This truck is killing me! So the guy I bought this '85 Bronco EB 351W from did who knows what to this truck. The stereo had been stolen out of it so wires were just torn out. The dash instruments don't work right. The back up lights never come on. The dome lights work, but the individual push button spot lights don't in the roof or the cargo area. The headlights flicker when hitting bumps and sometimes pop on/off. The clock works, but when hitting bumps or just driving, you can see it flickering too.
So here's what I think and what I've done so far.
For the dash instruments I've bought 3 different instrument clusters from Ebay, so have plenty of spare everythings if anyone needs something. I replaced the water temperature sensor, but it still pegs out within five minutes of driving.
I figure the fuel sensor may be out, but I don't know yet. I replaced the oil pressure sensor and it now reads correctly. I suspect the battery gauge is working.
I put on a new radio antenna and new stereo down to all new speaker wires. It sounds great, but if I hit the right bump, it goes out and comes back on. I don't think it's the constant power because the time doesn't reset. If I turn the headlights on all the way, the radio usually goes out and comes back on.
I replaced the headlight switch, which was a pain in the *** if you have fat fingers, but nothing changed. I looked for every ground I could find and giggled and shook them all but everything continued to work.
I read in an old post where some guy was having the same problem and he was told to look at the ground behind the kick panels. Took mine off and don't see any grounds in there anywhere.
Everything points to a ground problem. I've looked at the headlight grounds and they seem fine. Looked at the battery and alternator to see if there was a loose ground and still nothing.
I am electrically challenged. Does anyone have any idea where the truck might be shorted? Oh, almost forgot. I decided to ground the stereo ground wire by itself instead of where I had it to the cigarette lighter. Cut the wire and even with no ground the stereo still worked. Also, no change. Still cut out then back on when I turned the headlights on.

 

Since you several things going off and then back on I would be looking at the power side (B+) connections from selonoid to fuse block.

 

I'll check that out in the morning. What do you make of the stereo working even though I have disconnected the ground wire and it still worked?

 

Check for any shorts in the lines going to the cabin, the violence on the stereo steal might have messed the wiring, so check that area. I wouldn't trust any readings from the cluster until i'm 100% sure the electrical issues are solved. Ask for wiring diagrams from Subford in the contemporary F-150/250/350 section, those should come in handy.

 

Thanks Encho, the stereo steal was a real hatchet job. They even tore out the antenna. I've got the wire diagram from my Hayne's manual but the problem is that the wire colors don't all match up with what's really in there. What would really be helpful is if someone knew where on the body, especially inside the cab, where the ground wires are actually supposed to be grounded at.

 

Ask Subford, he remembers even the colors of the wires.

 

The ground path would be thru the antenna cable. At least you know that you have a good ground at the antenna base.

 

Ha, yeah there is that I suppose. Always a positive way to look at things.

 

Well, you were right Encho, I checked with Subford and took his advice and at least part of the problem is fixed. Turns out the ground from the battery to frame wasn't enough. Once I added a 10 gauge ground wire from battery to the body, the stereo works great and no more issues with the headlights. All I need to figure out now is the dome spotlight issue and the back up lights. I suspect the back up lights are the switch on the transmission, and the gas gauge probably needs a new sending unit. I'm still stumped by the temperature gauge. Within a couple of minutes from starting, it is maxed out but I know it's not that hot. I replaced the sensor but it didn't change anything. I think I might buy an electric aftermarket gauge and wire it direct to that.
So a HUGE thanks for the help!

 

for your temp sender, you sure the connector to the cluster is in all the way? those connectors get weird over time and need a little bit of love.

and yeah, backup lights are probably the MLPS or neutral safety switch. I kept blowing my switch (stupid non-motorcraft) and had to run a switch from the backup lights to the feed wire. Dunno about in 85, but in mine it was orange/pink (feed) to violet/black (backup lights) I think. Just put a multimeter on it, make sure you get 12v to the orange wire with your key in the on position and then check the violet/black for 12v with the truck in reverse. It also might just need adjustment.