79 f100 electrical problem
New User
Joined: Dec 2001
Posts: 17
Likes: 0
From: Canada
79 f100 electrical problem
Hi there all
Electrical problems in winter.... heard that one before.
The weiredest thing: I was driving (the 351M motor runs very well) and all of a sudden the body electical cut out, lights, instruments, everything. The motor was still going though. I drove home and switched the motor off. Then I could not get is started again. The starter does not even turn over.
Again, no lights, nothing. The only light that comes on is the oil light, seat belt light, not even the alternator light! Nothing happens when I try and start.
I have replaced the starter relay and volt-regulator, still nothing. I have traced the wiring for any short-circuits, nothing that I can see. I have taken out the light switch, still the same.
Please help. What could be causing this problem?
Your advice is appreciated
Thanks
Electrical problems in winter.... heard that one before.
The weiredest thing: I was driving (the 351M motor runs very well) and all of a sudden the body electical cut out, lights, instruments, everything. The motor was still going though. I drove home and switched the motor off. Then I could not get is started again. The starter does not even turn over.
Again, no lights, nothing. The only light that comes on is the oil light, seat belt light, not even the alternator light! Nothing happens when I try and start.
I have replaced the starter relay and volt-regulator, still nothing. I have traced the wiring for any short-circuits, nothing that I can see. I have taken out the light switch, still the same.
Please help. What could be causing this problem?
Your advice is appreciated
Thanks
Senior User
Joined: Apr 2001
Posts: 321
Likes: 1
79 f100 electrical problem
Hi,
You stated that you traced for shorts, but did you try looking for opens, like the fuseable links?
Do you have +12V anywhere on the fuse block inside the cab?
I think that the wires for the cab come in through the firewall near the brake booster. Have a look there and make sure all is ok.
I'm not sure, but do you mean that the oil light comes on, and the other two lights don't?
Do you have schematics for your truck? If you don't it might be worth getting a Haynes book, it's not totally accurate, but better than nothing.
Let me know if you find anything, perhaps I can give you more ideas to try.
-Brents
You stated that you traced for shorts, but did you try looking for opens, like the fuseable links?
Do you have +12V anywhere on the fuse block inside the cab?
I think that the wires for the cab come in through the firewall near the brake booster. Have a look there and make sure all is ok.
I'm not sure, but do you mean that the oil light comes on, and the other two lights don't?
Do you have schematics for your truck? If you don't it might be worth getting a Haynes book, it's not totally accurate, but better than nothing.
Let me know if you find anything, perhaps I can give you more ideas to try.
-Brents
Posting Guru
Joined: Jul 2001
Posts: 1,613
Likes: 2
From: Gainesville, FL
79 f100 electrical problem
79F100,
Keep it simple. The reason your problem happens in winter is because they salt the roads where you live, right?
You've got a dirty ground on your battery terminal or connection to block. It's enough to allow small current loads to operate but, the big current can't make it.
If that's all you've got to deal with your lucky.
Good luck, Have fun,
KingFisher
Keep it simple. The reason your problem happens in winter is because they salt the roads where you live, right?
You've got a dirty ground on your battery terminal or connection to block. It's enough to allow small current loads to operate but, the big current can't make it.
If that's all you've got to deal with your lucky.
Good luck, Have fun,
KingFisher
New User
Joined: Jan 2008
Posts: 0
Likes: 24
79 f100 electrical problem
Clean off all the battery connections real GOOD,then try it again.
Dennis
First Came the Chicken,
Then Came the Egg,
Or Was It the Other Way Around?
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
Dennis
First Came the Chicken,
Then Came the Egg,
Or Was It the Other Way Around?
78 F-150 429CJ C6 ,Silver w/Explorer Pkge
641/2 Mustang,Pre-World's Fair Car #8092
64 Fairlane S/C waiting for a 390-4spd.
68-Mustang.Sunlit Gold 80,000 miles
New User
Joined: Dec 2001
Posts: 17
Likes: 0
From: Canada
79 f100 electrical problem
Hi Brents
I have a Haynes book and have traced the wiring diagram carefully. I will admit I did not check the wiring with a multimeter (I don't have one) but I definitely have juice getting to the fuse box as the cargo, oil, brake and seat belt lamps work. Those circuits have fuses. With the ignition "on" the oil lamp burns. With the ignition on "start", the oil, brake and seat belt lamps burn. Thats it. So there is juice at the ignition switch.
Do you know where splice 208 is according to the wiring diagram, inside engine bay or in cab? I will replace my battery cables next.
Thanks for your response.
I have a Haynes book and have traced the wiring diagram carefully. I will admit I did not check the wiring with a multimeter (I don't have one) but I definitely have juice getting to the fuse box as the cargo, oil, brake and seat belt lamps work. Those circuits have fuses. With the ignition "on" the oil lamp burns. With the ignition on "start", the oil, brake and seat belt lamps burn. Thats it. So there is juice at the ignition switch.
Do you know where splice 208 is according to the wiring diagram, inside engine bay or in cab? I will replace my battery cables next.
Thanks for your response.
New User
Joined: Dec 2001
Posts: 17
Likes: 0
From: Canada
79 f100 electrical problem
High KingFisher
Thanks for the response. I will admit I did not check the negative battery connection to the engine block. I assumed it would be alright but I guess not. I am going to change both the pos and neg cables with thick cables as the factory battery cables are very thin. Why oh why do they put such thin cables on?
I realize that rust is a very good insulator at 12V and that the lights circuits take a lot of juice, so that might just be the problem. Thanks, I will check.
79f100xlt
On second thoughts, why then if there is a bad ground connection did the motor keep running when the lights went out?
Thanks for the response. I will admit I did not check the negative battery connection to the engine block. I assumed it would be alright but I guess not. I am going to change both the pos and neg cables with thick cables as the factory battery cables are very thin. Why oh why do they put such thin cables on?
I realize that rust is a very good insulator at 12V and that the lights circuits take a lot of juice, so that might just be the problem. Thanks, I will check.
79f100xlt
On second thoughts, why then if there is a bad ground connection did the motor keep running when the lights went out?
Trending Topics
New User
Joined: Dec 2001
Posts: 17
Likes: 0
From: Canada
79 f100 electrical problem
Hi Dennis
I will replace the factory battery cables with thicker cables, and will also put an additional ground directly to body close to the battery. As of yet, I have assumed the ground connections to be good, but I could (and probably am) wrong in assuming that. By the way the battery is brand-new. Maybe I must install a dual battery...
Thanks for your response.
79f
I will replace the factory battery cables with thicker cables, and will also put an additional ground directly to body close to the battery. As of yet, I have assumed the ground connections to be good, but I could (and probably am) wrong in assuming that. By the way the battery is brand-new. Maybe I must install a dual battery...
Thanks for your response.
79f
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
Top 10 Fords at 2026 Carlisle Ford Nationals
Joe Kucinski
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Posting Guru
Joined: Jul 2001
Posts: 1,613
Likes: 2
From: Gainesville, FL
79 f100 electrical problem
79,
Your right! Maybe? The motor to body ground is on the driver's side rear intake manifold to firewall. A couple of things can cause your headlights to fail.
1. Headlight switch
2. Hi/Lo beam switch
If only your lights failed then it's your switch. The dimmer effects only the instrument illum. lights not the gauges. Idoit light power is cycled through a relay. If you turn your ignition key, you should hear the relay click. I can't remember exactly where that relay is found under the dash? I believe it powers just about everything except the starter, which leads us back to your ground cable. Jump your solenoid with a screw driver and see if it will start. We'll go from there. Got to run.
Good Luck, Have Fun,
KingFisher
Your right! Maybe? The motor to body ground is on the driver's side rear intake manifold to firewall. A couple of things can cause your headlights to fail.
1. Headlight switch
2. Hi/Lo beam switch
If only your lights failed then it's your switch. The dimmer effects only the instrument illum. lights not the gauges. Idoit light power is cycled through a relay. If you turn your ignition key, you should hear the relay click. I can't remember exactly where that relay is found under the dash? I believe it powers just about everything except the starter, which leads us back to your ground cable. Jump your solenoid with a screw driver and see if it will start. We'll go from there. Got to run.
Good Luck, Have Fun,
KingFisher
New User
Joined: Dec 2001
Posts: 17
Likes: 0
From: Canada
79 f100 electrical problem
SHE RUNS AGAIN!
You guys were right about the 'ground' connection. Well, after putting in a new starter relay, volt-reg, and half dismantling the dash to trace wires, I put on a new neg battery cable and......you guessed it...started first time. And the lights also work. Damn, everything works again!
I guess I under-estimated the effects of a bad ground connection. I must admit the old cable was very rusty and the connection to the engine VERY dirty.
I bought a couple of feet of 1 gauge welding cable and new battery connectors, made up new cables, and added an additional short cable from the neg terminal to the fender close to the battery. I don't want to rely on the cable from the manifold to the firewall. My next step is a new starter cable.
This is great, thanks guys for a great website. My first time on a message board and my problem was solved.
I must say though, coming from the southern hemisphere, my first winter in Canada is going to be tougher than I thought...
Cheers
You guys were right about the 'ground' connection. Well, after putting in a new starter relay, volt-reg, and half dismantling the dash to trace wires, I put on a new neg battery cable and......you guessed it...started first time. And the lights also work. Damn, everything works again!
I guess I under-estimated the effects of a bad ground connection. I must admit the old cable was very rusty and the connection to the engine VERY dirty.
I bought a couple of feet of 1 gauge welding cable and new battery connectors, made up new cables, and added an additional short cable from the neg terminal to the fender close to the battery. I don't want to rely on the cable from the manifold to the firewall. My next step is a new starter cable.
This is great, thanks guys for a great website. My first time on a message board and my problem was solved.
I must say though, coming from the southern hemisphere, my first winter in Canada is going to be tougher than I thought...
Cheers
