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I Have A 1986 F250 6.9 Diesel And A Problem. I Have Replaced Both Batteries, Alternator, And About 3 Or 4 Voltage Regulators. Have Checked Wires, Batt Cables, And Connections Between All The Above All Good. When I Replace Voltage Reg Its Charges For A Couple Of Days Then No Charging Again.
I am having the same problem with my 85 f350, I am going to try to get it to the shop tomorrow. as soon as i find my problem, i will foward it to you. maybe it will be of some help.
Use jumper wire to test altenator BATT/FLD wires.If you have the three wire setup for the alternator make shure you have a good ground.For two and three wire setup make shure you have the FLD wire hooked up to the FLD on the alternator.Very easy mistake!
My wires were a mess on the fender under the hood.It took me two days to untangle and fix all the wires.Look for bad puffy wires and fix them,any loose wire fix that to.If you have a fuseable link that has two wires,and is bad you can slpit them up then fusable link them alone.Check the wires/connection on the voltage regulator.Make shure you have a good ground from the voltage regulator to the fender.On most of the wires i fixed had the burnt green on it.I did all that and in the end it was a bad FLD connection on a new alternator.
Look for bad light bulbs to.
Check for resistance between the alternator output wire and the positive battery terminal on the passenger side.
Internal corrosion inside the battery clamp drove me nuts several years ago on my 86.
Outside looked great, clamps were clean and tight, truck start like a new one, drive it 10 miles, shut it off, no start.
Look for any discoloration of the insulation or brittle insulation on the positive battery cable close to the passenger clamp.
Either is a sign of overheating, which point to the clamp having resistance internally.
New cables and it starts like a new one for 3 years now.