96 won't charge batteries, alt putting out 28v

[SCHMITTY]

Just finished a tranny rebuild for a kid who brought the truck over in pieces. Said it was having issues before he started taking it apart.

The grounds are clean, the battery connections are clean and tight.

The gauges and tach work fine.

The alternator bench tested and passed

The batteries read low and I can watch the voltage drop while the truck is running.

The large fuse on the driver side inner fender is not blown.

The tranny works nicely but I'd like to try and give the truck back functioning and this has me scratching my head.

Any ideas ?

[BigPapa]

The alternator should NOT be putting out 28V. Your problem is in one of four places, the alternator, the batteries, the cables and connections (including the big wire from the alternator to the batteries), or the PCM.

The batteries can put out 12V but be bad. They need to be load tested.

You will have to physically check all connections and cables. I have had cables show continuity but not be able to handle a load because only a strand or two was actually making contact.

You can bypass the ECM and "force" the alternator to charge. On the back of the alternator are two small studs, one above the other. Make a jumper wire and apply 12V straight from the battery to the top one. The bottom one is a ground. This will at least tell you if the alternator is actually capable of charging.

This method got me home one night right after my conversion when the alternator wasn't charging because I had a connector not making full contact and the batteries finally ran down. I used alligator clips on mine and keep it in the truck in case I need it again.

If these three check out OK, it's your ECM.


Here's a link to a similar thread that could help:

No charge, but tach works just fine? - Competition Diesel.Com - Bringing The BEST Together

[RDPsmoker]

Seems like the ECM doesn't know battery voltage so it's cranking the alternator up. Find the wire that reads voltage for the chraging system.

[SCHMITTY]

Thanks guys. I'll try that out

[RDPsmoker]

Use a meter and test resistance on all of your charging system. And make use of jumper wires to help bypass wires to diag bad connections/wires

[zstroken]

Originally Posted by RDPsmoker

Seems like the ECM doesn't know battery voltage so it's cranking the alternator up. Find the wire that reads voltage for the chraging system.

. Yes, seems like an open circuit between alternator and battery.

[BigPapa]

Originally Posted by RDPsmoker

Find the wire that reads voltage for the chraging system.

This would be the one going to the top small stud on the back of the alternator. If I'm not mistaken, the regulator on the '96 is in the alternator, not the PCM. The PCM just says charge or don't charge, not how much to charge.

[SCHMITTY]

The batteries passed the load test at napa. The owners kid tried to wire a up a lockup switch before the tranny finally died. There looks like some hokey splices by the pcm.

[jlbayes]

Originally Posted by BigPapa

This would be the one going to the top small stud on the back of the alternator. If I'm not mistaken, the regulator on the '96 is in the alternator, not the PCM. The PCM just says charge or don't charge, not how much to charge.

Pcm sends out a pwm signal for the alt control. Pcm regulates voltage based on input back. True for all 2nd gens.

[SCHMITTY]

This is the one area of the truck I haven't had to fix before. Thanks

[BigPapa]

Originally Posted by jlbayes

Pcm sends out a pwm signal for the alt control. Pcm regulates voltage based on input back. True for all 2nd gens.

Looks like I was mistaken. Thanks for the correction.

Maybe it's the rectifier that's in the alternator, not the regulator.

[Burn Down]

Is it possible they put a 24 volt alternator on it by mistake? 28 volts is perfect for a 24 volt system. I have seen 24 volt alternators put out 35+ volts when the regulators take a dump.

[SCHMITTY]

It was putting out 20 from the large stud on the back of the alternator, 11.8 at the batteries and dropping. I tried jumping positive to the top little post of the 2 on the back of the alternator- no voltage increase at the batteries or the large post on the back of the alternator. Now the alternator isn't putting out any voltage but if I test between the positive on the battery and either of the 2 little posts I get the same voltage as if I were to measure between + and - on the battery. In going to take the alternator in and have it benched tested at a different shop them the owner said he went to same with the batteries.

[SCHMITTY]

Just to be clear about the posts

[RDPsmoker]

So you had 20v at the stud and only 11.3v at the pass battery?

[SCHMITTY]

I did. Then when I ran a jumper from the positive post on the battery to the top little post it popped the alternator 120 amp fuse. But both little posts are showing that they are grounds when I test them.

[SCHMITTY]

I watched a YouTube video explaining the external regulator cheap fix. I'm going to see if I can make the alternator to put out voltage.

[SCHMITTY]

Did the jumper wires correctly. Alternator started charging. I'll probably wire in an external regulator. The batteries and alternator tested good at 2 different stores

[SCHMITTY]

Originally Posted by RDPsmoker

So you had 20v at the stud and only 11.3v at the pass battery?

Ya. I also got some weird readings between the stud and the little studs.

[SCHMITTY]

Now the tranny is in limp mode . Starts off in 3rd if I let it and won't go into overdrive