I just got back in from more troubleshooting. With a voltmeter I verified power at the starter relay where all the positives branch off into the harness but none of the wires at the ignition switch had any voltage in them so it does seem like there's an issue somewhere between the relay and ignition. Unfortunately I'm out of time today so will have to pick it up again next weekend. This commuting thing's starting to get to me. I'll read through your post some more in the comming days and hopefully get it figured out next weekend. Let me know if you think of anything else to look into.

Thanks again

 

I don't want to impugn... but are the accessory loads on the battery side of the relay? There should be a stack of them, the stack maybe being the cause of the lack. Most, if not all, go into fusible links immediately. The out side of the relay of course goes to the starter motor solenoid.

 

Sorry, I'm not sure I understand your question. Voltage tested good where all the positive wires (with fusible links) connect to the starter relay (on passenger inner fender). The positives are on the side farthest from the battery.

 

You have them on the wrong side of the relay. The 'far side' is only energized when the ignition key is in the 'Start' position.
They need to be connected to the '+' post of the battery, which would be the 'close' side. They'll get power, send it through the fusible links, & wires, and have juice at the ignition switch, the computer relay, fuel pump relay, etc etc etc.
tom

add: starter motor solenoid is down on the starter, built into the starter actually. It is part of the field, and pulls a 'shoe' into position by magnet, which also happens to close a rather large switch internal to the starter, which in turn passes a lot of power to the motor windings .... energizing the starter. The relay above, near the battery energizes the solenoid in 'start', when all that stuff happens. The other post of the relay is where all accessories & ignition, etc, get their power. They must be hard wired or the relay cannot function as juice cannot come back to it from the ignition switch. The output of the alternator is also fed into that 'spot' in the system, providing juice to all the consumers, and feeding any excess the battery would accept into the battery... until 'full' at which time the voltage from the alternator is reduced {field windings not fed so much} until just barely above battery, basically, and the alternator is used to run all the 'stuff'. maybe. I think.

 

I haven't touched those wires at the relay other than to clean them yesterday. They did not need to come out when doing the firewall work. Maybe I'm still not understanding something?

Edit: I think I just realized my misunderstanding. The positives are on the battery cable side of the relay which is the side of the relay farthest from the actual battery (hence far side not battery side). To confirm, all those positive wires are direct connected to the battery cable at the relay and thus should have voltage at all times, and they do.

 

The worst thing I can think of is that you ended up using the vehicle harness wires as your 'ground' when you were welding. That might not turn out well.
A second, maybe last, thought is that there is a majorly large connector that plugs into the firewall, or passes through, that got either disconnected, loose, or damaged by the work done.
Have you checked that the fusible links are intact? They are supposed to give out before melting the harness. They would be connected to the starter relay and the other end buried in the harness, but 'right there' to look at.
tom

 

I was wondering how to check those fusible links. Would they be noticeably melted? They weren't. I'm hoping welding didn't melt any wires. My welding negative was always grounded to the firewall itself, adjacent to where I was doing my work, and the harness was pushed as far forward as possible into the engine compartment. Fortunately there wasn't much harness left on the truck while I was welding so it wouldn't be that much to check.

Another possiblity, maybe, is some brake fluid spilled out of the master onto the open harness connectors that remained on the driver side of the engine compartment. It happened while I was reinstalling the brake pedal. Come to think of it I just added some dielectric grease and plugged them back in. I'll try cleaning those better this weekend.

This back and forth is really helping me think through it. Thanks.

 

Fusible links almost always bubble the insulation when they cook. You can check voltage drop from end to end, or resistance for continuity. All the fuses have two spots that are recessed where you can probe w/o removing them. Measure with meter set to handle 15 volts. If there is a voltage difference from side to side in a fuse, the fuse is blown or defective. You can check them all in less than a minute.
You should measure 12.x volts from any good fuse to ground with the key on. The yellow wire feeds a fused circuit for the warning lights when the key is on. Hmmm, yellow wire is 'key' ... actually, keyed to feed a lot of stuff from the ignition switch.
All that stuff on the drivers side inner fender is either ignition, evap control, alternator, front lamps, etc. If you don't have power to the ignition switch, you won't have power to most of the accessories, such as tank select, low fuel warning, lamp out warning, blower, turn stop hazard, power windows, radio, wipers, clock, speed control, dash lights, horn cigar, etc. The yellow continues on with a splice to the fuse panel to feed turn,stop and hazard lamps & speed control. There is one other feed to the fuse panel, BK/O for horn & cigar lighter. The headlights are on the BK/O feed also.
I'd check headlight function. If you don't have that, there is a problem with the harness as that is a direct link back to the fusible link. Ditto for the yellow to the ignition switch. They would be called 'home runs' in home telephone wiring, straight to the source, no detours.

 

Power is back. I was out in the garage for about 5 minutes probing the leads off the starter relay when all of a sudden everything just came on. I'd be lying if I said it wasn't too easy. The next few minutes were spent shaking every wire and connector around where I'd been testing in an attempt to get the problem to manifest itself again. Anyway, All systems are go at this point and I even started the truck up and let it idle for 5 minutes. The wife and I will bleed the brakes tonight then if everything's still good I'll put the dash back together. Let me know if anything comes to mind that might have caused the voltage loss. It was just too easy...

 

I can only say that I had a problem with the "stack" of leads on the battery side of the relay. There are so many that it was hard to get them tight and working, and once I did, I did not want to touch it again.
You might, being a Michigander, and having a vehicle that was bathed NaCL given the amount of corrosion, want to consider removing and cleaning the stack as it might have gotten a coating of corrosion that jiggled loose. I can find no mention of whether the battery was disconnected, but you were welding and could have induced current flow somewhere. I realize you mentioned where you grounded, etc, but who is to say that the ground carried absolutely every electron that flowed?

 

Yes the battery was out while welding. That stack of positives was removed and cleaned well last weekend after the problem occured, although they were not very corroded to begin with. It may just be one of those things that remains a mystery. Still, I think after it's plated and insured I'll keep the Bronco around home for a few days so if the problem happens again it won't be 150 miles from my garage. Big thanks for all the advice Tom, really! I'm a little dissapointed I didn't get to dig into the electrical system a little more. That kind of thing is always a good learning experience.

While we were bleeding the brakes I found a bad wheel cylinder so I'll replace both this afternoon... and that's it. Finished! Pictures to follow

 

All back together like nothing ever happened. I'll say it was considerably more work than I was expecting. Still, it's nice to know there's no more rust.







For now the Bronco's tucked back under its cover inside the garage until our Michigan weather improves.

 

It has lost electrical power again so I've got some more digging to do this coming weekend.

I finally got it plated and insured this past weekend and drove it about 80 miles around home with no problems. Then 6 miles into the long drive toward Grand Rapids it lost all electrical power. After a few minutes on the side of the expressway electrical suddenly came back and I was able to drive a mile before it cut out again. I had the Bronco II towed home, pushed it into the garage, and drove the Ranger to Grand Rapids with my tail between my legs.

Next weekend I'll do some investigating (especially the checks you recommended Tom) and hopefully get it figured out. Something I did notice while parked along the expressway: there did seem to be some electrical power, albeit very weak. When I turned the key the gauges would bounce a little (nothing else would happen) and pushing the rear defrost button its light would come on. But there was no current significant enough to work the locks, windows, starter, or anything else.

Wish me luck. Hopefully I can get this figured out.

 

The only thing I can do is relate a story. Got up at 6AM to travel 20 miles for an early appointment. It was dark and raining. Everything was fine for 10-15 miles, then the radio went silent. A few minutes later, the drivers side wiper quit wiping. Traffic with headlights on and shining through the drops was coming on strong. I thought I could still get to my destination, but then the engine started to lose power and run badly.
I barely made it climbing a slight uphill bit, and coasted into a gas station. Nothing worked. Called home and asked for a few tools and a ride home. Finally stopped raining and got light enough to look under the hood. Fiddled with this and that looking for something to blame. Last thing I did was try to twist the battery cable clamps on the battery posts.
The ground clamp was loose enough I could twist it. I tried the truck and it started. I drove home, cleaned and tightened the clamps. The wiper was another story, plastic pivot falling off the link.

Your loss of power seems to be for everything that demands more than a smidge of amperage. That to me screams of a loose connection. Well, shouts anyway.
I don't remember if it was discussed, but there should be an {likely uninsulated} braided cable that connects the engine block to the firewall, or to the frame rail. The inner fenders are plastic, so it would not work there. There are two large cables from the battery to the block, via the starter relay when operative, so the block should be well grounded. If the block cable is corroded underneath, and it's had some time sitting without vibration and motion to 'keep it clean and connected barely', and time without the elements adding humidity, etc. In other words, it sat still, didn't get moved, didn't get juiced - loaded carrying current - and just got more corrosion.
What is common are a few things. Battery terminals & cables, and the other end, the body / engine ground and + connections on the starter relay. Not too many more common spots. So, when ALL goes, you look to the commonality. IMO.
I'd be under the hood looking for grounds, etc. There just aren't the problem areas you can get with a newer vehicle. My sisters Honda CRV was eating alternators, or at least the light was coming on and the batteries were down. Replaced with re-man, again with another re-man, then with a brand new from Honda. Still had the problem of the light coming on after 10-30 minutes drive. She finally called me, and asked my thoughts.
I said check the volts at rest and with the engine running and when the light came on.
I looked at a diagram, and there is a separate module to regulate charging the battery that was magic to me. They finally traced into the loom and found a bad solder joint.
Point was, there was no longer a simple alternator & regulator, new bits were added, which complexifies the situation. Yours is simple, and you'll find it and wonder why you didn't see it from the beginning.
tom

add: I would check the grounds where you re-attached them to the firewall if you did, and make sure that there are 'star' washers under the terminal that would penetrate the nice paint coat you put on to insulate the sheet metal. If you depend on just the threads connecting, you limit the 'area' for transfer significantly. Is it possible you tried to prevent corrosion by coating too well and denying connection to the sheet metal??

 

Out in the garage now.

Ran wire with voltmeter from battery negative to body just next to where braided wire leaves body toward engine: minimal resistance so ground appears to be working.

Power door locks work well. Power windows aren't responding at all. Rear defrost button light works when pushed. Gauges flutter when key is turned. As far as I have found nothing else is working.

Really just thinking out loud, but if it means anything to you let me know. Will be out here a bit yet then again tomorrow early afternoon.