Hi all,

Recent 1992 E350 5.8L buyer, first time poster here. I'll completely puzzled and almost defeated with an alternator problem. It's a bit long-winded, but here's the scoop....

Got the dreaded Ammeter light on start-up one morning. I removed the alternator and had it tested which confirmed it was bad. Bought a new one and installed a new 3G plug, stator wire, and crimp connectors from the alternator wires to the vehicle's permanent wires.

These fixes seemed to work. However, after 50 miles or so, the alternator stopped working. I had it checked at 2 different O'Reillys and they said it was bad (I'm now suspicious of this diagnosis). So, I returned it.

I had the original Motorcraft alternator rebuilt. It "worked" for 12 miles before no longer charging the battery.

I'm now convinced the alternator is not the problem. Only thing I can think of is a wiring issue that gets aggravated when driving. I've read all 3 plug wires need to be "live" upon engine start. I'm not confident they are. The only wiring I haven't replaced is the vehicle's. Is that the next step? Or is there something else I should consider? Thank you for reading and any advice you can pass on.

 

Have you checked the battery? Was the battery totally discharged when you put in the alternator(s)?

 

Yes, I've used a multimeter throughout this whole process.

Battery is strong. 12.9 VDC. Any time I've driven it or started it on battery charge alone I've used a battery charger to top it off. I've never used the alternator to charge a flat battery.

New Alternator #1 delivered 13.6 VDC while working
Rebuilt alternator #2 delivers 14.2 VDC

 

The only variable I introduced just prior to having these issues was I installed a battery isolator. Nothing I've found online leads me to think the isolator could affect the battery/alternator. BUT, I'm desperate. So could it not just be a coincidence?!?!

 

 

No, it's 12.9 when I take a reading after the battery charger indicates it's no longer charging. Good idea though, I will check it tomorrow morning .

The battery is only 3 months old, so I'm fairly confident it's not the root of the problem.

 

Was the house battery (or batteries) discharged?

 

No, the house battery has never been significantly discharged. It's not under a constant load (fridge, etc.), so it's largely remained full.

Disconnecting the isolator doesn't provide an immediate solution to the issue, by the way. Just trying to connect the dots.

 

Reading in another forum dealing mostly with RV-type converted E-Series battery isolators seem to cause all sorts of problems when wired incorrectly or are defective. Frankly I'd disconnect or remove that device from your charging system and see how that affects failing alternators.

FWIW a "good" alternator should provide 14.3 VDC in order to work adequately. There is a small bit of tolerance to that but for the most part 14.3 is the target output voltage.

 

Interesting to hear the battery isolator COULD be the root of the problem. I will disconnect it and hopefully get the alternator up and running.

I figured some voltage was headed to the house battery, which was resulting in a lower reading at the start battery (but maybe that logic is incorrect).

Thanks to all for responding

 

Try running a separate ground to the battery. Many years ago(before all the electronics) I had a similar problem. A bad engine to battery ground caused the problem. My mech. friend at the time thought that the higher amps coming off the starter circuit (battery) forced the system to ground thru the bad ground. But that the Alt. didn't have enough amps to maintain the circuit. After we got it working I cut the crimp ends open(because the ground cable looked ok) and found nothing but green corrosion under the one crimp

 

If you replace or make new ground connections use something designed to prevent the point of contact corrosion---same sort of stuff used on battery posts.