marshman

Hello,
1991 F-150 5.8 with a C6.. 4x4.
So, the other day I was leaving a place, went to start the truck and it paused before the starter engaged. But it fired up right after and I didn't think much of it... until 5 minutes later.
5 minutes later I was pulling out on to the main road and the truck stalled - all power to the cab cut out. Still had headlights - but nothing that gets activated with the key would come on.
Waited 3.5 hours and got it towed home. While waiting I kept checking connections and played with the key, it never responded.
Got it home and just decided that since there was no reaction to the key and it still had full juice in the battery with good connections all around - I'd just order the ignition switch. I got a Ford part.
While I was tearing it down for the switch, I had to move the key to let me shift the selector out of the way, and while sitting there for a long period with the key forward, power would pop on and off in the cab.
I actually thought this made more sense for the switch and a bad internal connection.
Anyway, I changed the switch, turned the key and it was all blacked out sill for a 1/2 second, then it all lit up. I cranked it and it started. This is about 3 days after the first stall.
I let it idle for an hour, shut it down and back on, etc.,. no test drive at this point. It never failed though.
Today, I went out and decided to take it to the store - fired right up, ran fine, got about 10 minutes from the house - and BOOM - the same thing. Stalled, all power to cab gone.
I had the wife run me over a multimeter and long story short - it was acting like the key wasn't being turned - no reaction. In that state, power was where it should be and ended where it should end (as if the key was off).
About 30 minutes later, it all lit back up, and it started right up again. I drove it home, and now it has started several times and won't stall. But of course, I don't feel like getting stuck roadside again - and apparently, a short drive is triggering this.
So, my plan is to get it to die out again somehow and start tracing wire by wire from the ignition switch on out until I find where the fault is - and this will take forever....
My question here is - is there any other major component that would cause this that is known? Several searches performed, very little about my specific case found.
Thanks.

Steve83

Your description of what you checked, under what conditions, and what you found is too vague for us to help you.

My only guess is the big round firewall connector below/beside the brake booster. Back out its bolt, and inspect the large terminals for signs of heat.

marshman

What was checked - power at the battery, power at the starter relay, every ground contact within eyesight. Everything acts like the key is off. No power to the starter relay signal wire when key turned to start, nothing past the starter relay of course, everything else coming off the hot side of the relay has power right there. Fuel pump doesn't prime, radio doesn't come on, gauges don't react, dash warning lights don't come on, etc. Conditions thus far have been after a 5-minute drive, roadside in 100 degrees, then again after a 10 minute drive, roadside in 100 degrees, and finally it just now happened in my driveway after a 7-8 minute idle, no drive this time - again, at 100 degrees. I can't stress the heat enough - it sucks. The truck was cold start each time.

This time I was standing at the engine bay with the hood open and watched it die. It starts right up, purrs like a kitten, and then off like a light switch - literally. No gauges, no fuel pump prime, no warning lights, no starter signal to the relay, nothing that works in key acc/on position, no nothing but headlights. Then you wait a bit (between 3 days and 30 minutes thus far) and everything fires back up like nothing is wrong.

I just pulled the harness from the firewall, no visible signs of heat, yellowing, charring, corrosion, etc at the pins male/female side. As a matter of fact it was pristine, looks like I was the first person to open that. Then, of course, a bunch of dirt got in there and stuck to the factory dielectric grease. So I just wiped every pin, added new dielectric grease to every pin, and put back together. - also immediately checked for it to start right after putting it back on (to see if jarring everything might make it start which would then lead me back to this harness connection), but no dice.

Now, I am at home - truck in front of the toolbox, multimeter out, and other than random jumping around and guessing - I don't know what to do next.

Steve83

Get a 12V test light or digital multimeter (DMM), and start probing wires that SHOULD be 12V at all times. Start with the headlight wires (since they work) to make sure the tester works, and that you're connecting it properly. Then check the ignition switch & fuse block. Look up your truck's wiring either in a Haynes manual or this site:
http://www.bbbind.com/free_tsb.html
You don't have to give your real e-mail, but you do have to click WIRING DIAGRAMS when you see that button.


(phone app link)

Once you find a dead wire that should be hot, leave the tester on it, where you can see the tester. Then start GENTLY wiggling the wiring harness all over the truck WHILE WATCHING for a change in the tester (dark to light, or 0V to ~12V). When it changes, you're probably in the vicinity of the problem. Try to keep wiggling to change it BACK to the failure, to make sure it was you that made it change the first time.

You should also put all the truck's details & history in your signature:


(phone app link)

marshman

Figured it out! Slid the battery back to wiggle the fusible links and see what would happen. I found this riggery which I believe was bypassing a fusible link that probably blew sometime in the past. The wire they cut only has a few strands going into the crimp at the base of the "repair" at the top of the repair is that big yellow connector (orange circle) and it's cooked! I had 12v before that connector, nothing after - I ran a jumper real quick around this patch job and everything came on!

I have been repairing this truck for two years straight now from the previous owner - with every repair I have run it back to factory correct with factory parts. This connection looks like a single fusible link that splits off into two (yellow circle) - the piece they left at that junction is simply too small to try and do what they did, but better. It really needs the whole before/after replaced. Without doing a bay harness replacement, I suppose I can rebuild this - now to dig up the right wiring parts...

marshman

I should add - when I got the truck a lot of electrical was frying out - slow starter, etc. It had a chintzy ground cable going from the batt neg directly to a side bolt on the intake manifold. I replaced this with a factory ground cable back to the block, chassis, and the small wire back to the junction on the core support by the battery (which was just cut off before). So, between that and the few strands sending current through this, I am sure that's why it cooked (slowly).

Steve83

Read these captions:


(phone app link)


(phone app link)

marshman

UPDATE
Ok, so.. I first chopped in a 12ga wire with a fuse holder- with a 30a fuse, crimp connectors, made it a few blocks and it blew. I checked the harness along its route and couldn't Identify a fault (visibly). What I then did was switch to 10gauge wire, a fuse holder with 10 gauge wire, a 40a fuse, and all soldered connections - aside from a 10ga eyelet at the relay, which I crimped and soldered. While there, I Dremel wire brushed every straight + contact and ground. All the wires going to the hot side of the relay, battery post, etc - they all got wire brushed and dielectric grease between every contact. The postive cable had a replacement end that was rusty and corroded, so I chopped that out and started with a fresh one and clean contacts. After all connected up, I sprayed corrsion protector on batt posts and relay posts - and off I went for a test drive. Just drove for 90 minutes - no blow on the fuse, the wire is not hot, and all looks good. We'll see soon enough, but appears to be cured.