What is the minimum voltage we need to see at start-up. I have a CTS and it is showing 11.8 v and after the truck starts it stays there for roughly half a minute or more then it jumps right to 13.9 v. Is it the glow plugs dragging this down?

 

Healthy batteries will show approx. 12.7 after sitting over nite. When you start the truck voltage will drop into the 11's till the gpm times out and then will jump to anywhere between the hi 12's to the low 14's depending on load and batteries.

 

Thanks 69cj I just rechecked with DMM and 12.7 v before starting and 12.v after start and then back to 13.9 after time out of the gpm.

 

I never knew why that was happening when I first got my truck. I was watching the yard apes boosting all the trucks at the auction where I bought mine, they'd boost it, wait 30 seonds, pull the cables off, and the trucks would quit. I asked one of them why, and he said "the computer runs the alternator, and it won't let it charge till the motor's been running a while".

 

The yard ape was incorrect. There is no connection between the PCM and the alternator, except on dual alternator setups. On dual alternator setups, the PCM prevents the lower alternator from charging until the glow plugs turn off.

 

I always wanted to know why too.
Good info guys. Thx

 

What is the source of this info? I have a Ford bodybuilder's bulletin that says the PCM randomly selects one of the alternators while glow plugs are active. Did Ford reprogram the PCM to prevent lower alternator from charging somewhere along the way?

 

This is from the service manual for a 2004. It's entirely possible that it's different for other years.

I suppose it's possible that Ford changed the way this works with later PCM flash updates also.

 

Thanks for sharing that doc, David. Unless this bulliten was just wrong to begin with, I guess they did change something.

 

That's interesting. It wouldn't be the first time that I've found errors in the service manual.

It is also possible that the '04 and maybe later was different, since that bulletin was published in '03. Someone with dual alternators would have to get out the VOM and do some testing to know for sure.

I'm pretty sure my original post is correct, though. That the single alternator system is not controlled by the PCM.

 

I'll try to check mine some time and see what they do. Where do I want to get voltages from? The lower alt?

 

My year old Die Hard batteries sit at 11.4v before starting. Thanks to this thread I checked them. 13.7v after the gpm shuts off.

Time to head to Sears.

 

That's really a good question. Since the output (B+) of both alternators is connected to the batteries, I would expect the voltage there to be the same, regardless of which alternator is charging. I think you'd have to look at the voltage of the I circuit to see if it's being powered by the PCM. You'd either have to check them both at once (while the glow plugs are being powered, or do multiple starts while checking one, to see what they're doing.

I suppose their might be readable PID's from the PCM that indicate which alternators are turned on, but you'd need something like AutoEnginuity to read them.

 

13.7 after the glow plugs shut off is good. Unless you're having some other problems I don't see why you need to do anything.

 

I'm not too worried about the alternator but 11.4v with the truck off is considered a dead battery. If I'm not mistaken, a batteries resting voltage should be north of 12.5v to be considered good.