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'86 F250 7.2L electrical problem

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01-06-2007 11:07:09

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The story so far: I found this truck 60 miles away at a sale. It was getting dark so I ran the engine about 2 minutes and said I'd get a ride back the next day, probably buy it and drive it home. I couldn't find a ride so I called and was told another guy was looking at it. Two days later it was still available, but was now overheating. I drove up, bought it for $350, and had it towed home. I replaced the thermostat and on trying to start it the battery began to die. I found rust and corrosion on a very hot positive terminal of the solenoid relay and the 16ga fusible link to the black/red wire was fried but not open. (When that wire is open everything is dead including the starter.) The solenoid terminal broke off when I put a wrench on it, so I replaced the solenoid, both battery cables and put a 20a ATO fuse (just guessing) in place of the fried 16ga fusible link.

I used starter fluid, the engine ran about one second and died and all power was lost. I found the 20a ATO fuse open. The next day I replaced it with a 25a fuse, left for half an hour and on returning discovered the alternator very warm. I removed the fuse and the alt temp dropped to the outdoor temperature.

I can't find any wiring diagram that matches the truck wiring but I'm sure I got all the solenoid wires in the right place. If so, what could cause the original fusible link to fry, blow the replacement 20a fuse on starting, and the alternator to heat up when everything is off? Thanks in advance for your help.

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01-06-2007 12:11:22

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 Re: '86 F250 7.2L electrical problem in reply to Slowpoke, 01-06-2007 11:07:09  
Is this a 7.3 liter diesel?

1.) A 20 Amp fuse is NOT a suitable replacement for a fusible link. A fusible links is, in effect, a slow-blow fuse, and will tolerate quite an overload for a short period of time, while a common automotive fuse WILL NOT.

2.) The battery drain and warm alternator could be a wiring error, a bad rectifier diode in the alternator, or a shorted electronic voltage regulator that is keeping the alternator field current "on" with the engine shut off. That is a common problem on Fords of that era. Also, if this is a diesel, there MAY be a tie-in of the alternator to the glow plug controller, or glow plug lamp relay, with the possibility of problems in that area.

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01-06-2007 13:39:08

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 Re: Re: '86 F250 7.2L electrical problem in reply to Bob, 01-06-2007 12:11:22  
No, it's a gas engine. I have found that the blade type MaxifuseŽ is a slow blow fuse as is the 'fusible element', also known as a fusible link, but not to be confused with the wire style fusible link. I have not yet found any info regarding time on the ATO/ATC fuses. In any case wire fusible links are about $2.50 each, and the harness wire must be cut back each time to install it using a butt connector. I've only got 2" left before the wire disappears into a molded connector. The voltage regulator is in the alternator and the auto supply said it's replaceable. It's strange that all these things should go wrong at once.

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dave guest

01-06-2007 15:20:14

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 Re: Re: Re: '86 F250 7.2L electrical problem in reply to Slowpoke, 01-06-2007 13:39:08  
Voltage regulator is bad if alt is warm overnight. On 90 and 91 they are not internal. They are mounted on rad support panel. Keep looking.

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01-06-2007 16:15:02

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 Re: Re: Re: Re: '86 F250 7.2L electrical problem in reply to dave guest, 01-06-2007 15:20:14  
VR is supposed to be either on passenger side by coil tower or in alternator. Coil tower doesn't protrude thru water shield and there's no reg to be seen. Neither is it on/near radiator. Must be in alt. It only took a while to heat the alt. like 30-40 minutes. I'll take the alternator out and have it tested.

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01-06-2007 11:43:47

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 Re: '86 F250 7.2L electrical problem in reply to Slowpoke, 01-06-2007 11:07:09  
Just guessing from what you said, but probably a bad diode in the alternator...remove the largest wire from the alternator and just touch it to the terminal you removed it from, if you see a spark, the alt. is bad.. I'm not sure on that year truck if it has an external voltage regulator, if it has, check it too...

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01-06-2007 18:55:19

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 Re: Re: '86 F250 7.2L electrical problem in reply to Billy, 01-06-2007 11:43:47  
The VR is on the back of the Motorola alternator. I removed the alternator from the mounting but left it connected, set it on top of the engine and grounded the case to the negative battery post. There's a small plate on the larger of two connectors and it says "ground here to test", with an arrow pointing to a recessed screw. Of course that's impossible to see while mounted at the bottom of the engine. When I touched the positive battery cable to the + post the alternator would try to turn. So it's either the Voltage reg or a bad diode. I left the battery cable connected and grounded the test point but nothing happened. It's off to the rebuilder.

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