I have a noticeable vibration (hear and feel) coming from the alternator but it only occurs when the AC compressor is on. It is an 89 stock accessory setup with serpentine belt. The two large aluminum castings, one on each side of the engine.

I noticed this a while back. It happens at about 1200rpm and only when the ac is on. The alternator makes a loud, harmonic buzz that comes and goes as it passes through this rpm range.

I had the motor out a couple months ago and noticed the alternator bracket was broken. I had the bracket welded and put it back together. When I started the motor it seemed fine. I notice when the ac is on the alt makes the vibration and noise. I pulled the alt off and the alternator housing was broken. I replaced the alternator and the new one does the same thing.

One point is my alternator is re-clocked due to interference with the valve cover. Since it is turned I can't use the rear bolt to attach a brace. It appears there is a hole on the accessory bracket which could be used to attach a brace to the back of the alt.

The only other idea is the AC compressor may be going out and sending a harmonic vibration to the alternator. It is a fairly new compressor and doesn't make any noticeable or unusual noise.

Any Ideas or suggestions? Has anyone had this type of problem?

 

Is it possible that when the alternator bracket was welded, its no longer properly aligned, causing the belt to not track properly?

 

This might be the problem.

Also, could it possibly be the additional electrical draw on the alternator when the AC is turned on? The alternator in my Lumina makes a loud hum when I turn on the AC. It's a 7 year old, 1 year warranty Vatozone alternator so this may the reason.

 

My old 91 454SS did that too. I had a bad AC compressor but it was still the original compressor.

 

Thanks for the ideas.

I feel confident in the alternator bracket repair. It was the small tab where the small bolt goes. It was a clean brake and I made an alignment fixture to keep in in the proper location for welding. But the vibration persists so anything is possible. I see the broken tab and broken alternator housing (at the large through bolt) both as indicators that this problem has existed for a while. I just either didn't notice it or the broken mounts may have actually mask the problem by allowing the alternator to rest against the main bracket assembly. It is a spring loaded tensioner serpentine belt system so when the bracket broke it the tensioner just took up the slack. The hasn't been any unusual belt wear or rubber dust noticed anywhere. The belt still looks new.

Does anyone know (or remember) if there was a rear alternator brace from the factory? There is a bolt hole in the alternator and a boss with a similar hole near the top of the power steering pump, just in front of the first header bolt on the driver's side. Looks like a perfect place for a brace. I'm thinking about changing valve covers or modifying the ones I have to allow me to turn the back of the alt to the position that will allow me to add a brace. But it's a lot of trouble to go through if there is another reason for the vibrations.

 

Did you ever figure this out? I'm having the same issue myself..

 

Do you have the Alt to Exhaust Manifold bracket/Rod installed? See yellow circle in pic.

 

89-up don't use that bracket. GM redesigned a few of the support bars, seems like some documentation shows it's a early/late 1988 MY change.

 

I was able to add a brace which helped a lot. After installing different valve covers I was able to re-clock the alternator back to stock. Then, I took a random brace I had and modified it to run from the back of the alt to the front header bolt. Here's a pic.



The image is a bit dark but you may be able to see I had to cut and weld it near the bottom to allow it to nest under the header bolt. I replaced the previous bolt with the allen head stainless one. There is still a mild vibration at ~11-1300 RPM but nothing like before. At least I can't hear it or see the alt vibrating now.

 

Oh, one more detail which may help others. I bought an alternator for a 91 Camaro and it had a 1/4" bolt and nut for the charge terminal rather than the large block with a bolt. This was a little shorter style and I was able to further shorten the terminal stud so it doesn't hit the valve cover. Still leaves plenty of thread for the charge wires. With these tall Proform "Chevrolet" script valve covers and the alternator mod the charge terminal has about 1/4-3/8" clearance. Tight, yes but it works. I used a rubber cap for an insulator over the assembly. I can still remove the valve covers with this setup so it works well. No need to re-clock the alternator now. However, I do have to loosen the alternator and rotate it out to get the charge terminal nut and wires off.

I have a HSR intake but may see if I can add the top brace like Bob's pic shows. I hadn't seen that one before.

 

Bob was right, I was missing the bracket..I even dug around in my TPI parts bin and found one, however there is another problem:



Apparently the corvette intake is a bit different and uses a different bracket.

 

battman, can you use about a 1" bushing spacer between the intake and bracket?? Or is the bracket a tad tooo long to line up with the hole?

 

the boss on the water jacket has the angles off, I'm thinking I will just remove the boss that's in the way.. hopefully it will help with the vibration once installed.

Good catch Bob

 

Got the brace installed over the weekend and it still vibrates and resonates through the car.



I did notice with the AC on I have a lot of belt deflection from the AC compressor to the crank pulley. I'm now thinking that The A.I.R pump has been removed the long belt run maybe causing the vibrations..