Voltage question

[Shimy87]

I put a new alternator on car a while back and all has been fine. Volt gauge shows around 14 volts. Took car out today and cruising voltage is 11.7 stop car and idle and it goes back to 14.4 if I hold rpm in park at 2500 it drops to high 12's. Drop back to idle and back to 14.5 drive again and drops to high 11's belt is not slipping....an y direction on what to check?? Thanks

[Steve Wood]

If the small brown exciter wire is showing normal voltage when the key is on, engine running, then I am going to say it has a bad diode pack or a loose connection in it.

Some times, you get one that works like a one wire alternator and will start charging just off idle, but normally they will charge fine once over about a 1000 rpm.  Sounds like this one has a problem.  

[Shimy87]

Thanks Steve, isn't the 140 amp one from like a 94 impala a good choice?

[Steve Wood]

The best, imo. I don't like rebuilt ones unless it comes from a local rebilder. Parts houses rebuilds are spotty.

[Shimy87]

I got one ordered. Just so I know, the brown wire should show 14 volts when car is running? If it isn't whats the usual problem. If it is and volt gauge is below 12 whats that tell me?

[Steve Wood]

No, the voltage feed for the brown wire comes thru the volt light on the dash which serves to add some resistance to the circuit which lowers the voltage a little.  Feeding the alternator with fully battery voltage will burn the diode eventually, if not sooner.

If the volt light comes on when you turn the key on, then that circuit should be working ok as long as it does not come on after the engine has been started.  If it does not come on when the key is turned to ON, then it will not charge when the engine is started.

Upon rare occasion, GM put a resistor across the volt light in the dash so it would charge even if the bulb burns out.  Our cars often have a problem when the volt light bulb does not make good contact to the instrument panel (printed circuit), the bulb does not come on and the battery does not get charged.

When installing a GM alternator on old cars like jeeps, etc., I have run a wire from an ignition hot wire thru a dash light type socket/bulb and on to the alternator and it works just like the light in the panel.  And, I have also just installed a resistor in the circuit (think I used about 400 ohms) instead of putting a bulb in it.  This is basically what Casper's sells for their alternator fix.

Not all alternators require a voltage drop to safely excite them but the CS series does.

I had a Limited once with a digital dash that would charge about 50% of the time.  I cured it with a feed to the brown wire going thru a resistor back to ignition hot.  there had to be a poor connection somewhere in the circuit as it would come on and go off and you could hear the engine load up a little when it started to charge.