a thought

[tgreese]

The brake light connects to both the pressure imbalance switch on the combination valve (located on the frame) and the parking brake switch. It will come on if either there is a hydraulic fault in the brakes (bad) or when you set the parking brake.

[rocklaurence]

Might try and disconnect wires at both senders and see if guages still peg would tell you if sender or wire, just a thought

Do this first--its fast and easy. If it doesnt send the gauges to zero, Id guess that its the Plug-in power module at the back of the gauges. I believe I had this same issue and it was the power module.

[tgreese]

Mmm. Not sure what Rocklaurence is thinking here. There is a radio noise suppressor (likely an inductor) between pins 14 and 15 on the back of the cluster. It's just a jumper - no active parts to fail.

He may be thinking of the CVR "constant" voltage regulator. It's not constant; instead it's a pulser. It feeds all the gauges except the ammeter, so if it's bad they will all show it. On the 1st generation cluster, it's a little steel can on the outside of the cluster. The 2nd generation cluster puts the CVR inside the temperature gauge. I'm sure a '77 is a 2nd generation cluster.

Look at page 1L-46 here https://oljeep.com/gw/82_tsm_2/82Wagone ... rPlant.pdf - this has both a picture of the back of the cluster, and a schematic diagram.

[will e]

What's the oil pressure gauge doing?

[Normanator]

What's the oil pressure gauge doing?

Oil pressure gauge stays full left at 0 until you start the engine, then she goes up to almost 80 (80 is full right). My last jeep would do this until the engine warmed up, then drop to 40 or thereabouts. Haven't had this one warmed up since the gauges have been in.
I know the engine isn't actually overheating because I can put my hand on the block or heads and its warm, but you can keep skin against it without getting burned, and the thermostat (new) isn't open yet.
I have had this engine up to temp before the gauge cluster was rebuilt and it's never had a cooling issue, so it has to be something electrical. I will go through some of these ideas this evening and get back!

[rocklaurence]

Yes, your correct. I was referring to the CRV and I believe it drops instrument voltage down to approximately 5vlts.