77 Pontiac Trans Am 400 V8 A/C vibration issues

[johnv]

When I restored my 77 T/A the A/C was converted to 134a. A 5H14 Sanden compressor was installed along with a new factory condenser. The system was flushed, a new O/T installed, 8 hour vacuum pulled and it held before charging. Pressures at 2000 rpm were 225 and 25. Ambient was around 75F. Discharge temp was 40F.
The issue is a bad vibration that occurs just off idle and at around 2750rpm. The vibration is felt and can be heard though the entire vehicle. As soon as the A/C is turned off, the vibration and noise are gone.
When the vibration occurs you can see it in the belt. According to Continental the belt tension should be a 100 psi. I adjusted it form 55 to 90 but was afraid to go any tighter as the belt was extremely tight. At that tension the vibration was less but still there. The belt has been replaced.
I have made sure that the compressor mount is as solid and tight as it can be, and that the pulleys are parallel and in line. I installed a new drive pulley since the wear pattern in the groove was not uniform.
I am on my second compressor since the company that the system was purchased from recommenced that. The company is a major supplier of conversion kits.
Originally there was a muffler in the discharge line, but it was not used when the system was converted. I have the muffler and am wondering if that would help with possible pulsation from the compressor. As I understand it, typically the Sanden compressor doesn't require a muffler.
The condenser is not parallel flow. I am wondering if changing to that would help.
Lastly, since the distance between the two pulleys is pretty far, it would seem that a back side idler would possibly be beneficial. I have searched for one that could be retrofitted to the system but haven't found one yet. I have not seen any vehicles similar to mine that requires that though.
The company that the conversion kit was purchased from says that everything is textbook, they have never run into that and have no idea what is causing the issue.

[bohica2xo]

Old V belt systems require a lot of tension. The idler bearing in that Sanden is up to the job. Lots of heavy equipment is still using V belts

Go ahead and use full factory tension. The A6 that came on that car had similar construction, and perhaps a little more rotating mass.

If you want to add an idler on a long belt span, use an inside idler. External idlers on V belts are hard on them, reducing the life of the belt.

I can recall many big Cadillac and Lincoln cars with belts as tight as banjo strings - because those owners hated hearing a belt squeal.

[tony1963]

On later applications, GM did use belts where the underhood design did not permit such large expanses between pulley locations. What you are seeing is the belt whipping due to the torsional loads of the compressor.

Go ahead and use the full tension and keep in mind that the belt will relax a bit after applying the load. After a few weeks of driving, check the tension again.

The later model vehicles adopted poly V belts (a wider V belt) and also eventually serpentine belts where the tensioner absorbed these forces. On Mercedes vehicles, you can also find a shock absorber on the belt drive on the diesel models to control the forces.

[bohica2xo]

John:

Which pulley is on that Sanden right now? Number of grooves and outside diameter?

[johnv]

There are two groves. The OD of the pulley is 5.2”

[bohica2xo]

Try boosting the tension first.

There is a high mass pulley used on some class 8 diesel trucks for that compressor.

[Tim]

I was going to say to provide the 4-digit Sanden number, and I can provide the data. dia, centerline, and so on...

[johnv]

The Sanden compressor number is 5H14. Thx.

[Tim]

The Sanden compressor number is 5H14. Thx.

That is the body style. It should have a number like 7312, 4884, and so on. Many compressors use the same body but have different pully assemblies or head configurations. The 4-digit number identifies this characteristics.

[johnv]

I’ll try to tension the belt to the 100 psi. I’m in a number of car groups that use Sanden compressors and haven’t found any with belt tension near that high.
In my original post I mentioned using a muffler or a cross flow condenser. Wouldn’t either one of those reduce the pulsation in the system and as a result minimize the belt whipping?
It just seems there is something else going on besides belt tension.
Don’t get me wrong, I really appreciate everyone’s input here. I have been trying to figure this out for 2 years now and am just frustrated.