Hello! I'm certainly new on this forum but not new in working on Ford trucks (even though I might be a bit young). I have owned a 1985 Thomas school bus with a Ford B700 chassis for four years now. I have had a few problems with the chassis before hand but I was able to figure those out with some local help. This one, however, I really need some help other than the previous mechanics of the bus. Recently I was driving it when I had to go through a very rough road. I believe these bumps may have caused my problem. When I pulled up at one house I noticed that the left headlight was VERY dim. When I was trying to diagnose the problem the next day I determined that there was a short between the high and low beam wires for the left headlight. What I need to know is where the wires split for the right and left headlamps. The problem is most likely after this split because the right side still acts normally. While I can wire the left side into the right side wiring before it reaches the light, I would like to avoid doing this because it would be routing twice the normal energy down wires only meant for one headlight. Any ideas how I could avoid having to do this? What would be ideal is to find where the short is and correct it. This could be done if I knew the wire colors and locations so a wiring diagram would be ideal if any of you have one that you might be able to scan in. I'm sorry for rambling, but any help is greatly appreciated! Thank you!!!!

Mike

 

Sorry, I can't post a wiring diagram. Those are covered by copyright laws.

If the bright and dim were shorted together your headlights on both sides would have both filaments on. Your problem is most likely the ground wire connection for the left headlight.

 

Both filaments are on, on the left side healdight. I looked at them with a welding mask (so I wouldn't ruin my retinas ) and the right hand, properly working headlight switched between the high and low filaments when you pressed the switch. The left side, however, had two dimly lit filaments on the whole time. That's why I've been thinking that it's a short on the high and low beam wires after they split for each side.

Also, does anyone has any idea where I could find a diagram for a vehicle this old? I have all the manuals for this truck chassis though none of them contain wiring schematics.

 

Two filaments lit dimly would indicate a bad ground. The current is passing through the other filament and back through the harness to the other light and then to ground. The current is actually passing through three filaments in series before it "hits" ground. Three times the resistance = one third the current. That's why they're so dim.

 

Try here,

 

Thank you all for your help. I'm certainly glad I didn't rewire it to find that, that still wasn't the problem! One minor but important issue...where might I find this ground? It's a school bus so there's tons of equipment on it not normally on the truck chassis so there's grounds all over the place. Where were they typically mounted on these trucks for the left headlights? Thanks so much!!!

 

Ok, so I did find the ground to the left side. I disconnected it to see if the headlight's behavior changed at all and when I did the whole left side turned off and the horn (the low toned one) turned on. Are you still thinking a ground is my problem?

 

So... when you removed a headlight ground wire... the horn went off ?


That's weird.

Have you gone over it and cleaned connections while you were looking for the problem >

 

I replaced the connector directly behind the bulb. The individual plugs were popping out of it so I replaced it with just three push on plugs instead of all three in that main plug Ford used. After I disconnected the ground and found out what happened when I did, I cleaned it off and the scraped a bit of paint from the surface around it, then put it back on. (I also sprayed it with oil so that it wouldn't corrode.) That is about all I could clean. The rest of the wires for the headlights don't have any terminals or splices until it reaches the split for the left and right. They are also taped up in electrical tape for much of the length so I would find it unlikely that it was cut by something (and also makes my high/low beam short theory make a little less sense).

 

So take a piece of jumper wire & run it from the ground wire on the headlight plug to the ground post on the battery and see if the headlight gets brighter.

Grab a test light, and with the alligator clip attach it to the power headlight wire ,....and touch the probe end to ground and see if the test light ,lights up.

 

I took your suggestion and used a jumper wire between the ground and part of the frame and looking at the reflection on my hand I could see it would light up when I touched it to the frame. Thank you so very much for helping me narrow this down! Do you have any idea why the wire from the headlight would go bad like this? The ground serves the left parking lights, marker lights (the ones on the fender), left headlight, and apparently the horn. If those all work and not the headlight it must mean the wire somehow went bad. It doesn't look as if it was chaffed by anything. Any ideas? Thanks again for helping me get to the root of the problem!

 

"Do you have any idea why the wire from the headlight would go bad like this? "

Old age ring a bell? You have found the problem-a bad ground. You have two choices:

Try to figure out how the factory ran it's grounds. This could get involved, since some of the body is mounted in rubber bushings. So you could have a ground strap bad all the way on the other side of the engine compartment, that grounds the front sheetmetal, which is probably where the headlight grounds are bolted to.

Or, just run a new ground from the sheetmetal to the frame, like you found worked. There is nothing wrong with this, the more rusty sheetmetal is grounded, the better it works.

Like one of the other posters said, some of the lights worked because the current was flowing improperly through other circuits. The smaller bulbs draw less amperage, so the re-route of current because of the bad ground wasn't as noticeable. The headlights draw a lot of current, so that's when the problem showed up the worse.

 

Thank you all for your help! I was able to fix it today. The problem was a splice where all the grounds for the lights on the left fender and the horn came together and attached to the firewall as one ground. I noticed this splice that looked a little raggedy and when I took off the tape around it the solder just fell apart in my hands. I resoldered it and now everything works. Thanks again!