Voltage Drop

[michael.francis.5876]

I have a 1976 Chevy K10 Custom Deluxe with a 400 in it. Whenever the truck is at idle everything runs fine but as soon as I put it into gear the voltage drops instantly. I changed the alternator and it didn't do anything. I also checked all of the black ground wires and cleaned up the ones I found but it still does it. Anyone have any ideas on what to try next?

[Captkaos]

What do you mean the voltage drops?  What does it drop to? And from what?

Chris Lucas
73-87chevytrucks.com
squarebody.biz

[rich weyand]

Which alternator is it?  (current rating)

When this happens, what accessories are running?  Is it only with the lights on, etc?

[michael.francis.5876]

I don't have the actual voltage but I look at my gauge. It runs around 14 like it should and then drops below 13 depending on what I have running. If I have lights, wipers, heat on high, and wipers going it drops almost to the red danger zone which I think is around 8. It doesn't normally sputter like its going to die but I wouldn't think that it should drop that far. Alternator is a 60ish amp which should be more than plenty. The thing is that when it is park I have all of the same stuff running but for some reason when I put it in gear it it drops and the lights dim and the blower slows, and the wipers slow. If I don't have the lights on it isn't as bad. Continued slow driver at basically idle for long periods of time like I did the other day in the blizzard it sputtered and almost killed the engine a few times. I had to gear down to low and keep gas pedal down.

[michael.francis.5876]

If no accessories are on it does not appear to drop at all. Just when I start using heater, lights, wipers, etc...

[rich weyand]

You either 1) blew a diode in the regulator, and got a bad one for a replacement; 2) have your idle speed set too low -- should be 700 in PARK; or 3) need a bigger alternator.  You could also have the wrong pulley on the alternator because some PO changed it.

In any case, the alternator is losing lock on the regulator voltage when you shift into gear and the idle speed drops due to the load from the transmission.  Any 10SI or 12SI alternator will have the same form factor, fit in the same bracketry, and wire up exactly the same as the stock alternator in your 76.  You can get 94-amp 12SI alternators over the counter at any auto parts store.

[michael.francis.5876]

My alternator is hooked up on the left side of the engine with a short belt hooked of the center pulley. I think the idle is actually high but I will have it tested. You really think I would need a bigger alternator with have everything stock? How would I tell if I have the wrong pulley?

[rich weyand]

Left side/right side is always specified from the driver's perspective facing forward.  Is it on the passenger side or the driver's side?

The alternator belt SHOULD run around the alternator pulley, the water pump pulley, AND the crankshaft pulley.  That could be your problem.

[michael.francis.5876]

I had to double check. It is on the passenger side of the vehicle and is hooked up to the water pump pulley and the crankshaft pulley. I have a 2.8" diameter alternator pulley and the other 2 are 6" pulleys. That gives me a ration of 2:1 (6/2.8=2.1). Do I need to run a 2" pulley on the alternator to get it to a 3:1 ratio?

[rich weyand]

The pulleys on my 78 are 6", 6" and 2.5".  Sounds like the wrong pulley on the alternator.

[Irish_Alley]

2.8 are you including the lip of the pulley?

[michael.francis.5876]

Yes I measured lip to lip but also that is what size of pulley it says it is which from my research is stock. Looking at the alternator it puts out 32 amps at 2000 rpm and 64 at 4000 rpm. Wouldn't I need my alternator to spin at 2000 rpms at idle? That would mean assuming 700 rpm engine idle I would need to actually push my idle closer to 750 and go with a 2.25 pulley to put me at 2002 rpm on alternator (6/2.25=2.67×750=2002). Am I thinking about this correctly?

[Irish_Alley]

now rich how did you measure yours? i dont see .3 of a inch doing anything even if theyre measure the same way. ill talk to my alternator guy in the morning if i can remember. but he told me what rpms they energize before i cant remember that, i know mine is set up now to work at 200 rpms just encase it gets taken off the truck and used on some farm equipment

[rich weyand]

I measured across the lips.  Basically, the outer surface of the belt.

Simplest cure for this whole thing, I think, is get a higher-capacity alternator.  A 94-amp 12SI is only $70 at OReillys with a core.  Ask for the alternator for a 1984 Chevy Camaro with the 305 G-code engine.  That's what I got, so it probably comes with the smaller pulley as well.  I have no problems at 700 rpm (550 in gear), and I have high-current headlights, fog lamps, a 600-watt subwoofer, and electric radiator fan!

Mounts on the same bracket, same physical size, no wiring changes.

[michael.francis.5876]

The local napa parts store was able to source one made for the truck that's a 94 amp. The camaro one was gonna be 2 to 3 weeks. I will pick it up tomorrow. I really hope this cures the problem. Picked up a new belt and will test the battery for peace of mind.