1968 Idiot light

[lnevill]

Bonnie and Clyde is getting close. Haven't fired the rebuilt 390 yet, but I am happy to report I was able to apply DC power to all electrical circuits and no smoke was let out of anything! All lights, foglamps, markers, signals, etc work, except for the oil pressure idiot light....

Ground the lead at the sender and nothing. Change the bulb with the alt bulb and it works fine. It's my understanding that the output of the voltage regulator goes directly to the bulb and the sending unit acts a ground switch: open with pressure, closed with no pressure. Is this correct? Should I have ~ 5v at the bulb and end of the lead (when not connected)?

Thanks!

Lee

PS: I'll get some pictures posted soon.

 

[rvrtrash]

That's how an idiot light would normally work, but I'm surprised you have one instead of a gauge. Is your block grounded to the body or battery? Did you use Teflon tape or pipe sealant on the sender? Both will keep the sender from grounding. Did you use an idiot light specific sender or the one for a gauge? Is the key in the "Run" position when you're checking? If I think of anything else, I'll add it.

Steve

 

[Mosesatm]

Steve,
It's an S-code 4-speed car so it may have a tach.

 

[rvrtrash]

Tach equipted car, got it. I thought about this more as I was out doing yard work, and an indicator light would be fed by the ignition switch, not the voltage regulator or constant voltage unit. It would have 12.6 V on the terminal with a fully charged battery, engine not running. I had just woke up when I first read the post, but were you saying the oil pressure bulb worked when put in another circuit, or the circuit worked when you used a different bulb? I initially assumed you meant the bulb was good.

Steve

 

[lnevill]

Yes, S-code 4 speed with 8k tach, sorry I left that out.

Bulb is good. It worked when swapped with the alternator idiot light and the light that was working in the alt slot, no longer worked in the oil light receptacle. No voltage at bulb receptacle (yes, with switch on) so must be an open between ignition and receptacle.

This is the original wire loom that went through the hail of bullets, but the previous owner had it restored and this is the only circuit that is not working. I was pretty impressed actually with how well this wire loom fit and worked, given the fact that it had been shot full of bullet holes!!

thx
lee

 

[rvrtrash]

I guess the next place I'd look is the plugs at the instrument cluster. Maybe they forgot to put a wire back, put it in the wrong place, or sometimes the wire will back out of the receptacle if it's been messed with and the little keeper has been flattened out. If nothing else, check the voltage at the plug on the wire harness side and see if you have it there. That will narrow the search some.

Steve

 

[lnevill]

Bad regulator...

The power source for the oil light is the regulator located on the cluster. The alternator light goes to 12v. I checked the temp and fuel gauge by grounding them, but that will send the gauge to max regardless of the voltage regulator output. The VR output is 0 volts. Not good!

Thanks for the inputs!

Lee

 

[Mosesatm]

I don't know......it looks like the fuel gauge also ties into the regulator so if your fuel gauge is working properly the problem should be somewhere else.

 

[rvrtrash]

The power source for the oil light is the regulator located on the cluster. The alternator light goes to 12v. I checked the temp and fuel gauge by grounding them, but that will send the gauge to max regardless of the voltage regulator output. The VR output is 0 volts. Not good!

Thanks for the inputs!

Lee

I guess I'm unable to help. Good luck in your quest.

Steve

 

[lnevill]

Mosesatm,

Yes, the VR powers all of the "analog" gauges as well.

All I had tested is the fuel gauge and temp gauge at max by grounding the lead at the sender. This is a simply voltage divider circuit and if you ground to down leg, it will read max regardless of the VR output. It is all the places in between max and min that require the voltage. I've never had gas in this tank or fired the engine to warm it up, so I was fooled by the gauges reading max when tested.

Moral of the story: grounding the wire at the sender is only half the test. Really need to check the gauges between min and max to confirm everything is right!

thx
Lee

 

[lnevill]

VR definitely fried!!

Root cause appears to be a mix-up when the harness was restored. The oil pressure idiot light and the alternator idiot light power sources were switched at the receptacles.

Lee