jayhawk261

On our 91 Z28 305TPI 5-Speed car, we just took the steps necessary to the the air conditioning up and running. It works great. This, of course, exposed the next problem. When the AC is on, the alternator vibrates enough to make a lot of noise that can be heard outside the car and felt through the car. All bolts are tight. Now, this car has a set of headers on it. Is it possible that there was a secondary bracket for the alternator that mounted to it and the exhaust? If so, I'd say it was missing due to the header install and I need a bracket to solve my problems. Anyone have any ideas?

Thanks!

And no, it is not the internal part of the alternator making the noise (although it might start that after too much of this). You can watch the alternator and bracket vibrate. Also, it only does it at idle. Once above idle, it goes away.

HF_monster

I dont believe that their was a bracket to the exhaust but I could be wrong. If it was me I would check the belt tension or a possible miss alignment of the pulleys as a source of the problem also check to make sure you dont have a broken mounting bolt on the alternator or a cracked mounting pad. just some possibilities

just had a thought did you put in anything to bump up the RPM's when the AC is turned on?

jayhawk261

I didn't put anything in as I assume it should do it automatically. I suppose it's possible that feature isn't working at the moment. I didn't listen when turning the AC on to check if it actually bumped it or not. If it doesn't, then I have an electrical issue. Anyone know where the sensor is for that purpose?

HF_monster

I believe its controled by the computer on TPI IIRC but not too sure but if the RPM is low enough it can cause a vibration from the load the compressor creates. If you have a schematic for the pin out of the ecm it will tell you their should be a wire that tells the computer when the AC clutch is engaged and the computer adjusts the RPM for the added load.

oif0709

Listen to hear if the motor bogs down when you start the ac up. But it wouldnt cause just the alt to vibrate. The whole engine would vibrate trying to stabilize. Take your mounting bolts off the alt and look at them and try again to see if you cross threaded them because if you did it doesnt matter how much you tighten it, it will still be loose.

jayhawk261

I'll take a look at the bolts. I have never removed them before, but the alternator looks like it was replaced shortly before we bought the car. I suppose they could have done some damage.

TTOP350

iTrader: (8)

There is a metal rod/bracket from the alt case down to the 1st stud on the dr side exhaust manifold. Most guys pitch it when doing headers because it needs a spacer and a lil grinding on the rod.
I would check the aluminum bracket to head and block mounting bolts also.

jayhawk261

That's what I was thinking might be the problem. Maybe I can find one in the bone yard and get a longer header bolt.

TTOP350

iTrader: (8)

I dont run it on any of my cars and I dont have the problem. I'd check those silly bolts..

jayhawk261

Yeah, I'll get them checked out and see if it can be as simple as that. I sure hope so! It's kind of annoying!

jayhawk261

Found it! Turns out the bolt holding the PS lines to the alternator bracket was missing. Had a perfect fit in the bolt bin and no more rattle!