bad altinator?

today I noticed that the volt gauge on my truck was fluctuating alot. When there was little to no engine load the guage read where it usually does, but if I worked the engine the gauge would show the volts drop off way more than it used too(still in normal range though) and if I let off the throttle it would shoot back up. Is my altinator going bad or is it something else? The batteries are 8 months old and all the connections are clean and tight.

 

update with info I left out: alternator is a Delco 35si, the belt only has 50k on it and still looks like new with no evidence of slipping. The pulley on the alternator looks good also.


Is there a way to check the belt tensioner to see if it's still good? The belt seemed pretty tight.

I'm still leaning towards the alternator itself, at idle or cruising with little to no load on the engine the volt guage is only showing 13.5 volts, under load it will drop to around 13-13.25 volts. One time it did drop under 13volts for an instant, but then shot back up. I know it doesn't mean anything, but one of our other trucks has auto tensioners we've never had one go bad in the 5yrs we've had it.

 

I checked all the connections and they are tight. I then went ahead and unhooked everything to check how clean they were and everything looked good. I took everything apart and cleaned all the connections in Nov. when I replaced the starter and firewall relay.

 

can a guy replace just the voltage regulator or do you have to replace the whole alternator? I have a spare alternator so I might just hook it up and see if there is a difference.