engine and drivetrain Odd brake light issue- any ideas?

[Simon]

Oddly, my brake lights only work when the car is warmed up. When I leave for work in the morning with a cold engine, no brake lights (turn signals and back up lights work fine). By the time I get to work, the brake lights work fine. Any thoughts? I've put in a new brake light switch with no change.

Simon

Sahara '74 automatic, weber 32/36, petronix

[jetboy]

i am thinking its got to be in the mechanical side of things, - but if you already replaced the switch and it still does it? if you just reach in with your hand and pull the switch out and in do the lights work? maybe your brake lever and the switch are mis-aligned, so it gets caught- but if you use the brakes enough it frees up??

the electrical side should actually work better when its cold....but its possible that you have a bad ground, that when it sits gets coroded and crap, and loses contact, but when you drive it gets bounced and vibrated enough it "cleans" itself and makes contact again- finaly pushing some juice through all the rust etc thats holding it back.

sorry for the lame advice-- thats a tough one.

[Mike Self]

pedal linkage between the booster and the firewall. Reach in with a screwdriver and snap it on and off while an assistant watches the brake lights. If it works when snapped, but not when gently released, I'd give it a shot of WD 40, snap it a few times and I'll bet you'll be back in business. If not, new ones are cheap and easy to replace. Make sure it's adjusted properly; if left on, the brake lights will melt your lenses in a few minutes...

cheers

mike

[gracer]

The linkage could be sticking and not letting the lever make contact with the switch. G

[Simon]

So this evening, with the engine cold and the lights out, I attached a volt meter to the two leads to the brake light switch, 12volts measured. Manipulating the switch failed to yield lights.

My understanding is that for voltage to be measured across those two leads, the ground must be ok. However, the groud could be at the far end (ie, beyond the light bulb as it should be) or a short before the bulb. If the ground wasn't ok, there wouldn't have been a voltage. Hence, the problem must lie with the item between the leads, ie the switch, or a transient short in the wire leading to the lights.

Does this sound logical?

Simon

[Simon]

So, I made a jumper and by-passed the brake light switch. Unfortunately, no lights. Hence, there must be a ground/short before the power reaches the brake lights. The brake light switch is likely not the problem. I think I would have prefered a wonky switch as it would be easily fixed! A transient short is not going to be fun.

Simon

[SteveJ]

First check and clean all the fuses and contacts in the fuse box. Do this with the battery disconnected.

Second check-

Turn the key off.

Wedge the brake pedal with a rod against the driverseat.

Pull each light housing out in the trunk.

Clip the negative lead to a GOOD ground.

Turn the key to the on position and see if you have voltage at the quick disconnect connector at the backside of the taillight housing.

After that, start checking the lamp sockets for contact with the lamp and corrosion in the barrel of the lamp socket.

Also verify a good groud between each lamp housing and the ground connectin in the trunk.

[Simon]

Everyone, thanks for the advice. Steve, I will try your suggested testing scheme over the weekend.

Simon