I have a 78 Ford F-100. I replaced the bulbs in the side markers and turn signals; then the headlights stopped working. My whole electrical system seems to be screwed up. Any thoughts? The tail lights flash when the emergency flasher is on, but not the front lights. The lights in my instrument panel: some work, some do not. My gas gauge just suddenly stopped working. I know I need fuses, relays, etc., but where do I start?
Thanks for any help, this is driving me crazy!

 

get a gauge and start checking to see where you have power and where you don't

 

Would you please recommend what kind of gauge I need? I'm obviously very new to this stuff.

 

Be sure to check all of the fuses first.

 

Yeah, fuses are all good.

 

I'm just thinking outloud here, but I would start at the block connector at the firewall and work your way toward the headlights. Look for a break in the wire or something grounded out. You might also tap on the relays and make sure the connections are good. Also, I don't know if they used many fuseable links on that year but that might be a possiblity.

 

You have several different problems, try to take it one at a time.

If the headlights stopped after changing the signal bulbs, then check the connections to the headlights and signals - especially the ground wires (should be one behind each headlight). Make sure connections are clean and tight.

The front should flash too with the emergency flashers on, do the front turn signals work (they use much of the same wiring)?

Instrument panel lights - check the bulbs and connections. Must remove the instrument cluster for this.

For gas guage, look over wiring along frame to rear tank. Make sure wire is not broken, connections are clean and ground to frame is clean and tight.

Take it one problem at a time and we'll help you get it all handled...

Marty

 

Cddyal,
73Custom knows his stuff! As an electrical engineer in a past life, I have learned not to eat an elephant all in one bite! Therefore, I recommend that you first go the Autozone website and download a wiring diagram.

For basic electrical troubleshooting, a tester light will do. A test light looks like a small screwdriver or awl, with a light in the handle, and an attached grounding wire. You connect the ground wire to the negative lead of the battery or clean metal surface. When you probe a hot wire with the tip of the tester, the light in the handle illuminates. With this tool, you can tell which wires are hot, and which are not. You can buy one of these at any automotive supply store for about $2.

For more sophisticated troubleshooting you will need a multimeter. You can set the multimeter to volts DC, ground one lead, and then probe with the other. The multimeter will show the amount of voltage going through the wire.

Troubleshooting tips: For the head light circuit, I would start at the starter relay. Make sure the black wire with an orange strip is hot. If so, turn on the light switch and check the red wire with a yellow strip located on the high beam switch. This wire should be hot also. If not, the problem is in the switch or the wiring going to it. Use the tester to determine the problem.

If the wire mentioned above is hot, check the red wire with a black wire coming from the high beam switch. It too should be hot if the low bean is selected. Press the high beam switch, when the high beams are on, the red wire with a black strip will no longer be hot; the power is switched to the green wire with a black strip. If this is working properly, remove the socket from the back of the headlight, and see if the same wires mentioned above are hot there, according to the different dimmer switch positions.

If all for the above wires are functioning properly, check the black wire on the lights for proper grounding.

Write back and let us know what you find.
John24255

 

have you checked your dimmer switch? When I first bought my 74 f100, I would accidently bump the switch with my foot and my headlights would go out. I replaced the switch and havent had any problems since. Hope this helps.

 

This may or may not help







Larry

 

I had almost the exact same symptoms with my 77 F150 except the headlights worked ok. In my case, the wiring was bad in the frame midway between the cab and the bed. Apparently, one of the wires got a nick in it and over the years, corrosion set in. When the corrosion finally got bad enough, the wire got hot and caused the insulation to melt off of all the wires in the harness. This caused the fuel tank sender wire, taillight wires to all be connected. That in turn caused all kinds of 12v feedback into the lighting system and virtually everything was adversely affected. The scary part is that when I pushed on the brake pedal and sent 12v back to the taillights, the 12v was also going down into the fuel tank sender which usually only sees millivolts of power!

Also, the headlight switch has a circuit breaker built into it. It's possible that a short in the harness could be causing the breaker to trip and that would definitely kill the headlights while still allowing the running lights to work.

Ok, with that said, crawl under the truck, and find the wire harness in the frame on the driver's side. find where it comes down from the cab and very carefully examine every inch of it all the way back to the taillights. It's possible that you have a bad spot in it like I found. The spot was easy to see because the wire harness wrap and the wires themselves were noticeably melted. Because you are seeing a fuel gauge problem, if you have the same problem that I did, I would expect the bad spot in the harness to be between the cab and where the wire for the fuel tank sender branches out of the harness.

I am very curious to hear if you have the same problem.

- Ron

 

Thanks, I appreciate the comment... Experience is a good teacher.

Marty

 

Thanks for all the help and suggestions? I'm going out today to buy the gauges to test my electrical connections, and I will be putting all the advice to good use over the weekend. I'm actually quite anxious to see what the problem is!

 

cddyal,
Please remember that electrical problems can be challenging, but not insurmountable if you apply sound logic, patience, and above all perseverance. Also, many of the problems you think are big, can have a very simple and common solution, i.e. a broken wire, loose ground etc.

It sounds like a fun project; I wish I lived closer to help you resolve it. If you start to get frustrated, remember you have lots of friends on-line just waiting to help.

Best of luck,
John24255

 

cddyal,
When purchasing your test equipment, you don’t need to drain your bank account to complete your mission. I believe autozone sells a small multimeter for about $6-$7. You may find the same thing at a BigLots or Harbor Freight for much less. Again, the test light should work find for what you need to do. These are just a few bucks at your local automotive parts store. I believe BigLots sells them for $2.99, and they include a test light for checking the spark output as well as a continuity tester.

I don’t suggest you buy one for this application, however, if you happen to have a friend who has an automotive analyzer, they usually have voltmeters and Ohmmeter built in. Chances are if you ask to borrow it, you may even get your friend to help in the troubleshooting process. In the case of electrical troubleshooting, I believe the wisdom of Ted Nugent applies--two heads are better than one! You are probably too young to have heard or remember that song. I don’t think he was talking truck repair, but heck if the wisdom fits, use it!

Best of luck,
John Huff 24255