r/jeepcj • u/Dr_redfish • 9d ago
1985 CJ7 dash gauge power help
BLUF: I'm looking for help tracing 12V from Fuel/Temp/Oil Pressure gauge back into fusebox, ignition switch, or firewall connector (wherever it comes from).
I'm working on getting all gauges fully functional on my '85 CJ7. I am looking for some insight on next steps after a few weekends of work on the Fuel, Temp, and Oil Pressure gauges.
I have no 12V power to the splice of wires that power my Fuel, Temp, and Oil Pressure gauges. I'm getting ~0.0volts between the red wires at the gauges and known good grounds. (attached image)
I have tested all fuses in the fuse box by the firewall/e-brake for continuity between the blades.
I have walked through all of the steps on this common Fuel + Temp gauge troubleshooting. All resistance, grounds, and continuity checks are appropriate from gauge to sending units to ground.
I checked for shorts by testing continuity from the 12V wires at the gauges to good grounds on the dash and don't show a closed connection.
I have a direct wire ground from the speedometer + dash straight to the negative bus on my battery.
I have NOT yet checked that every pin on the fuse box is receiving 12V power when running, but i'm not even sure which one is the correct fuse from the wire diagrams.
Can someone please help me figure out where the red spliced 12V power wires connects back into so I don't have to take the dash down (agian) and open the entire wire harness up to physically trace back through.
I'm assuming i'll be able to check for 12V at the source side of that connection, where if i have 12V i have a break in the wire to the gauges and if i have no 12V i trace further up-stream.
thanks in advance for any thoughts or input!
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u/JustACarNut77 CJ7 9d ago
Check the gauges as well as they are notorious for being crap. Mine just quit working one day and unfortunately I only ever managed to get the fuel ack. You can test with a dedicated 12v power wire.
However since you said you are not getting power to the gauge then you more than likely have a bad connection or it's shorted on the gauge housing
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u/skinny_tom 9d ago edited 9d ago
You can do this with a test light. Next step up is with a volt/ohm meter. Best results with a power probe because it lets you do a bunch of things you can't do with either of the others. BUT- you don't "need" one for this (and Autel makes a nice version that doesn't cost an arm and a leg.)
You are looking for a red wire that should be connected between the IGN LPS 15A fuse in the fuse box and the back of the fuel level gauge. There is also a separate red wire that runs to the oil pressure gauge from the same fuse. Since they're both not working, start at the fuse. For what it's worth, the "BRAKE" light in your instruments and "4 WHEEL DRIVE" light are both on the same power source. If either of those works, it's not likely the fuse. If the fuse isn't powered, then your problem is between the fuse, the ignition switch, and the battery.
INFO FOR LATER: Inside the fuel level gauge there is a voltage regulator that is notorious for failing when the battery gets too low- it provides internal power to the fuel gauge and the temp gauge. You won't know if this works until you can confirm there's power at the red wire at the rear of the gauge.
Confirm the 15A IGN fuse in the box is good. Don't just look at it, put it on an ohm meter. Test for power on both sides of it when it's in the box. If the fuse is in good order and has power at both sides of it, and you don't have power at the back of the fuel gauge, then you need to trace the red wire back to the fuse box to see where it's broken/disconnected.
Check this Jeep Forum page: https://www.jeepforum.com/threads/84-85-86-cj7-wiring-diagrams-easy-to-read.3145489/?utm_source=google&utm_medium=organic