Long story short...I now have a ghost in the machine. Several times now I have lost all power (juice) to my 94 7.3idi f super duty. Everything-even the flashers and horn have no 12v. This has happened after trying to start it, and even after its running a short time (last time was on the highway). After waiting.... sometimes 15 minutes, sometimes much longer, it seems to return to 'normal' and I can start it up, as if a circuit breaker went and then reset itself. My question is before I start chasing every darn wire with a meter is there a main circuit breaker or a relay somewhere that can take out everything ? I don't see anything in the fuse/wiring diagrams I have found online, haven't seen anything in the forums either. If there is no main breaker, is there anything associated with the starter or the glow plug/starting system that can "soak" the system ? I have always gotten a whole lot of clicking in the starting cycle, like a breaker is tripping, but its never been a problem. Any insight would be appreciated !

 

Ford uses the starter relay for a junction spot to get power to the cab. Make sure the nut and connections are clean and tight. I believe there is a fuseable link in that connection, althow if it is burned out, I have never heard of it reconnecting on it's own. Also look at the main wiring harness connection fron the engine to chassis, as this can over heat and corrode causing hit and miss problems.

 

Honestly tracking that down is going to be a royal pain in the ***...
I had a drain on my system, took me like 6 months of trial and error to get rid of it, and I'm still not sure what it was. Hit up a local (established, the longer they've been around the more experienced usually) battery shop and see if you can re-create the situation. They deal with stuff like that every day and will be able to sniff it out much faster than you can, and if you make your face known and become a regular it'll help down the road with pricing.

 

I have been steered to the ignition switch/actuator, was told a rod from the keyswitch extends into it and it's only about 20 bucks. Thing is, why would it be running fine and then just die, seems to me if this is bad it would just never start.

 

You know what, it could be loose/damaged. I'm not leaning towards it as they can be in a truck from the factory and still work perfectly fine after 25 years of use. 22 years old on mine, I figured it went 18 months ago, got a replacement and as it turned out mine was still fine. Hope it's quick and painless though.

 

Emergency flashers & horn don't go through the ignition switch. A broken actuator wouldn't kill them.
For a system-wide power failure, I start at the batteries & work towards the power distribution box.

 

Batteries and Alt were my first thoughts tbt.

 

So I've ruled out the ignition "switch", both batteries are good (12.73v), the alternator seems fine, there is no voltage drop at the batt's after sitting for awhile, so there doesn't appear to be a short to ground. There is 12V to the big term on the solenoid on the firewall, but no juice to anything. I started poking around underneath by the starter til I got goobered on, would appreciate if anyone could point me to a main schematic/wiring diagram before I go back under, 94 F Super Duty 7.3 IDI.

It acts like a resettable circuit breaker or maybe a skinned wire going to ground from vibration once running, then she dies. I'm thinking between the starter and the main power box on the fender (as suggested) the fusible link I will check out if someone can tell me what it looks like and where it is ? I hate going underneath any more, too darn old, but I'll do what I gotta do to get her fixed, usually runs like a clock, been rock solid reliable. Thanks for the assist !

 

I dealt with a similar issue. Fusible link was the culprit. It was the one at the starter relay. It runs from the starter relay and splices into a wire that runs to the alternator. Mine had a molded box into the wire which denoted the amperage. It was burned through. I replaced the fusible link with one from Autozone and was up and running.

 

I'm dumb as a box of rocks or blind...cannot find anything that looks like that. There is a heavy wire running from the starter relay to the alternator and an area on the wire that might be a fusible link. I will take a pic & post it to see what youall think before I strip the insulation off it.

 

Start at batts don't just look at the conections,,remove and clean them,, all...the longer post on the starter relay is used a binding post for all main electrical conections..the fuseable link is a rubber looking bump on the wire about 6 inches from post..it may not look burnt..it usually has a amp rateing on the link..resitance formed in a dirty conection, makes heat...heat increaces resitance...so it acts like a intermitant switch...

 

So I found 3 different fusible links at the relay terminal post, 1 was hidden by a factory wire loom, they are all original to the truck and I chose to replace all 3. Here's a photo of them, in case anyone else encounters this problem:

I also found a pretty good diagram of the glow plug system where other links are located:

For those of you looking to source the links, AutoZone does not have the 12 gauge any more, they only have 14 and 16. I found the 12G at NAPA, they sell a 6' length of it, enough to make 6 sets (you need 2 6" long wires to make up each link). Some terminal connectors, butt connectors and voila ! Sadly, I must now go under again, found a leak in the rusty oil pan while tracing out the starter wires for damage. Once that's done, I will fire her up again and let you all know how things go. Appreciate the help on finding the fusible links, gotta say at first thought I was going on a wild goose chase.

 

Thanks..that is helpful...

 

ARGGGGH! I get everything squared away, 3 new links at the relay, batt terms and wiring all cleaned up, the oil pan leak is fixed (The Right Stuff truly is an awesome product) go to fire her up and ZIP-Nothing, Nada, no 12v. I'm p.o.'d. so for grins and giggles I jumped the relay with a pair of pliers-it fired right up ! Shut it off and now seems fine-started right up like there is nothing wrong. On the wiring diagram there is another link by the engine harness, I'm going to look for it, it looks to be for the glow plugs/wait to start light.....would appreciate any other suggestions on what may be wrong or where else to look for fusible links or a breaker/fuse that resets itself.

 

If you know for a fact your ignition is functioning properly and you jumped the starter relay, it would appear as though your relay is suspect. That or something between the ignition and starter relay.