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So, My LT1 has headers, with none of the three brackets attached to the headers. I will fix that later.
But, why is this thing vibrate at all?
When you listen to it with a stethoscope, its quiet. But, under load, in gear it vibrates enough, that you feel it in the driveline.
Lets just assume all pulleys, harmonic balances, belts, etc are all new, to eliminate thinking outside the box.
Think, why does this compressor vibrate... Four Seasons has been making compressors forever, as I assume they made this one.
GM has been installing these forever, so why the bandaids of the 3 brackets.
Does it vibrate more with the AC on or off? I'm going to guess that it needs the extra brackets due to the fact an AC cycles on and off a lot. Sometimes under load. That jerking motion of it cycling may require more support. I'm not sure if the brackets are intended to prevent vibration or for additional support.
There is high stress on the compressor bracket when it cycles on. GM didn't install the braces without a reason, I suspect they are tp prevent cracks in the bracket after many cycles. On my car I can feel no difference when the compressor cycles other than a slight idle speed difference. Have you done a tuneup lately? Oxygen sensor replacement?
There is high stress on the compressor bracket when it cycles on. GM didn't install the braces without a reason, I suspect they are tp prevent cracks in the bracket after many cycles. On my car I can feel no difference when the compressor cycles other than a slight idle speed difference. Have you done a tuneup lately? Oxygen sensor replacement?
do you ha r headers?. Still have the 3 black rod brackets.
Yes, tune up about 2000 miles ago. I run heated O2s in this 93.
rums real nice, even with medium size cam and 396 cu in. .
I did run it uo to 3000 rpm with ac on, watched it cycle. Had hand on compreasor. It does vibrate a little. Under driving load I am sure ita a bit more stress.
All pulleys , tensioner new, including belt. It was doing this before the change too.
I am not saying what is the problem just discussing harmonics...
The compressor when on is a load causing drag on the pulleys (think about the parasitic horsepower result). This will try to distort with axial forces and most likely is hitting a harmonic (frequency at which oscillation increases) at your 3k to 4k range. A heavier pulley wheel on the alternator for example would change the harmonic frequency just like a crank balancer. Brackets would also change the harmonic frequency. Usually, and most desirably, the design will push the fundamental harmonic to the lowest frequency possible (0 hz would be perfect). Also, there are always multiple harmonic peaks so sometimes shifting the band to operate between peaks is necessary.
You should be able to 'see' deflection on the serpentine belt when it oscillates if the compressor is your source of vibration if you have a go-pro to mount. Are all belt driven add-ons bracketed two place in front and one in rear? Pretty standard on heavy loaded items. Maybe there is an alignment issue requiring a shim?
Yes, alternator and compressor are mounted down front and rear.
Great write up on whats goung on.
its livable, but running it to ground is what I will do.
i may mount the brackets that I can on the headers. I have spacers etc to get at least 2.
I am having a similar vibration issue and would appreciate a photo of the three brackets mentioned above.
The car is a 95 LT1 manual, American Racing headers, 3" exhaust with cats, down to 2 1/2" and Corsa mufflers. With AC off all is fine no vibration even at RPM below 2000 in 3rd or 4th gear or above, and at speeds around 30-40 mph (not that I drive it in 5th or 6th at those speeds). But, turn the AC on and it vibrates so much I feel it in the drive train and the car shakes. Actually feels like the vibration is coming from the exhaust itself. Vibration is not felt as much at speeds above 50 or 60, but it is still there.
I'm going to look for brackets tonight. Are they on the compressor or the headers?
Here's a pic of my 92. There's a bracket on the alternator and ac that goes to the top of the intake and then another small one going down to the exhaust manifold. I think that small one going to the exhaust manifold doesn't fit with headers unless you heat and bend it. Most people just leave it off, which could be the vibration issue.
I don't know if they changed it later. On my 92 there are 2 small brackets on the exhaust manifold and the larger upper one. In that later pic I see the alternator has a nub for that upper bracket and one of the small brackets. It doesn't look like anything is attached to the alternator in that pic, though. So it looks like there's a difference between the 92 and later ones. Doesn't surprise me as the 92 valve covers are one year only. So are other odds and ends on the 92.
Just looked on my car. #8 in the compressor diagram above is bolted to the compressor but the other end just hangs out into free space. #6 was cut off and only an inch or so remains connected to the compressor. If I read the diagram correctly they both attach to the same location on the exhaust. In my case that is now a hex head header bolt that is too short to allow for the braces to attach there. I'll have to replace that bolt with a stud and nut, leaving enough stud sticking out so I can get the braces attached.
Problem is that the headers may not allow space for a nut to fit on the stud and be torqued with a socket. I could indent the header tube with a hammer but risk bending it so near the flange and head. And, a sensor is welded in at that location.
What if I welded a bracket to the header tube and bolted the braces to that? Sounds like a bad idea. Other suggestions?
Sorry, I just caught it that there are 3 braces. #5 in the second diagram attaches to the alt. It is also missing on my car. Am I correct that it attaches to an exhaust bolt, but to one forward of where the other two braces attach?
Thanks for the help and PMs with photos. Here's a shot of my solution. I now have two out of three braces installed and vibration problem is corrected. Need to get a crow foot wrench to torque the nut that is up against the header flange. I guessed on the 26 ft lb torque when I installed it. Realized later that I could have used a crow foot.
If what looks to be a 'tubing' spacer were a hex threaded 'tall nut' the same thread as the stud I would think it might be a bit more secure, less likely to loosen because of the stress of the 2 added supports.
If what looks to be a 'tubing' spacer were a hex threaded 'tall nut' the same thread as the stud I would think it might be a bit more secure, less likely to loosen because of the stress of the 2 added supports.
Good catch, and thanks again for the help. It is a 1 1/4 long steel spacer that is the perfect length but not threaded. It's a close fit on the stainless stud, but not tight. Couldn't find a coupling nut at the hardware store that would work, or a spacer that I could thread and I wanted to drive the car. I'll replace the spacer as soon as I get the coupling nuts. See McMaster Carr note below.
For reference: The stainless stud is 3/8 - 16 (not that you have to use stainless). I had to cut the length down and inserted it into the head only the same distance as the header bolt was. The coupling nut I'm getting from McMaster Carr is 1 1/4 length but you can use a shorter one and add a couple of washers.
McMaster Carr has threaded couplings that should work better. Link below. McMaster-Carr
Good catch, and thanks again for the help. It is a 1 1/4 long steel spacer that is the perfect length but not threaded. It's a close fit on the stainless stud, but not tight. Couldn't find a coupling nut at the hardware store that would work, or a spacer that I could thread and I wanted to drive the car. I'll replace the spacer as soon as I get the coupling nuts. See McMaster Carr note below.
For reference: The stainless stud is 3/8 - 16 (not that you have to use stainless). I had to cut the length down and inserted it into the head only the same distance as the header bolt was. The coupling nut I'm getting from McMaster Carr is 1 1/4 length but you can use a shorter one and add a couple of washers.
McMaster Carr has threaded couplings that should work better. Link below. McMaster-Carr
OR add appropriate 'jamb nuts' to establish height.
AC Compressor vibrations.. Discuss why
There is high stress on the compressor bracket when it cycles on. GM didn't install the braces without a reason, I suspect they are tp prevent cracks in the bracket after many cycles. On my car I can feel no difference when the compressor cycles other than a slight idle speed difference. Have you done a tuneup lately? Oxygen sensor replacement?
