Hey guys,
I have a head-scratcher here. All blinkers, taillights and brake lights work perfectly without the headlights on. When I turn the headlights on, the driver's brake light no longer works. Taillight is fine and all of the others continue to work properly, but when I step on the brake pedal, the left refuses to work. I should also mention that the left headlight is dimmer than the right. Is this a ground issue? I did install a harness with relays for the halogen headlights this past summer and I'll remove that to see if that's the issue, but that'll have to wait a couple of weeks until it gets back from the paint shop. Until then anybody have any ideas?
Thanks
MrG

 

Sounds like a ground issue to me front & rear.
Start at the battery.
Negative to motor pull and clean both motor & cable.
Motor to frame cable do the same. If you don't have one add one.
Motor to body do the same, clean body and cable ends. What don't have one, add one.
You could also add a ground from bed to frame just to make sure the bed is grounded.

Now at the dim head light follow the plug wires and 1 wire is a ground, clean where it screws down. Do the other side while at it.
The rear lights should be grounded thru the light housing to the bed, see why I said cable from bed to frame?
Make sure the housing is clean and screwed down good.
Let us know how you make out after doing all that.
Dave ----

 

Yep....
I totally agree with Dave....
Most of the time these older wiring harnesses develop bad grounds over time. Rust is the usual culprit.
Check EVERY ground point. Those jokers are everywhere!
Just follow the wiring and every time you see a wire screwed to metal, take it off, clean it and the spot where it attaches to shiny metal.
Re-attach and go to the next.

Good luck!

 

Ok thanks guys. I'll do that when it comes back from the paint shop. It just seems odd to me that the brake light works fine when the lights are off, but then has a ground issue when they're on. I DO think my dim headlight is probably a bad ground. I used an existing screw that holds the rubber grommet (for the AC lines to the condenser) in place. It has a metal ring around it and it's screwed into the radiator support, but it's probably not a very good ground. I'll be sure to update when I get the truck back and clean all of the grounds up.

 

Seems like a normal ground/earth issue at the left rear socket to me too. It's at least worth checking out.
Could be a corroded or loose fit in the socket, could be an imperfect grounding through the lamp housing, or it's dedicated ground wire if yours has one. Not all Ford trucks do however, as some use only the housing/reflector as the connection to the body.

Worth a try...

Paul

 

It was probably a decent ground when the truck was new, but over the years has deteriorated just like many other things. But the first thing to do with any of these old Fords is to verify that the factory grounds were not removed by some previous owner.
The main battery cable-to-engine ground is obviously there and good, or the engine would not be able to start. But there should be a secondary wire coming from that negative battery terminal to the body. To supplement the main ground we usually say a 10ga wire is sufficient for this body ground, but you can use anything you want really. Just consider 10ga a minimum.

And if necessary, due to there being some rust between body panels, it might be a good idea to attach a secondary jumper wire between the fender where your main body ground is, to a point on the radiator support. Since the radiator support panel is where the headlights, turn signals, and depending on the year, even voltage regulators, horn relays, and other electric components all get their ground path from, it's a good idea to make sure that the core support knows that it's still a highly respected member of the body panel family!
If nothing else, your headlights will thank you for it.

Speaking of headlights and upgraded wiring... Updating your headlight circuits to use relays is usually a good thing on these old trucks. Most modern vehicles use them, and it is quite a good upgrade to the old trucks, especially where original wiring is concerned.

Paul

 

I don't have the battery neg to body, so I'll have to do that. Thanks for the input

 

I'm pretty sure mine can't be grounded through the taillight housing as it's all plastic. But I'll look for/clean all grounds. Thanks.

 

On my 79 behind the plastic tail light housing the wiring harness has a ground wire attached behind housing to bed metal , remove an clean an use star washers , there is a ground wire like this behind most light housings front an back ,,also engine block should have grounds to body an frame , there are many places to keep the grounds clean and these old trucks have to have them in good shape .
Also the bulb sockets can go bad , as the ground wire runs from bottom of socket an turns to a metal tab to side of socket for bulb to make contact , an the sockets can get corroded on the ground or the ground tab gets fatigued an breaks , , if you look at the socket opening you will see the ground tab which runs to bottom of the socket an touches the ground wire , they are prone to go bad , I had to replace both rear sockets on mine an the front as well as they worked very poorly or not at all , with the new sockets they work perfect

 

Thanks for the info. I was already thinking of replacing all sockets as I found that the PO completely removed the two read market sockets. It's no wonder they don't work!