'93 SR, 133k. I bought it last fall and replaced:
T-belt
all tensioners
acc. belts
H2O pump
ignition stuff

It runs like a champ, low oil consumption, no smoke (yet). Previous owner had just replaced the alternator (POS Bosch reman).
Problem: Every few weeks all the lights will fluctuate intensity at ~ 4hz with accompanying slight siren whistle from alternator. I've "fixed" it three times now by futzing with the hot line connections between alternator and battery, but it keeps coming back. Lots of searching concludes the fluctuation is charge current going un-damped from the alternator straight to the electrical system rather than charging the battery.
Anything else I should try before junking the alternator? Battery is charging fine and I've had no problems with that.
<img src="/forums/images/graemlins/confused.gif" alt="" />

Thanks in advance!

All I'll suggest is check your grounds from the battery. I've had some pretty wierd stuff happen resulting from poor grounds. It is something that is very easy to overlook. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Thanks Nathan, I have experience with bad grounds. Pulling the motor on my '89 Carrera was so easy I pulled it for valve adjustments! But the block strap decided to pass the starter current on to the 18g horn ground <img src="/forums/images/graemlins/scared.gif" alt="" /> after one pull. (poof!)

Your alternator is about to fail.

I'm up to reman unit#5 (love those lifetime warranties...). The symptoms you describe happened to me before unit#2,3 quit. #1, #4 failed in more dramatic fashion.

My theory: Reman units can't cope with being so close to the header and getting hot. My next step is a used OEM unit.

Paul

reman units <img src="/forums/images/graemlins/barf.gif" alt="" />

I'm not familiar with your exact truck but check to make sure there is not a heat shield missing.