I have a slight problem with the marker lights and tail lights on my new 1970 F350.

When the headlights or parking lights are on the tail lights are not on. The brake lights work. The rear turn signals do not come on when the lights are on. The fronts turn signals work. The rear turnsignals work when the lights are off. The clearance lights on the side of the bed do not work at all.

Yesterday all the lights worked. It was tonight and dark outwhen I discovered this so I didn't mess around with it a whole lot.

My guess is a ground or a loose wire...
Any help will be appreciated.

 

Abe,front-to-back, or back-to-front, start locating and cleaning up every single grounding point you can .Sometimes the rheostat in the headlight switch can be cleaned to make better contact, worth doing if the truck has sat awhile.Here in my damp climate,contacts in flashers corrode and stick.Pull all the bulbs and clean the sockets and bulbs themselves with a small wire brush.That's a good starting point.

 

The seals in the hoist cylinder and grease in the hinges might be insulating the bed from the chassis ground. Try running a 12 ga ground wire with good clean connections between the bed and truck frame.

 

I went out tonight... turned on the lights.. they all worked including the bed marker lights... I turned on the ignition and all lights worked.. I put on the turnsignals and they worked, both with and without the lights on...

So I must have gotten something wet when I washed the cab or the gremlins were there and left today sometime...

Thoughts... advice?

 

This may sound crazy but wiggle your ignition switch the next time they go out. Particularly if you have a lot of keys on your key ring.

Failing that you might want to check with the guys on the '67-'72 forum.

 

That's worth remembering.
Thanks.

I'm an engineer by trade. Intermittent problems can be a beast to troubleshoot.

 

They have been working great since I posted this on the 23rd.

They should have a good ground. The PO run a ground wire back to the Neg terminal on the battery.

But here is a question, more out of curiosity: the clearance lights (on the roof)do not come on when I turn the parking lots or headlights on.They only come on when the ignition switch is on. Is this stock of did the PO wire them up that way?