1996 F250 7.3 w/auto transmission.
Batteries went dead. Had them tested. One was bad, the other headed that way. Put in 2 new Interstates from Costco. Engine starts but volts never come back up. I know the alternator stays offline for a while 'til the glow plugs stop drawing power. How can I tell if the alternator is bad or if there's a switch bad that tells the alternator to come back online? Any help appreciated.

 

check voltage output of the alternator with the engine running. if it is the same 12 volts as the battery is, i would pull it and have it tested.

 

Agreed! Does your battery light come on?

 

Instructions

1
Set the voltmeter's electrical switch to DC.

2
With the engine turned off, open the hood of your vehicle.


3
Hold the end of the red lead coming from a voltmeter to the positive post of the vehicle's battery. Hold the black lead coming from the voltmeter to the negative post of the battery. The voltage should be displayed on the voltmeter's screen. Remove the leads from the battery posts.

4
Start the vehicle. Again, hold the leads of the voltmeter against the battery posts. Read the amount of voltage displayed. A properly functioning alternator should show a battery voltage ranging from 13.5 to 14.2 volts.

5
Rev the vehicle's engine to 1,500 rpm. Take a third reading across the battery's terminals. The voltage should increase if the alternator is functioning properly.

6
Replace the alternator if the readings remain at or are below 12 volts.

 

Wait a couple minutes after you start it, glowplugs can really pull your voltage down!

 

I did check with a voltmeter and the voltage never rises like it used to. I thought there was a timer someplace that prevented the alternator from generating while the plugs were on to keep it from burning out. I was wondering how to test if that timer was bad. If there is no such thing then my alternator's a goner. The voltage just stays the same even after the plug warmup time - about 11.4 volts.

 

You should check the connector and wires on top of the alternator, they are easy to check but a bad location right up on top like that. Have the alternator tested when you bring it in for a replacement.

 

Check your terminals and wiring too. I had a bad positive wire, it was oxidized a good 5" up the wire past the terminal, and the terminals were shot. New battery harness fixed the charging issue for me.

Also, check the connection from the alternator to the starter relay solenoid. There is also a writeup on one of the build threads on replacing that wire with a larger gauge wire too(3G upgrade). I am still going to need a new alternator, but if you do, look at upgrading to the mean grean alternator, it's a bit pricey($350 last i checked) but it's well worth it if you have a big draw like sound system, radios, lights etc, or you plan on adding some of those things.

Somebody correct me if I am wrong, but I'm pretty sure GP times are based on the connection from the engine temperature sender to the PCM to the GPR

 

This. The alternator does NOT go "offline" or "online"; it is (or should be) generating at all times that the engine is running. But the load of the glow plugs drops the voltage and makes a reading meaningless when they're running. So as said, just wait 'til they cycle off.

Another remote possibility is that the GPs are not turning off. If the relay or the wire from the relay to the PCM is somehow grounding out, it will be energized whenever the key is in RUN. So if the voltage does not rise after a couple of minutes, you could try running the glow plugs for several seconds, then disconnect the relay before starting, and measure the voltage after starting.

 

Ah, that's what I was wondering about. I'll check it. Thanks