So this is concerning my 1990 Blazer. I replaced the battery with a brand new one and just bought a brand new alternator and installed it today. The volt gauge still reads funny though. It reads a normal 13.5ish at idle in park. The needle is steady. But when I'm driving with my lights on and the heat on and I'm sitting at a light, the volts drop to almost 9 (red zone) and my lights dim and it feels stuttery and like it's going to stall and shut off. If I turn everything off, everything reads fine for the most part but it still feels stuttery. There's no tach, so I don't know what the RPMs are. I don't know if it's a corroded connection on the wires or not, I didn't look at them really. Also, would a smaller pulley speed up the alt? I'm not really sure what's going on. I've replaced a bunch of stuff with brand new parts but I'm still having issues. There's not much left in there to replace.

 

How did your tensioner feel when you put the belt back on, did it put up a fight and rest good and tight against the belt? With the engine off, grab the fan on the alternator with both hands and try to spin it. If you can, you're getting belt slippage. When all the accessories are on, there's a tremendous amount of load put on the alternator, which causes a drag. Plus, belts can be more prone to slip at higher rpm's. And yes, a smaller pulley speeds up the alternator. Is the new pulley the same size as the old one?

 

I had to use a pipe over the racket to pull back the tensioner to release the belt, it wasn't hard to pull back (I'm fairly strong though, I work out, ha), but it wasn't easily pulled back either. The belt was nice and tight when I put it back on. I almost thought maybe it was too tight. I'll have to test your suggestion tomorrow, as it's snowing right now with already 2 inches on the ground. I know having everything on causes draw, but it seems like in the past couple days it started doing that thing at idle while in gear. I replaced the alt with a new one (100 amp I think?) that appeared identical, just new and shiny.

 

Were you having this same problem before you changed the battery and alternator? Check the plug on the alternator, one of them is the exciter wire that trips the internal voltage regulator. Make sure it isn't corroded, or that the metal tab isn't pulled back in the plastic part of the plug giving you a half-azzed connection. You might as well check all your battery positive and ground connections while you're at it. Are any other lights in the dash flashing crazy? That would be a sign of a possible bad ground.

 

I was only having a similar issue a year ago when that alt went bad (I had it tested and it did test bad). But I kinda cheaped out and bought the cheapest reman. But the issue stopped after that was installed. But it started back up again not long ago so I bought a new one today, not a reman. I was thinking maybe the wires were corroded. The battery connections on the actual battery looked fine, but I forgot to really look at the ones that attach to the alt. The headlights and dash lights only dim in kind of a flickering way at idle while in gear, that's all I notice. It's not super obvious, but I noticed it right away. I'll have to look at all the connections again tomorrow in the light and try to clean the ones that get bolted on the alt.

 

Check your battery ground connections to the frame and body too. If you have a bad connection there, things can go flakey. If it's still acting up after all that, where the voltage drops at idle, there's another test you can do. That plug in the alternator connects to the voltage regulator. Look at the back side of the alt and see if you don't see the shiny back side of the regulator through a hole, adjacent to the plug. If you take a small screwdriver, touch the tip to the back of the regulator while also touching to the body of the alternator, you should hear the alternator make a whiney load sound. What that does is activates the voltage regulator and puts the alternator into a full charge state, only while the screwdriver is there though. Don't worry, you're not going to blow or short anything out. But don't leave the screwdriver connected, it's only a momentary test. Have someone watch the gauge in the dash while you're doing that to see if the needle spikes. If it does, it means 1) the rest of your charging system is ok and, 2) either the exciter wire isn't working, or the voltage regulator is bad.

 

Thanks 69. I'll have Case look at it and we'll test it out. While driving it's fine, the volts stay normal. It's just when idling in gear it gets goofy.

 

thanks, ill see if i can figure out that test method tomorrow. ill just go over everything for her and check for corrosion. im thinking the idle might be set too low, is it possible to adjust it on tbi? it sounds like its around 400 in gear. the iac works normally otherwise, it idles up about 1500 when cold and first fired. it was ok for the most part until i installed a set of foglights. i dont think to 55watt bulbs are going to cause this many issues though

 

WWMD? Hit the alternator with a hammer. Massey knows what I'm talkin about.
The idle on the TBI isn't manually adjustable.....technically. There is an idle screw under a hardened steel cap on the drivers side, but if all is working as it should, you shouldn't need to fiddle with it. Have you checked the timing? Advancing it will speed er up. The IAC valve (on the passenger side of the TBI) is another thing to look at. Remove it and see if maybe the passages are carboned up, that would restrict idle air flow. How many amps do those fog lights draw anyhoo? And is the discharging problem happening with those both off and on? How about using the rear window defroster? Those can suck juice like no tomorrow. Jeez, that was a lot of questions!

 

no to the defroster. the lights cant draw more than 10 amps. i have them wired on the parking lights. im not sure if timing is adjustable on that engine. my 4.3 has a roll pin preventing manual adjustment, but hers could be different since its tbi. most people have issues with them idling high, but never low. during start up and when cold, it revs to about 1500 like normal, then slows down to normal. its just when in gear, stopped, with lights on it does it. and the idle sounds too low. im going to inspect the bushings to make sure theres no play in the throttle plate. if i have to, ill drill that plug out and adjust it