Lights turn off when engine is running??? (1 Viewer)

[iliketomakestuff]

I had an old post that contained this, but other things as well (which are now resolved.)

So, 1976 FJ40. Lights work. Start the engine and the headlights turn off (running side lights stay on). How in the world would that be happening?
I've got a headlight switch replacement on the way.

I've changed fuses, cleaned fuse panel contacts, replaced some point to point wiring to make sure wires are good.
(FYI, this was happening before I upgraded to the OEM headlight circuit, and still happens after)

Any ideas where to start?

 

[45Dougal]

Both low beam and high beam?
Does your blue high beam indicator light up?

 

[Paul_A]

Voltage out put or bad battery? I had a tractor that died when ever you turned on the head lights. New battery fixed all.

 

[iliketomakestuff]

Voltage out put or bad battery? I had a tractor that died when ever you turned on the head lights. New battery fixed all.

Battery is new, alternator is good

 

[iliketomakestuff]

Both low beam and high beam?
Does your blue high beam indicator light up?

I just checked. No high beams

 

[77mustard40]

Clean your fuse box connections and report back.

 

[iliketomakestuff]

Clean your fuse box connections and report back.

They’ve all been cleaned and checked. Nothing obviously faulty, dirty or loose

 

[45Dougal]

Can you read 12v at the headlight connector?
I'm thinking that maybe you don't have a good ground at the headlights and the body.
Check volts on the connector, and the body wrt the -ve battery terminal.

 

[iliketomakestuff]

Can you read 12v at the headlight connector?
I'm thinking that maybe you don't have a good ground at the headlights and the body.
Check volts on the connector, and the body wrt the -ve battery terminal.

I’ve got solid continuity between the ground terminal of the headlight connector and the negative terminal of the battery. I also get 12 V from the positive terminal of the battery to the ground terminal of the headlight connector when the lights are OFF. When they are ON it goes down to .2V

 

[45Dougal]

Sounds to me like you don't have a good ground on your headlights.
The continuity that you are reading is probably misleading you.
Fit a patch wire to ground them.

 

[iliketomakestuff]

It could be, but the upgraded headlight harness has a wire that goes directly to the ground terminal on the battery. So I feel like that’s a pretty solid connection directly to the circuit.

 

[45Dougal]

It can't be that good if you're reading 0.2V between 12V and ground.

I suspect that your continuity reading is back through a bulb filament and to ground through something else.

When you turn them on, the ground is open circuit and so it floats to the same voltage as the positive side of the bulb.

 

[iliketomakestuff]

It can't be that good if you're reading 0.2V between 12V and ground.

I suspect that your continuity reading is back through a bulb filament and to ground through something else.

When you turn them on, the ground is open circuit and so it floats to the same voltage as the positive side of the bulb.

Oh great call! I unplugged both bulbs and continuity was lost! Thanks! I’ll work on a ground patch and report back

 

[iliketomakestuff]

Oh great call! I unplugged both bulbs and continuity was lost! Thanks! I’ll work on a ground patch and report back

Scratch that. User error for me.
Let me set a new baseline.

I do have a solid continuity from the headlight plug to - battery terminal.

With lights on, engine off I’m getting 12.98v between the headlight plug and battery.

With lights on or off, engine on I’m getting .1v between the headlight plug and battery.

 

[45Dougal]

Scratch that. User error for me.
Let me set a new baseline.

I do have a solid continuity from the headlight plug to - battery terminal.

With lights on, engine off I’m getting 12.98v between the headlight plug and battery.

With lights on or off, engine on I’m getting .1v between the headlight plug and battery.

Do you get the same voltage with both headlights unplugged?
Somehow that ground wire is getting 12v on it, and it can't be a good connection, because otherwise it would be smoking.
Maybe that ground wire is actually connected to the alternator, and appears to be ground when not running?

Unplug the alternator and retest your continuity?

 

[iliketomakestuff]

Do you get the same voltage with both headlights unplugged?
Somehow that ground wire is getting 12v on it, and it can't be a good connection, because otherwise it would be smoking.
Maybe that ground wire is actually connected to the alternator, and appears to be ground when not running?

Unplug the alternator and retest your continuity?

To clarify, I’m getting 12.98v between + on the battery and the headlight plug ground. That’s correct, right?

 

[45Dougal]

The headlights are behaving like the charge warning light - maybe the wires got crossed?

 

[45Dougal]

To clarify, I’m getting 12.98v between + on the battery and the headlight plug ground. That’s correct, right?

Yes it should be.
But that should never change.

 

[iliketomakestuff]

OK, alternator unplugged.
Engine off, lights on I get 11.98 V between + battery and ground on headlight plug.

Engine on and it drops to .1 V

Engine on, lights off and it’s still at .4 V

Hooked alternator back up and ran the same test with the same results

 

[45Dougal]

The "continuity" setting on your meter is the most misleading function ever devised..
With everything switched off, what is the resistance between the ground pin on the connector and the battery negative terminal?

What is the resistance between each positive terminal on the connector and the negative battery terminal?

What's the resistance between the engine block and the battery negative?