I have a 86' gmc stepside shortwide sierra classic... down to the point. i bought it about 3 months ago, and this probelm has been there the entire time. Someone swapped the motor out from a stock 305 to a 350, they didn't put the airconditioner back in... well


when in park, everything is fine... steady idle.. blah blah well, when i put it in drive, the alternator will shoot up from 14 voltz to 16voltz.. maybe thats fine... but when going down the highway, and after it is warm in park, the idle will jump down, lose voltage, and dim every light in the truck. It couldn't be the alternator because it was replaced about a month ago and is still brand new... unless there is a problem in the alternator?

the electrical system that the person put in it i can't say is the best. of course if it were, it would probbably not be doing this ... but there are 1 or 2 grounds that are leading to nothing, and no one can really find a power steering belt that will fit this motor w/o the compressor.


one more detail... when in drive.. when it drops in rpm like i said before, you can hear this humming sound, kinda like a faint whistle. i think it could be the power steering when it isnt getting enough rotations cuz the rpms are too low?

if you need to know anything else.. just ask

 

Day late and a dollar short here but what the heck. Sounds to me like you need to hook up those body grounds. If your not getting a good ground, it can screw up everything. So, I would disconnect the (-) negative battery cable; hook those grounds to the frame and body somewhere; re-connect the battery and see what happens. Also, check for any other loose or disconnected wires. You never know.

Hope that helps if it's not too late,


Brad