charging system

[up4x4]

I have a 95 gmc 1500 w/5.7, whenever the truck is running voltage is good @14. but then when I stop at a light with headlights or heater blower on voltage is dropping down to 10-12, and lights dim, blower slows. This is all occuring with new battery and new alternator. Even had the system checked at a local shop and they said charging system checks good but also noted the same voltage drop. They told me to check the ground strap from engine to frame which i did. Even removed it and cleaned the connections and still no change. Any advice? Idle speed in gear is @ 625-650. w/ bad voltage 10-12, in park 725-750 w/ good voltage @14

[Giant]

Sorry, I would have said bad ground too. Maybe a new ground cable would help. I had the exact same thing happen to my 94 caddy. would drop so much voltage (even while going down the road) that the cluster would die and I've even had the car stop on me. My caddy would idle fine, but once you put it in gear you could see the lights dim and hear the engine drop. A couple times when I did that it would drop power all together and stop.

[HyperBuzzin]

Kinda sounds like something is shorting out somewhere.

 

Was the battery load tested?

Even a brand new battery can show good voltage, but still not be any good once a drain is put on it.