Hi everyone,
I'm having some trouble troubleshooting an issue with my alternator. I have a 140A alternator from Summit Racing (GM10si style) on my 400. Complete rebuild of my truck is roughly 2 years old. I have a heavy duty fuse and 6g wire from the alternator to the battery. My problem is I think my alternator isn't charging to its full capacity (but I could be wrong, I'm not an electrical guru by any stretch of the imagination). As I'm driving down the road I can watch my voltmeter read a steady 14v until i turn something on. When I turn on A/C it will drop to 12v (this may be in part due to the draw from a Mark VIII radiator fan). Then if turn on my headlights (wired through a relay) it will further drop to 10v. If it hit the switch to my off-road lights (also wired up using a relay) my voltmeter reading continues to drop. Seems to me that even with these draws my alternator should be able to handle the extra load just fine. Yesterday I attempted to get more out of my alternator by running a wire from the "F" lead on the alternator to the battery but it didn't make any difference. There was also a lead labeled "R" which from my reading should just connect to a dash light but I couldn't find the correct wire so I didn't do anything with that on.
I've been driving the truck for the past few years and never had a single hiccup when firing the engine up. No evidence that the battery has been drained. The A/C is a recent addition though and I doubt I've ever run the A/C and lights at the same time so far for any extended period. The off-road lights are also a new addition. Last night when testing it I turned the A/C off and just turned on the lights. It went from 14 down to 12 with the headlights (hi or low beams) then when switching on the off-road lights it dropped to 10v and my headlights dimmed significantly.
Well, that's my story. Any help would be greatly appreciated! Thanks all!

 

What gauge and length are your power leads and grounds and how do the terminations look?

The self regulating alternators (gm style) aren't known for holding the same voltage across wide amperage loads... But you sound like you're running some seriously high amp accessories all at once, so it is possible you're exceeding the capacity of the amp. You'll need a multimeter that can measure current draw (clamp style) to know for sure.

I'd start with checking the wiring.

 

Checked into the wiring and found this: Wiring up the GM 10SI Alternator

A note at the bottom about amperage output at low RPM and about underdrive pulleys makes me want to ask, What pulleys are you using?

edit: found this and stuff and theory about the alternator: http://www.pirate4x4.com/tech/billavista/Wiring/Part2/ in the article, he mentions stuff about remote uh, remote voltage sense! And talks about the disadvantage/advantages of a 1-wire setup.

Also, your reference to F and R terminals I think are non-existent and should be labeled 1 and 2 respectfully. You mentions that you wired up R (terminal 2) which is the remote sense which should go to your power distribution (Left Starter Solenoid/Relay Lug which your battery lead should be on) and the F (terminal 1) should be wired up to the ford wire that fed the original alternator. Terminal 1 wire goes through the ALT light in a light cluster (and not sure about the Gauge Cluster wiring as it is significantly different). If this cannot be done, then wire terminal 1 to a switched ignition source.