Brake warning lamp issue

[dalegend]

The brake warning lamp is coming on and staying on with the headlamps on. The light does not come on at any other time.

With the headlamp switch in the off position or the park lamp position the lamp is not on. The lamp only comes on with the headlamps on.

I would assume if it was a short to ground somewhere in the wire to the from the warning lamp switch to the ignition switch or from the ignition switch to the warning lamp bulb it would be on at all times.


Any ideas?

[Moose Monkey]

Just on with low or high or both beams? Might be ground problem in headlight circuit.

[dalegend]

I will check into this. The only wiring done on the truck over the winter was to replace two wiring harnesses. Direct replacement harnesses and all plugged in so no splicing was done. One harness from the cab to the rear lamp harness that runs the length of the frame and the rear tail lamp harness.

The one thing I notice is that the left rear tail lamp had never been as bright as the right rear. Have tried new bulb but it did not fix it. Not sure if this can be related. Bad ground at the lamp?

Haven't touched any other wiring all winter

[dalegend]

Lamp is on with high beams or low beams.

[Andy4639]

Simple. The brake light switch is a ground activation circuit.

I would disconnect the switch under the hood and see if the light goes out. If it does then the head lights are some how grounding through the brake switch wire.

If you have everything working other than this one issue take a look at the back side of the ignition switch. Then the fuse block. The brake switch light on the dash runs to the ignition switch then to the fuse block then out to the switch on the brakes. It maybe shorting in the ignition switch plastic connector (those are bad about melting) theen the fuse block. Per the drawing I posted.
Is the temp gauge acting funny it's right beside the brake switch on the ignition switch connector. It's a ground also.


http://67-72chevytrucks.com/vboard/s...185856&page=16

[davepl]

I'm still stymied as to how applying power to the headlight circuit could complete the ground circuit for the brown brake wire. That's the problem, they're two different types of circuits. So leaking power from the headlight switch to the brake light wouldn't do anything. And turning on the headlight switch doesn't create a ground potential anywhere. So weird.

[Andy4639]

Originally Posted by davepl

I'm still stymied as to how applying power to the headlight circuit could complete the ground circuit for the brown brake wire. That's the problem, they're two different types of circuits. So leaking power from the headlight switch to the brake light wouldn't do anything. And turning on the headlight switch doesn't create a ground potential anywhere. So weird.

That's when everything is right in the system.

We have no idea what he has yet! It maybe he needs to disconnect the brake switch and see what happens.

[dalegend]

Lamp stays on with the wire to the warning switch disconnected

[Moose Monkey]

Tail light does sound like ground problem at Mount point (bolts thru light assembly) . What was the other wires replaced?

[Moose Monkey]

Just for giggles,try grounding the brake switch wire with a test light. Both without an with lights on/off

[leddzepp]

Just for fun, next time it happens, stomp on the brake pedal REALLY hard and see if the light goes out.

[VetteVet]

Originally Posted by dalegend

The brake warning lamp is coming on and staying on with the headlamps on. The light does not come on at any other time.


Does the light come on normally when the key is turned to start?

With the headlamp switch in the off position or the park lamp position the lamp is not on. The lamp only comes on with the headlamps on.

I'm trying to remember if the 67 dash lights come on in the park position or just in the headlights on. I'm thinking that the dash lights are grounding the brake warning light position somehow when they are on.

Try jumping power to the PNL LTS fuse with everything off and see if the BW comes on. If it doesn't then turn the key on and jump the fuse.
Post back wit the findings.

I would assume if it was a short to ground somewhere in the wire to the from the warning lamp switch to the ignition switch or from the ignition switch to the warning lamp bulb it would be on at all times.

This is correct, which you can verify by grounding the BW switch wire at the master cylinder and turning on the key. This is what the key does when the key is put in start. This is what Andy is getting at.


Any ideas?

Originally Posted by davepl

I'm still stymied as to how applying power to the headlight circuit could complete the ground circuit for the brown brake wire. That's the problem, they're two different types of circuits. So leaking power from the headlight switch to the brake light wouldn't do anything. And turning on the headlight switch doesn't create a ground potential anywhere. So weird.

And so am I, The two circuits are exactly the opposite.
The headlights are switched on the positive side and the brake warning light is switched on the ground side.

There may be a wiring mistake on the cluster plug that would give a ground to the BW bulb when the dash lights are on. Otherwise I don't see a connection between the BW bulb and the headlight switch other than the dash lights.

[VetteVet]

There should be no path to ground for the brake warning bulb except through Pin 2 on the cluster plug. If you could pull that pin wire then it would isolate the ground to the cluster. This is true for both the light cluster and the gauge cluster. Which do you have?

[57taskforce]

Is it possible that it's light bleeding thru cluster? I.e. The cardboard tube that isolates the indvidual bulbs in the cluster maybe is missing or damaged? Going out on a lib here. I'm not in front of my cluster to see if this is even possible but thought maybe I'd throw it out there.

[Andy4639]

Vett both the brake light wire and the head lights come off the ignition switch harness also right? They could be contacting each other in between some place. Maybe even in the plastic connector on the key switch itself. I had to replace my once due to over heating and melting of the plastic. I never did find the cause but it's never done it again since I installed the new connector.

[VetteVet]

Originally Posted by Andy4639

Vett both the brake warning light wires and the head lights come off the ignition switch harness also right? They could be contacting each other in between some place. Maybe even in the plastic connector on the key switch itself. I had to replace my once due to over heating and melting of the plastic. I never did find the cause but it's never done it again since I installed the new connector.

Hello Andy

According to the wiring diagram, the feed wires for the key and headlight switch are separate wires although they both connect with the engine bay wire from the alternator battery junction. Look in the middle of the diagram where the red wires join. One to horn relay, one to ignition switch, one to the fuse box, and one to the headlight switch. All connected at the junction with the wire from the engine bay.




This one shows the red power wires to the key switch (top) and the headlight switch (bottom right). If we could post both diagrams together and line up the wires it would be much easier to see. Ignore the black arrows they show the accessory wire to the fuse panel that powers the wipers,heater fan switch and turn signals.





The brake warning light is just an open circuit to ground, and it must be grounded for the light to come on. If it gets shorted to a positive wire then the light won't come on because there is 2 positive sources on each side of the light, and no current can flow through it to light the filament. This is how the charging light works in the idiot light dash.

If there were another power source to the light on the brown wire side, when the key was turned to start, the brown wire would become grounded and the light might illuminate, but the other power source would go directly to ground, and we know what that means.

Your ignition switch connector probably melted because of a large amp draw through it over a long period of time. That's why the factory went to the relays instead of running all the power through the ign switch.

If we get some answers from the OP we'll know more about what's wrong.

[Andy4639]

Agree we need more info from him.

[Moose Monkey]

Any luck yet?