I'm not trying to make things worse but those low voltage starts could have taken the ficm out also.

 

I agree with you there. What I was trying to get at is if the alt is good then you should be able to disconnect the batteries and the truck stay running. Yes the alt is really not made to do this. It's more for just keeping the batteries charged, and the added stress of not having the batteries in while running is bad on the alt. You should never use the alt to charge the batteries.

 

That was next on the list or were you reading my mind????? Must be a Fayettenam thing???????? Geez I hated that place

 

Just make sure your hotel is next to a watering hole or liquor store.

 

The "alternator doesn't start until the glow plugs turn off", is an oft-repeated myth. It's simply not true. It is slightly valid in one instance, that of dual alternators. ONE of the alternators is not turned on until the glow plug cycle is over. However, the other one is. And on a single alternator vehicle, the alternator is turned on when the key is turned on.

 

I read both documents, but I'm telling you what many tech's have told me and what I've read on my voltage meter.....MY alternator does not "produce" until my GPCM kicks out the glow plugs. Never has. I can hear the fuel pump ramp up, see the fuel pressure jump up, and hear the motor change tone when it gets full juice.

The single alt. document is a little more "ambiguous" than the dual alt. document in that it doesn't explain PCM involvement as well if at all???

 

I've noticed it takes about 45 seconds to 60 seconds for the voltage
(read via ODBII w/Scangauge2) to jump from 11.6/11.8 to 13.9/14.1 Volts after the engine starts.

Could the lower voltage be a result of increased load rather than
the alternator isn't producing power? Somewhere, I thought it
was explained the fuel pump is pulling down the voltage @ startup?

A fuel pressure gauge is my next mod.

 

If anything it's the glow plugs pulling the huge load at start up. My fuel pump only has a 20A fuse in the line and the alternator outputs a LOT more than that.

My voltage is very similar, in fact I'm going to go watch it as I start it in about 10 minutes just to see what it does.

 

The single alternator description doesn't explain the PCM involvement because there isn't any.

Personally I believe that the glow plugs draw enough current to draw down the voltage until they shut off. I've seen the same thing others have described, lower voltage until they shut off. But the descriptions and electrical diagrams are pretty clear that the alternator powers up with the key.

The single alternator wiring diagram is below. It basically says the same thing.

 

Here is a bit more information that I looked up that might help. According to the 6.0L Bible, under the Glow Plug section, each Glow Plug should draw 10-12 amps. That's 80-96 amps just for Glow Plugs.

http://dan.prxy.org/Truck/6L_bible_h.../Page_068.html

The stock alternator is 110 amps, as far as I know.

So not even considering the rest of the electrical draw on the truck, the stock alternator is near capacity just running the Glow Plugs.

 

ok so let me add to this.
the pcm does not controll the alternator. its controlled off the internal regulator.
the regulator will mot become active till the gpcm timer is off. do not mix up the gpcm timer for the gpcm light.
so far as amp draw on glowplugs that info is old and no longer part of the testing for a glowplug concern.
yes the glowplugs pull a crap load of juice. with a vat 40 I have seen 300 amp draws cranking a 6.0 over.

NEVER disconnect the batteries on a running engine to see if you have an alternator issue. the batteries act as a supressior to voltage spikes. A very easy way to kill a pcm, or any other module.

 

Several interesting tidbits in that "verse" of the bible too. I'm running a higher amp alternator than stock--it bench tests at 140 output and my voltage sits at 11.6 then jumps to 14 after a "time". I know it's glow plug related just not sure how or what dictates the alternator to "come on" when.

 

I understand that the alternator field is controlled by the voltage regulator built into the alternator.

What I don't understand is what provides the current to the "I" circuit to turn on the voltage regulator. According to the documentation, it's the HEC via the charging indicator light. According to what everyone else seems to say, it's the PCM.

 

ahhhh. ok I got ya..
it is in fact the HEC that controlls it. not the pcm.
the "I" circuit is not turned on till the gpcm timer goes off (up to 2 min depending on eot).

hope this make sence to the lot of ya

 

My lesson for the day. Thank you very much sir!!!!!!!