Alternator problems at idle on L78 Camaro

[Jeff H]

I just noticed that all the lights are very dim at idle but when I rev it another 2-300 rpm's they brighten up(this is on my 70 L78 Camaro). I checked the alternator and it's an 834 unit with the stamped 2.75" pulley. It seems to be spinning too slow at idle. Does the correct 837 alternator with the machined deep groove pulley spin at a higher speed at idle or could I have other issues? The car has an HEI distributor and I'm not sure what else isn't stock at this point. Can anyone tell me what size od the stock crank pulley and alternator pulley should be for the L78 setup? I want to keep it as close to stock as possible so that may factor into the solution. Any thoughts?

[Hylton]

Change the regulator inside the alternator.

[Mr70]

Make sure your belt tension is really German...Good'n tight.
That used the stock Deep groove Fan pulley #6060 with an outside dia. of 6 inches.
The stock Deep groove Alt.pulley is the #831 with an outside Dia. of 3 & 3/16.
I have these if you need them.

[mrrec]

Those old 37 amp alternators - even in perfect shape - won't keep up with the headlight load below about 700 crankshaft rpm. They never did.....
Dave

[Jeff H]

Yeah I figured as much. But the gauge lights are real dim even without the headlights on so it's not really the headlights that are causing the dimming. It seems to be really low output at idle. I'm thinking the wrong pulleys might have is spinning too slow but the pulley on it now is actually smaller than the stock deep groove pulley so that doesn't make sense.

[Mr70]

If your belt tension is tight,are all your connections clean from corrosion too?..That would put a strain on the whole system.
Mine did the same thing,until John P. rebuilt my Alternator.
I don't know if he beefed up the internals from stock,but it keeps the dash lights bright under 700RPM.

[L78steve]

Did you check battery condition?It's the batterys job to fill in when the alt.can not.It may have the capacity to start the car but not have full voltage.If a battery is not fully charged before it's put into service it will never be 100%.

[Hotrodpaul]

It would be interesting to put a voltmeter on the battery terminals while the system is under load and have a willing participant slowly increase the engine speed from idle. If the Alternator is charging, you should see around 13 volts, if not, it will be less than 12 volts. Might be a good test for the Alternator.

Paul

[Kim_Howie]

Check your grounds in the dash area

[Jeff H]

It looks to be a new battery but rather weak(555 CCA) so I topped it off on the charger for 16 hours and the lights are definitely brighter at idle now. Rev it a little and they get a little brighter but not a lot. It's reading 13.8-14 volts at idle with no lights on and drops to 13.6-13.-8 with the lights on. Then it will jump up to almost 15 volts when you rev it a little. So I don't think the alternator is a problem. I will want to get a correct 837 alternator eventually but this one should work for now. It needs a stronger battery IMO.