Is there an adjustment to correct this? it all stared in the pouring rain at Carlisle on friday the car died.. would not restart, found an anternator and installed but only reads 11.7 at an idle, bump the idle to 700 rpm it goes to 12.5.. still low.. my VOM said 12.5 when at idle and the gauges show 11.7? it does have the underdrive (2) pulley set but did not have this problem before. checked the fusable links and the c-fan fuse all seems normal...made the 190 mile trip back home.. just still suspect..

 

No adjustment that I know of. I have same problem. Gauge is about .3 to .4 volts lower than actual reading at battery with a Fluke DVOM.

FWIW, I am on my third alternator.

 

The dash voltmeter reads the voltage AFTER the ignition switch contacts and there is a normal 0.3 volt drop across the ignition switch contact, so the dash voltmeter will indicate a voltage that is 0.3 volts lower than the voltage across the battery teminals. There is no adjustment for the dash voltmeter. 11.7 volts at 700 rpm idle is too low and is likely to be the fault of the alternator. Normal alternator voltage is 14.3 volts (dash voltmeter) engine cold and this drops to 13.3 volts engine hot. Watch for the red battery symbol light on the dash, it lights up when there is no alternator ouput current.

 

Walter, if you are using rebuilds, turn the alternator over, and look where the half moon plastic cover with the holes is.
It is held on by some expand-it tabs. pry it off, and look at the three stator wires as they come out of the windings.
If they are soldered, you are fine. If they are crimped on with some metal tubes that are crushed,you are in for another rebuild sooner or later.

I found that the rebuild shops clip the leads off, and to compensate for the short wires, they add metal extensions and crimp them on. Over time, these areas build up resistance, and give a low alternator output, both volts and amps.

In the summer time, the voltage regulators in the alternators are temp compensated to keep from overcharging your battery in hot weather.
Unfortunately, the material the corvette hood is made of holds in the heat. The extraheat allows the voltage regulator to deduct more volts, and the added heat also causes the windings at the connection joints I mentioned to expand and contract. These movements tend to loosen up those connections to cause the problem.

Removing the alternator just prior to summer, and re-soldering these 3 places tends to allow the new solder to withstand the summer heat for another year.

Also, the plastic insulator on the positive lug tied directly to the battery tends to soften with extreme heat, and loosen the nut inside the alternator, and cause a high resistance and extreme heat on that nut tied to the voltage regulator. This arcs the stud and creates a carbon tracking that adds more resistance.

The answer to this is to remove the alternator once a year, and ....you guessed it,.....tighten that nut.

This gives you an opportunity to check bearings and the solder joints, observe brush wear, and see the arcing first hand.
Checker has a rebuild kit for $47 if things don't look right.

In addition to that, Summit sells a rear alternator housing that has cooling fins cast right into it.
I bought one of those and installed it, and it seems to make the diode trio last longer.

In addition to that, cheap rebuilders can install smaller diodes that don't carry as much amperage, and can cause the alternator to fail prematurely.

 

Coupeguy:

The first couple alternators were rebuilds. They failed within 18 mo's.
Most recent one is OE Delco and has lasted much longer so far.
I will take your advice and check those connections.
My dash voltage readings are higher than those posted by Bodean.
Your explaination of the disparity between the battery voltage and dash gauge is valid.

Thank you,

 

Thanks for verifying what I saw on the dash read out and my VOM .. now I Guess I need to find out why 2 alternators that check good do not charge my system?