So about two weeks ago the alternator in my '89 went out. I ordered a reman Motorcraft from RockAuto and installed it. Had the battery checked (which is only two years old) and it checked out fine. About 4 miles down the road the replacement alternator quit. *#^%*!! I had RockAuto send a replacement and I went ahead and replaced the battery cables and the ground straps since they were all looking a bit questionable. Before I go firing this thing up again, are there any other possible alternator killers on these trucks I should be aware of? I had read these trucks were known for their grounding issues. Hence my replacing the cables. I even put two ground wires on the negative side. One to the block and one to the frame. I just don't want to fry another alternator if I've over looked something.

 

corrosion on the bracket and then if you have a 3g upgrade you should run a ground from back of case to the battery. if you dont have 3g, oh then make sure the connector is making solid connection to the different pins

 

What is a 3g upgrade?

 

You need to install a new pigtail of sorts. The wire connector that hooks to the alternator. They recommend it be soldered on where the new wires on the pigtail that hook to your old wires. Hopefully some of the parts places still handle this pigtail.

Did your first replacement alternator burn up at the wire connection on the alternator?

 

kinda looks like one of these(note the 2 connector things on the back side of it)

 

The Light Green/Red wire provides exciter voltage to turn on the alternator. More than a few times that circuit gets intermittent which makes the alternator appear to be bad. That voltage comes from the ignition switch that is hot in the Run position. From the switch the circuit continues through the charge lamp in the instrument panel then on to the alternator. The charge lamp has a shunt resistor around it in case the lamp filament opens but many times the connection has gone bad over the years.

Verify there is power on that Light Green/Red wire when the key is in the Run position. It goes through a few connectors so be sure to check those as well.

 

Ah, ok. The new alternators had a permanent harness that had to be spliced in but not for the voltage regulator. The original alt didn't actually burn up perse, but it did stop charging.

I just got everything back in and ran the truck for about 5 minutes. At initial startup the system was reading 14.45 volts. Battery was 12.7 before starting. I turned the headlights on and ran the ac on high and the output dropped to 13.3/13.4 volts and went back up to 14.1 after turning it all off. The oil pressure gauge and battery gauge jump when I turn the blower motor on for some strange reason. I thought new ground straps would have resolved that but apparently not.

I just read about the 3g upgrade. I may go that route if the second replacement craps out. It'll give me an excuse to add more lights on the front...hehehe

 

Maybe the company that rebuilds these alternators was aware of the problem of the alternators burning up by where the main wires plug in. You can probably disregard what I said in my previous post.

 

its by design