Well, this sure isn't what I was hoping for as my second post to the forum. I bought a great 76' SuperCab with a 390 4 barrel Wednesday (the 30th) and while driving around the neighborhood this evening the truck just lost all power and came to a stop. No lights, no engine, nothing and I cruised to an easy stop (fortunately at the end of my own block). I thought I smelled a bit of electrical burn for a moment after I came to a stop but could not track it down before the smell disappeared.

Some early symptoms - on drive home from purchase, the turn signal light function was in and out. The starter was also turning slowly and sounded tired/weak. The ignition switch was a bit loose and previous owner said you had to make sure the key was not all the way to the right or accessories would not work.

Some early checks/fixes- replaced battery and battery cables and checked grounds. Starter began turning stronger and turn signal lights were back. Thought I had it going well.

Any advise on where to start checking would be greatly appreciated (I'll be busily searching the forums also). I'm loosing serious points with my other half as I type.

 

Since you do not have lights(I am assuming no headlights?) I would suspect you have a problem over near the starter solenoid.

On the large terminal on the solenoid where the bat + cable is connected, there should be some smaller wire(s) also hooked to the large terminal. This wire or wires are what feed the whole elec system of the truck. There may be colored rubber things also in the wires. These are fusible links, so they need to be checked to make sure they are not burnt in two.

As this wire leaves the solenoid, it comes to some splices where it goes to the alternator, and also splits to other fusible links that feed the headlights, the ignition switch, and the fuse box.

If it's completely dead with no headlights, I would start over there, and unwrap the harness if you have to.

 

Franklin2, thanks, you are correct. No headlights or any other power from battery forward. I'll start over at the solenoid and those fusible links. If the links need replacing, do I just splice in new fusible links or do I need to start replacing the harness (after tracking down whatever short caused this failure in the first place)?

 

Just replace them, you can buy them in the store. I would also go to radio shack and buy a cheap soldering iron and some solder, and also some shrink wrap. Solder any splices you make in the wiring and then slide the shrink wrap over the connection, and you will have a nice job that will last a long time.

 

Franklin2, thanks again for your advise. Cheap soldering iron, solder and shrink wrap are now on hand. Having spent some time looking through the electrical section of the forum I see you are a frequent contributor with a huge amount of knowledge. I started trouble shooting the power failure this afternoon after getting the truck back to my house and found the fusible links just off the solenoid are still good. I also checked for continuity on all my grounds (block to body, body to frame, etc.) and they are good. I'm now in the process of sorting out the lines further and really need to get a wiring diagram before I go much further. I'm going to document my progress here so I have a record of what I've done/found/screwed up.

So far it seems power is getting into the cab of the truck but is lost somewhere behind the dash. When I bought the truck last week, one of the first things I noticed was a mess of wires hanging under the steering column as well as some obvious aftermarket additions including an old cruise control system, an old brake controller and a more recent stereo addition. Some look to have been wired well and others . . . lets just say, not so well.

The truck also has an old (apparently aftermarket, but I'm not certain) dual battery set up that was disconnected when I bought the truck. Although a battery is sitting in the driver's side battery box, it is not hooked up and I do not plan to try hooking it up as the truck ran fine without it and the system looks funky. The battery wires are hooked up to a battery isolator that is in turn hooked up to the alternator, but the battery connection ends are all taped over. I'd like to complete the removal of this mess but as of yet, I can't tell if any part of it is still serving a purpose, just getting in the way, or causing problems.

The brake controller was connected to what appears to be a factory or dealer supplied harness as it has Ford emblems stamped on the tape around the harness. That harness came all the way from the solenoid, entered the cab on the driver's side, and was fairly easy to trace. It also had one wire simply cut and hanging loose (one side in the engine bay and one side in the cab). The side in the engine bay was hot and was open and loose. The brake controller also had a hot lead coming into it that was still live. The hot lead to the brake controller terminated at the brake controller with no splices feeding anything else. I traced out all the brake controller wires, took a photograph, labeled them and disconnected the old brake controller to get it out of the way.

The majority of the mess of wires that was hanging under the steering column appears to be related to the cruise control and connects to a yellow "Dana Corp" box about 4"x4" with adjustment screws for "low speed" etc. I had tucked this stuff back up under the dash prior to the drive when everything went dead, so something in this mess could have been the cause of the short but I cannot find any burnt connections of blown fuses amongst that equipment.

The fuse box has absolutely no power. So, I'm trying to figure out what wire should be providing power to it. I pulled the fuse box off the fire wall so I can see all the wires going into it. I also pulled the gauge cluster out and the radio out so I could see more of what's going on behind the dash. I'm looking for a wiring diagram and next time out to the truck I need to start unwrapping the harnesses near the solenoid to see if I can figure out which wire is supposed to be feeding the dead fuse box.

 

Here are a couple of diagrams from autozone, but these early diagrams are missing a lot of features and have errors in them, so keep that in mind. You might be able to go to your local library and find some manuals with diagrams, or they might have a online service that is free you can access.

AutoZone.com Repair Guide Image


AutoZone.com Repair Guide Image

Look at the second diagram. If you can believe it, they say some of the main power wires are black/yellow and black/red. You can see them coming from the starter solenoid.

This battery isolator could be a real problem, since it's spliced into the alternator wiring and the alternator wiring is part of that main power wiring. Isolators are a good thing, but some people's wiring practices are not.

I am not exactly sure how your particular year is wired, but if it's like most all other years, the headlight power is fed from over at the pass side fender area. If that is true on your truck, it would seem the problem is still over there somewhere.

There should be a main power wire feeding the ignition switch, and also one feeding the fuse box. On the later trucks they had one main wire, and under the dash they had a splice, where it split and fed the ignition switch and the hot all the time part of the fuse box. Then there is another main wire coming from the ignition switch to the fuse box, and this feeds the "hot with key in run" section of the fuse box.

 

Found a burned fusible link near the solenoid that I had missed on the first series of checks. Replaced that and the lights came back on and everything started up.

While tracing out the wires near the solenoid, I found some ugly looking splices involving the disconnected battery isolator that will have to be removed. This whole experience has been a good opportunity to clean up old crap from previous owners, so when I get it all back together tomorrow, it should be good to go for a while.

 

I have just joined because of this thread. I too am having the same problems. My problems may be caused by my doing and using a friends ban information.

I have a 1976 F150 with 390 engine and my starter relay was chattering when I tried to start my truck I was told to turn on the ignition and jump across the far left (+) side of the relay to the far right (-) of the relay. If it started, then the problem was the relay.

I did that and my truck started. While it was running I went to the store. When I got back to my truck and tried to restart it, same chattering and then same jump effort across the relay. I was in front of the truck leaned over the battery. When I tried to jump across the relay once again, I saw out of the corner of my eye, towards the alternator, a flash and then a puff of smoke.

Now I have now power to any of my gauges, my headlights, or my after market fuel pump which is wired to the ignition on side of the ignition switch.

I have a new fully charged battery and new relay and new ignition switch. Tonight, I checked continuity of three different fusible links on the right side of the engine bay near the relay with a volts/amp tester. All are good. I have power from the positive side of the relay to the small "s" terminal of the relay. Tomorrow I will pull my alternator and take it to Auto Zone to have them bench test it.

Is there anything else I can try?

 

You technically do not need the alternator for the truck to start and run. It only charges the battery after it's running. So if you get it squared away, it still may not start.

Since you saw a puff of smoke over there, I would carefully inspect the wiring. I believe on your year truck there is a splice in the wiring were the large alternator wire, the main wire that feeds the truck, and a couple of others are welded together from the factory. This splice is in under the tape of the factory harness, and has been known to get corroded and go bad. Since you saw smoke, I would look the harness over carefully for any melted spots. If it's this main splice, that could kill the whole truck.

 

I was getting ready to go out and work on the problem. I will do as you suggested with the wiring first and post back here my findings.

Last night before it got dark, I pulled extra, no longer needed after market accessory wiring off of that side of the engine compartment and noticed a heavier gauge wire with a larger ring type connector laying loose near the new relay that I just installed. It is a fusible link.

Right after I replaced the relay was when I got the spark and smoke. Now I wounder if I accidentally left this wire off when I reconnected the wires to the relay.

I printed the Auto Zone diagrams that were earlier in this thread. While I am checking the wires like you suggested, I'll see if I can determine where the other end of the loose fusible link wire goes.

Thank you for your help,
Mike B

 

I know the later trucks, and I believe the late 70's trucks, used a large yellow wire to feed the main power to the ignition switch and the fuse box. It may be a different color right at the large ring and the fusible link, but it should eventually turn to yellow if I am right.

 

It turns out that the loose large gauge wire with the ring connector should have been on the positive side of the relay along with the positive that came from the positive side of the battery. It terminates at the BAT terminal of the alternator. Along its length is a splice of a yellow wire, as you stated, that supplies the power to the rest of the truck. I guess I should have taken a really close up picture of the relay and all the hooked up wires before before I removed them.

I did not find any burnt wires and since I had to remove the alternator to have room to get to the wires on the back of it, I am going to have it bench tested at Auto Zone.

Your replies to me and to the previous person of this thread have been a very great help. I really dont like to pay a shop to fix my vehicle if I can do it myself.

Thank you,
Mike B

 

Hooking up the loose wire to the positive side of the starter relay helped. Yesterday I took my alternator to Advance Auto Parts and it failed the bench test. I had purchased it back a few years and it had a lifetime warranty. They replaced it, no questions ask. I put everything back together today and it starts great.

I am really glad I found this forum.

Thank you for your help, Mike B

 

I have this exact problem. All was fine until I replaced the alt. The harness on the back of alt had a bad field wire so I replaced that also. Now there is no power to anything. Just doesn't make sense. All F links look good. Replaced ign switch still nothing.

 

Do you have a meter or testlight? Get one or the other, and check for voltage at the back of the alternator on the large bat terminal. If it's dead, you have probably blown one of the fusible links. That happens when you change the alternator without disconnecting the battery.