I have what I consider , a low charge @ idle on my 86 A4 Z51 coupe. Above idle it charges from 13.8 to 14.4 depending on battery state. However, at idle it can drop down below 12 volts especially once the battery is charged althought the ALt light does not come on.. I have measured the voltage @ the battery and I know there is a difference of ~.5 volts between there and the dash meter. I have two working alternators, the original and a 140 amp version. both alternators do the same thing , only the larger one is worse. BTW, the car has never failed to start or acted like the battery is weak(battery is < a year old). Could this be a poor ground problem? I have checked the battery terminals. So far , I haven't have any real problems but I this situation tells me that I am about to get left on the side of the road .

 

my 86e does the same thing.
I raised the idle 100 rpm, and it's on the ragged edge. I believe the answer is a pulley change to increase the rpm of the alternator.
Thinking about that, I'm thinking that since everything turns so slow, (1500-1800, with it running at 1100 in traffic) that might be the ticket.
you might lose a half horsepower, but the driveability goes up.
Chris

 

Was your volt meter higher or lower then your dash?
I'd still look at the alternators. Assuming your idle is correct, I'd get your them tested. Most shops will do that for free. If that is okay, and your battery is new, then I would look at the wiring.

 

Most shops that check the alternator on the car want it revved up a little off idle. See if you can measure any voltage drop between the alternator and the battery? (Measure from the battery positive post to the back of the alternator) You should be seeing less than 1 volt drop in the cables/connections, if you see much more it's time to clean or replace cables or lugs? Corrosion can build up inside the crimped on lug.

 

When I bought my '91 I swore something was wrong. Everyone told me to use a volt meter, which I did, and I really needed to stab the termianls just to make a connection. According to the meter I was charging just fine. But I never liked what I saw on the dashboard. It seemed to hover around okay or at about 12 volts - maybe just higher. I was out in the heat and I noticed each time I went to start the car it was getting harder to start. By this time the car's meter was way under and I just made it home. Installed a new alternator and I have to say that I am at 14 volts on the dashboard at all time. AC, radio, lights, doesn't matter - at 14 volts approx. I've heard that low batteries make the alts work too hard and it kills them. I know you have two units, but I think it sounds like you need a new one. You said you thought you would end up on the side of the road - same thoughts I had. For $150 and not much time you could put one in yourself. Good luck

 

My 88 was doing the same thing 3 months ago. The battery checked out and every so often the alternator light would come on. I installed a new alternator. They tested it under load and said it was good but the issue remained......a 2nd "new" alternator and no problems since.

Moral? New ain't necessarily good.

read post number 4

http://forums.corvetteforum.com/show....php?t=1614075

 

Depending on load, battery voltage at idle is normal. The last member who hooked up a Clampon got about 30 amps at 600 rpms and higher rated outputs rarely produce any better until you reach 1000 to 1200 rpms. With the a/c on, it's probably drawing 20 amps and once your add the cooling fan, radio and what have you and it's essentially all tuckered out. GM issued a Bulletin about this issue Years ago - telling it's Dealers that they weren't going to reimburse them for any more alternator replacements if the only complaint was low voltage at idle (or that the lights were dimming at stop lights!). As to your dash readout, signal is through the ignition switch and cleaning up the switch contacts sometimes regains some of the difference (I picked up about .4 volts with a .6 drop). You can try a smaller pulley (about 5mm less is the best I've found), explore the aftermarket - www.alternatorparts.com advertises a couple that produce more amps at low rpms, reprogram for a higher idle - perhaps 800 rpms, or just quit worrying about it and understand it's simply the way it was built. Most cars don't have voltimeters - those with lighter electrical loads generally fare better, but hook up a scanner to any GM (Ford or Toyota for that matter) and you usually find that they're all around battery voltage when loaded and barely cranking.

 

Volt meter read higher than dash. thanks

 

A bad battery WILL kill an alternator, slowly but surely. It was explained to me that the alternator is only supposed to maintain the battery. It is not designed to charge a weak or dead battery. Doing so causes the alternator to heat up, resulting in all sorts of nasty things inside. Mine had two wires on the armature burnt through.

If you want to keep your car totally stock or "concours", I suggest having the alternator rebuilt. I had mine done for $102. The guy cleaned and brushed the outer case and replaced the original stickers with new ones. It LOOKS brand new, but it's original.

What a difference a working alternator makes. The car seems to run better.

 

I drove the car last night and paid close attention to what occurred. At start up, the dash read 14.4 volts. When I turned on the lights there was no change. I then drove about 8 miles at varying speeds up to 65 mph . During that time I stopped @ two lights and the volts dropped to 13.7 then back up to 14.1. I was waiting for someone and idled for about 10 min with the lights out volts stayed around 13.5 to 13.7. The drove about 10 miles in stop and go traffic. Once the car was fully warmed up or the battery was recharged , the volts dropped @ idle eventually down to 12 .0.
This is what is driving me nuts. The battery always starts the car briskly. What I can't figure out is why it charges normal volts after start up and then once the battery is recharged , the volts drop at idle. One thing I failed to mention earlier is that this has not always been the case. At one time the problem didn't happen. but it has been going on for 9 months or more. and the car continues to start and run normally except for the low volts @ idle.
The alternator that is in it now did not show this problem when I first rebuilt it (new rectifier and diode bridge). Everything else checked out.
What I am going to do once I get some warmer weather is to go through the entire charging system again. My best guess @ the moment is that there is a poor ground somewhere .
Thanks for all the input as all suggestions are welcome and will be followed up.

 

Could it be that once the alternator gets hot, it produces less? Try cooling it down after running and see what you get? Ice packs or something?

 

Yes, amps (output) drops as it heats up. Resistance also increases - especially in 20 year old wiring. I seriously doubt there's any ground problem - It's either there or it isn't. I'd suspect that the fan is kicking on. Assuming it's from the factory, it's probably sucking a lot more out of it than when it was built and again, old wiring with higher resistance takes a lot more oomph to get enough electrons flowing. Excessive draw often reveals itself with discolored wiring. Examine the connectors. If any are discolored, you can try soldering terminals (they're only crimped) which might stop some of the voltage drop.