My 1970 F-350 tow truck died and the electrics are dead. I was driving it the other day and when I put the two over head strobe lights and the two big spot lights in back on all at once the truck quit completely and died. It jolted to a stop and when I went to fire it up after turning off the lights it started and ran fine. Today I was in the truck and turned on the lights and they went on for a few seconds and then everything died. Now I have no lights or power anywhere on the truck and nothing happens when you hit the key or anything else. I took the ground cable off the battery and engine and completely cleaned and sanded all connections. I cleaned the positve terminal also and checked as many connections on the truck as possible. Once when I was trying the ignition it cranked but it was in gear so I turned the key off and I now have nothing again. The battery reads 12.34 on the meter and a horn that is direct connected to the battery works fine. There has been no sign of shorts and no smell of burning wires etc. Does anyone have any idea where I should look next?

 

Check for continuity across the fusible links coming off the battery side of the starter solenoid.

 

I'm assuming the starter solenoid is the same as the starting motor relay in the wiring diagrams? And is the fusable link #38 or #152 in the diagram that goes to the voltage regulator or alternator?

 

That's correct, there should be two fusible links.

 

I drove down to take a look this AM and what I found is some wires coming from the alternator that are plug ins that lead to another wire harness. Are these the fusable links? Are they replaceable? I was pressed for time so I when I could not pull them apart easily I left them for later. Could taking these apart and cleaning them do anything? I noticed my voltage regulator is not the standard big box like the old days but more of a thin solid state looking thing. Looks like three wires coming back from the voltage regulator.

 

The fusible links are square pieces of rubber surrounding the wire, they'll sometimes feel squishy or flimsy if they're bad, and yes they are replaceable.

 

No, I've seen nothing like this on my trucks. Would an old 1970 truck have these from the factory? I don't recall ever seeing one. I read a post somewhere that mentioned a ground strap from the back of the engine or manifold to the fire wall. Should I check for this and could it be the problem? The starter solenoid looks new on my truck but is it possible this could be gone and causing the problem?

 

I just went down to work on the truck and it does have the ground strap. I have a edelbrock streetmaster intake and there is a post where the strap mounts and the other end screws on the firewall. I took it off and cleaned and sanded the ends and reinstalled and I also unplugged the voltage regulator and cleaned the plug. I also checked the starter motor and everything is tight there. Still nothing.
A guy in town has a 1971 F-250 and sure enough he has the fusable link you show in your picture. I'm wondering if my problem might be a ignition switch problem or something else under the dash.

 

I'll ask another member (fmc400) to look at this thread, he's an electrical guru, hold tight....

 

Thanks, I really appreciate all the help. These electrical problems can drive you crazy!

 

Here is a complete wiring diagram for your 70 F350. Hope it helps...............

Sorry about the size, but if I reduce it the details won't show very well.

 

If the truck is completely dead (no brake lights, headlights, etc) it's safe to rule out the ignition switch, because there are several items (including those I just listed) that don't go through the ignition switch. This would indicate that the break has occured upstream of the ignition switch.

Unfortunately in situations like these, the only thing you can do is start at the source (the battery in this case), and check for voltage along the path until it drops off. A test light is a handy tool for this kind of go-no-go testing; your multimeter will work just fine too.

As MH suggested, the first place to look in this situation is a fusible link, which are marked with the small piece of rubber as shown in the picture. I would suggest taking one last look to make sure there really isn't a fusible link in the path that's dirty and hard to see, or covered with tape. If your truck really doesn't have fusible links, then in order for the truck to have fused out the heavy-gauge power feed powering the truck, you surely would have smelled burning insulation or seen smoke. That's why I would still be inclined to believe there's a fusible link that has fused out.

If there really are no fusible links, you'll have to start tracing the wiring and seeing where power drops off. Can you describe the major wiring modifications that have been done to the truck? Since it's a tow truck, I imagine the basic factory wiring configurations that we are most familiar with don't necessarily apply in this case.

 

Fabman thanks for the wiring diagram. Fmc as far as I can tell from looking at my friends dead stock '71 f-250 there is not much difference in the wiring set up on my F-350 except for the missing fusable link. I will check tomorrow to see what is in place of the fusable link because I'm sure there was not one there.
I remember years ago starting fords with a screw driver if the solenoid was bad. Should I try this to see if it is the solenoid? Is there a good way to test the solenoid? When I put the meter on the battery side of the solenoid it reads 12.34 like the battery as you would expect. I had my son turn the key to start as I tested the starter side but nothing changed on the meter. Does this indicate a problem with the solenoid?

 

You are correct that carefully jumping the battery-side of the solenoid to the 'S' post is a good way to test the solenoid because it takes the wiring out of the equation. However, this won't work if the solenoid is bad. There is no way to start a vehicle if the solenoid is bad, because the solenoid's job is to connect the starter directly to the battery. However, this trick is useful if the solenoid is good, but the wiring to it has some sort of issue.

However, you've stated that the entire truck doesn't have power. In that case, the solenoid won't click because it's not getting power to the 'S' post from the key in the first place. Good solenoid or bad solenoid, it won't work in that situation. You can verify that the solenoid is not receiving power by removing the wire boot from the 'S' post (the small post), and checking for voltage at the wire when the key is turned to START. If there's no power there, then there's no power for the ignition switch to send there in the first place. That would be inline with all the other symptoms you are describing.

You can certainly test the solenoid as described in the first sentence, but the solenoid won't have anything to do with the problem you have described here. All the solenoid does in that respect is provide a junction point for the battery cable, alternator output wire, and the wire for the truck's main power feed. In this situation, really all you can do is follow the wiring until power stops showing up where it should be. I do not know what exact year fusible links started to be used. It's also possible a previous owner had one collapse, and just replaced it with regular wire. So it's absolutely possible you don't have any fusible links.

 

Just went down to look at the wiring again, there is no fusable link. There is a black wire coming from the alt. voltage reg. harness that would normally be the fusable link but it is just a plain wire. It does have what looks to be a factory eyelet at the end where it connects to the solenoid.
Coming off the battery side of the solenoid is the above mentioned wire, the positive battery cable, a yellow wire the goes into the harness that snakes behind the battery towards the lights and a fairly heavy gauge wire (black with a grey line) that runs up to the firewall and enters the cab just below the the main harness and connects to a smaller harness that goes over the heater box towards the drivers side.
When I laid a screw driver over the positive side of the solenoid to the S post the starter engaged. Man, it felt good to hear something happen after all this time playing around. The question now is do you think it will start or is it likely there is no spark to fire the engine since everything else is dead?