ALTERNATOR

[Dave Blystone]

I THINK I HAVE A ALTERNATOR GOING BAD.  AS I ACCELERATE MY VOLTAGE DROPS TO 12.6.   ON DECELERATION IT GOES BACK UP TO 13 .3 WHICH I THINK IS BATTERY VOLTAGE.   IS THERE A EASY CHECK?
THANKS,
DAVE  
CAT  3126 2001 PATRIOT

[Edward Buker]

David,

It may be that there is a high current demand going to the batteries or elsewhere in the coach. Under the high current demand there is voltage drop in the coach wiring that is giving the 12.6 reading when the alternator is putting out a lot of current and when the engine decelerates to low RPM the alternator current output drops and the resultant voltage drop in the coach wiring harness is also reduced possibly giving you the higher reading.

You could measure the output at the alternator at idle or slightly above idle and at higher RPM and the voltage output should be similar at the alternator in the range of 13.8V to 14.4V or so. If that seems O.K. then add some load turning the headlights on and off and the voltage should stay in that window.

If the alternator was being asked to put out high current beyond its capability due to batteries with a bad cell or a very depleted battery bank was involved you could also get lower output voltage at the alternator until the charge demand levels in the batteries were within the alternators current output capability.

later Ed

[Gerald Farris]

David,
First check the serpentine belt, belt idler pulleys, and belt tensioner because a slipping belt can cause the same problem. If the belt system checks OK, check all of the connections at the alternator. If the connections all check OK, make sure that the batteries are fully charged, then check the output voltage of the alternator with a good volt meter (not your dash gauge). If the output voltage of your alternator is still what you stated in your post, replace your alternator.

Remember to never start your coach and drive off with discharged house batteries because this will burn-up your alternator. However if you have discharged house batteries, start your generator and charge your house batteries with the inverter until they are about 75% recharged (30 minutes to an hour, depending on charge level) before or during your drive.

Gerald