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Headlight switch turns on oil light
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sweatlock View Drop Down
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    Posted: May/19/2022 at 7:25pm
Okay, this is a really weird one and I need some help -  

1969 AMX, all new wiring harnesses throughout bought from APD, new headlight switch (old one did the same thing), all copper sheet grounds in place on the plastic dash PLUS an extra ground wire going to the firewall up to the copper ground for the clock/instrument panel. 

The problem is that when I pull out the headlight switch knob to turn the headlights on the red ‘OIL’ idiot light comes on; the light doesn’t come on with just the running lights on (first pull-out on the headlight switch). 

Did a site search, nothing. Did a Google search, nothing. 

This is disturbing to say the least when driving at night. 

Has anyone else experienced this? What the eff could it be? 
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tufcj View Drop Down
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Points: 4086 		Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2022 at 7:54pm
Wire misplaced in the harness, high beam indicator maybe?  Short in the circuit board on the dashboard?

Bob
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Points: 35 		Post Options Post Options   Thanks (0) Thanks(0)   Quote PeterS3 Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2022 at 7:57pm
The only thing that I've had that was similar to this was my trailer brakes coming on when I turned the left turn signal on. That was caused by two wires melting together after I welded a crack in a frame on an old truck.

It's going to be a pain to track down. You're probably going to have to unwrap lots of little places in the wiring harness, and cut the wires to do continuity tests between the oil light and the head light switch.

I've had good luck with adhesive heat shrink tubing, because it has hot glue inside the shrink tubing that keeps the water and salt out. Any time you cut a wire, make two solder joints with a 1-2 inch wire in between the cut ends so that all the stress of the wiring harness moving around isn't on that one wire joint. 
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Does the oil indicator light up with headlights off, ignition on while engine is not running?

If so, that is good, but when headlights are on, and engine runs... the issue is ground path. The wire to the oil pressure switch is getting grounded by headlight switch. It is possible the headlight connection on the harness could have a crossed wire.

If you have the original harness, you can compare.
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Originally posted by sweatlock sweatlock wrote:

new headlight switch (old one did the same thing),
 
 
Does the headlight switch have the spade terminal on the back to connect to a ground wire in the harness?
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Points: 3313 		Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2022 at 8:17pm
Originally posted by BU1 BU1 wrote:

Originally posted by sweatlock sweatlock wrote:

new headlight switch (old one did the same thing),
 
 
Does the headlight switch have the spade terminal on the back to connect to a ground wire in the harness?

Yes
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Points: 3313 		Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2022 at 8:20pm
Originally posted by 304-dude 304-dude wrote:

Does the oil indicator light up with headlights off, ignition on while engine is not running?

If so, that is good, but when headlights are on, and engine runs... the issue is ground path. The wire to the oil pressure switch is getting grounded by headlight switch. It is possible the headlight connection on the harness could have a crossed wire.

If you have the original harness, you can compare.

Oil light comes on with headlights off, engine off, ignition switch on. 

Oil light comes on with headlights on, engine/ignition switch off. 

Oil light does not come on with running lights on, engine/ignition switch off. 
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Originally posted by tufcj tufcj wrote:

Wire misplaced in the harness, high beam indicator maybe?  Short in the circuit board on the dashboard?

Bob
tufcj

New circuit board as well. 

Both you and 304-Dude have mentioned checking the connector wiring at the headlight switch and comparing it to the original - I still have the original wiring harness so I’m going to have to do that. Hopefully that’s it. 


Edited by sweatlock - May/19/2022 at 8:27pm
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mixed up View Drop Down
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Points: 2177 		Post Options Post Options   Thanks (0) Thanks(0)   Quote mixed up Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2022 at 10:45pm
sounds like you have gremlins in the system
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Oil light comes on with headlights off, engine off, ignition switch on.
- Tells us the oil pressure switch might be triggering the light.
-- What happens in this situation if you disconnect the wire from the sending unit?
--- If the oil light doesn't come on when the wire is disconnected then the sender circuit is probably correct.

Oil light comes on with headlights on, engine/ignition switch off.
- This tells us there is probably another signal to the light coming from the headlights.
-- Is there any change if you press the high/low beam foot button?
-- And again, what if the sending unit wire is disconnected?

Oil light does not come on with running lights on, engine/ignition switch off.
- This makes me think the errant signal to the oil light is not coming directly from the headlight switch.

If your oil light still comes on when headlights are turned on, even with the oil pressure wire disconnected and ignition switch off, I'd say something is mixed up in the gauge wiring with the headlight indicator wire, especially if the oil light comes on and off with the changes in the high/low beam switch.
Bill O.
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