Technical A/C Diagnostic Help?

We have a Vintage Air under dash unit, with a Sanden 508 compressor running R-12.

Works very well will blow 35* air all day long. Problem is on the highway around 2800 rpm the low side will go down to 10 psi or less, and then compressor makes a rumbling type sound. Adjusting the thermostatic switch to cycle the clutch only results in warmer outlet temps.
At idle and around town everything is fine 30 psi low and 210 on the high side. No bubbles in the site glass. Adding more freon only brings the high side higher.

When engine is shut off it takes like 15 minutes for the low and high pressures to equalize. Replaced the expansion valve 3 times with different brands, all the same as the original. Last nite removed the expansion valve and backed out the adjustment about 1/2 turn. Still the same.

This system is 20 yrs old and this problem started after changing rear gears from 2:73 to 3:27. We have the same set up in another vehicle that will make the same noise at those rpm's. Except that has a overdrive and will cruise around 2500.

Any thoughts or ideas? I'm baffled.

 

When the low side pressure drops, what happens to the high side? and airflow out of the vents?

 

High side stays about the same. Duct temps seem ok. Does not have problems making cold. Its the noise from the compressor.

 

What happens to the low side if you spay some water on the condenser?

 

Have you replaced the receiver dryer?

 

Yes about 4 yrs ago.

 

Lowers the high side. Plenty of air flow. Fan clutch engages according to ambient air temps.

 

Doesn't Vintage air have a tech line?

I would hope they have knowledgeable people that could answer your question pretty quick. HRP

 

Please let us know when you find the cure.

 

Contacted them and 4-seasons. Not much help, pretty much just customer service phone answering type.

My gut feeling is the expansion valve. Don't understand why it takes so long to equalize pressures after shutting off compressor.

 

Blockage in the dryer will do this.

 

If it were a restriction in the drier one would think that the suction pressure would be much lower than 30 at idle and around town on R12. Consequently, the discharge pressure would be high. Just curious, have you checked the evaporator coil on the outside of the coil to make sure it is nice and clean? Maybe a piece of paper got sucked up against it?

 

At 10 psi with R12 you have a good amount of liquid refrigerant returning to the compressor if charged properly. A compressor is a gas pump, not a liquid pump.

 

Guess I will ask about the moose in the room since no one else is. You have a vintage air system. Why are you running R12? Vintage air kits are R134a.

 

Good point Milner!

 

Yep I know its a 20 year old system but Ive never heard of Vintage Air ever offering a R12 system. Their whole company came about as an alternative to getting a new R134a system for vintage vehicles that had AC that was R12.

 

A quick check of the Sanden 508 shows it is a R12 compressor.

 

If the evaporator has accumulated dirt and dust over 20 years, or has restricted air flow for whatever reasons, it could be icing up at higher RPMs. You might not notice at lower RPMs. An iced or frozen coil will allow liquid refrigerant to slug the compressor which could very well be the noise the OP is hearing from the compressor at pressures that low, but you would think you could identify that by feeling the suction line right at the compressor.

 

Will check that, it seems to blow plenty of volume of air.

It could be older than 20 yrs. Will check on that.
When it was originally installed, it was just the under dash unit, with a gm a-6 comp and a condenser from the same donor 1973 Chevelle. The condenser is huge compared to modern ones. About 10 yrs ago the Sanden was swapped in due to a bad clutch. No metal in system back then, and on the expansion valve, when we adjusted it last nite.

How about the compressor having too much volume? From what I read the 508 is 8.4 CI per rev, and the 709 7.09 per rev. The A-6 is 12, but maybe it did not rattle like the 508 is doing.

Grasping at straws..

 

Agree!, and that is most likely all the noise.

But why?

 

If it has been working, something has changed. You shouldn't have to result to major component design changes to fix the problem. I certainly don't think the problem is the gear change. That is a coincidence that the problem started after changing the gearing I think.

 

Well, we have 2 cars with basically the same set up. The other one has a 6-71 blower and loud exhaust, cannot hear a thing while driving. Revved up in the drive, yeah it makes the same noise, and the low side dips near 10.

 

So is your only issue the noise and the fact that it takes 15 for the pressure to equalize?

 

The noise is the only concern. It's loud and with a cruiser annoying.

Just seems odd that the pressures are so slow to respond.

In 40 yrs of new car dealership wrenching crap, just never saw the pressures just sit there with nothing happening.

 

pressure equalization will be slow. Not instant. My converted El Camino you shut it down and you hear a whistling as the pressures equalize through the system. My old truck does the same way. My mercury is the odd one it doesn't whistle it just has a slight hum.

As far as the noise goes it could be a faulty compressor.

Also you said pressure sit there with nothing happening. You saying the pressure doesn't change when the AC is running it just sitting static?

 

Also if your low side is dropping to 10 when reving sounds like you have the same problem I am dealing with on this 15 dodge truck. it has trash in the system from the descant bag breaking open and clogging the system up. Low side will suck down to 10 psi high side would start to climb to 250 psi but then when the low side starts to suck from 30 psi down to 10 psi high side drops to about 230 psi and stays there. Lines warm and no cold air.

Another thing is it could be not a full charge if the low side and high side is low like that.

 

How long would you think it takes for the pressures on yours to equalize?

Compressor has been swapped out with new in the box 508's.

 

I have spoken with Vintage Air tech guys several times and found them to be very helpful. Call and leave a number, they call me back.

 

Hmmm I never timed it I would say the whistling comes and goes once or twice but if you listen closely like I have when I do a ac service charge on them I would guess it takes about 20 to 30 seconds for the pressures to be fully equalized after being ran for a performance test of the vent temperature.

Being new in the box doesn't mean much now a days I hate to say.

 

How much freon do you have in the system. VA made systems that used 12