RyanPerrella

I have been looking for the correct way to test vacuum for the AC system and find conflicting information on the procedure of the test.

This is what ive done so far.

Replaced the expansion valve and dryer.

I then hooked up the manifold set to the low side and high side ports. The center yellow line is hooked up to the electric vacuum pump.

I started the pump and opened the high and low side ports and made sure that the center yellow line was open so that the vacuum would even reach the manifold set.

Within 5-10 seconds the low side gauge showed 30 vacuum.

I then let the pump run for 30 minutes to remove all the moisture.

Now this is the confusing bit. I want to leave it for 30 minutes to see if the vacuum holds but see conflicting information on the net.

1) i see to close the high and low side valves and let it sit 30 minutes (this is listed on the tag for the gauges)

2) another thing i read says to close the center yellow line and let it sit 30 minutes and makes no mention of the high or low side being open or closed. (this is the directions found on the vacuum pump site) http://www.redhillsupply.com/how-to-vacuum-pump-ac.htm

3) I also read something that says you should close the high side, no mention of low side or center


What i did: once the vacuum pump ran for 30 minutes i then closed the high and low side and closed the center yellow vacuum line. I let this sit for 30 minutes and still have 30 for the vacuum.

But now i think my test is flawed because all the valves are closed.

WHAT IS THE CORRECT WAY TO DO THIS VACUUM TEST?

RyanPerrella

Ive now also run the vacuum pump again to get to 30 vacuum (still not up on the units of measure for this) and simply closed the high and low side valves as the directions on my manifold gauge set show.

Its been 20 minutes and still at 29-30 and holding........ARE WE READY TO ADD REFRIGERANT?

Steve Tacheny

Yes. That should do it. When you close the 2 valves it closes off the passages to the yellow hose and leaves the gauges connected to the the blue and red hoses, the low and high side so it is showing the vacuum in the system.

Steve

aggravation

Purge the refrigerant at the yellow hose connection at the manifold because you will introduce air into the yellow hose when you go to hook up the refrigerant tank unless you already had the refrigerant tank hooked up in the chain and sitting on the scale.
If you are using one of those little cans with a charging hose built in from the auto parts store disregard this post....

Steve Tacheny

Purging the yellow hose before pushing refrigerant into the system is a good point and absolutely necessary. Thanks for adding that

Steve

dr bob

Ryan--
If you saw full vacuum within seconds of starting the pump, it's quite likely that you are not really connected to the car well enough. In one end of the blue and red hoses, there's a depressor that pushes on the pin of the schraeder valve in the charge port. The hoses can be easily reversed so that there is no depressor in the car end, or maybe the depressors have fallen out or been removed for some reason. Or maybe they are too short to open the valve. Result is what you are seeing, where the pump quickly evacuates the manifold and hoses, yet is not drawing any air out of the AC system in the car. The system is large enough that it takes a minute or so to get decent vacuum, and a bit longer to get serious vacuum. You can easily verify my diagnosis by closing the red valve to the high-side port, disconnect the hose from the high-side port, then use a small probe to open the schraeder valve there. You'll hear the air getting sucked in as you open the valve. No air means there's no vacuum. It will be handy to have those valves working correctly when you decide to the add refrigerant and oil, too.

Otherwise, your procedure is OK.


I pull vacuum on the car for at least 12 hours, and usually try to leave it connected overnight to pull as much air and moisture out as possble. If there's moisture at the bottom of the condenser or evaporator for instance, the process of vacuuming it out means that the local area will be cooled as the water flashes to vapor. ON a really hot day this is usually not a problem, but on a cooler day it will take a while for the metal to pull heat from the surrounding air sufficient to flash all the water. I have a great pump and I can afford the extra electricity needed to keep it on there overnight, so I do. I know you are in a hurry to move the car, so maybe the more common hour of vacuum, test and charge method is more in line with what your schedule demands.

RyanPerrella

Bob,

I saw full vacuum within 15-20 seconds actually. It did NOT take me minutes to get full vacuum. Could it be i have a better pump then you do?

I would love more specifics on how to purge the yellow line? As it stands the gauges are still hooked up both valves closed and its holding 29-30. Even after a few hours now. I am currently trying to figure out why the AC compressor doesnt engage when i jump the low pressure switch and am trying to find out what the "freeze switch" looks like which is supposed to be near the expansion valve.

please, bring on the purge process, once i get the clutch to engage via the low pressure switch with the car off and the ignition on i want to proceed to filling the system.

The freon is in small cans BTW.

And as for compressor clutch ideas, i do get it to engage when its jumped to the 12V power on the battery jump post.

RyanPerrella

Bob,

Great tip on checking to see if the gauges were hooked up properly.

I closed the high pressure valve and then as i started to remove it from the port i heard hissing until the hose was fully removed. I then opened the schreader with a probe and still heard it. So is it safe to assume that it was hooked up correctly.

I have no issue with leaving the vacuum on longer. Right now i still need to fix an airbag fault and fix the compressor clutch issue so i can leave it on vacuum for quite a bit longer. Maybe not 12 hours but everything i read suggested from 15-20 to 30 minutes. Nothing like multiple hours. But if you suggest and my schedule permits then i can definately do that.

Steve Tacheny

Dr. Bob is right. Longer evacuation of the system is definitely NOT a bad thing.

Steve

dr bob

Ryan--

The freeze switch is an aluminum box, maybe 1.5" sq, middle of the same tray where the expansion valve lives, agaimst the forward face. On my car (same year) there's a yellow-sheathed capillary tube running from a hole to the evaporator cavity to the freeze switch. Find that line and you can follow it to the switch. Two wires attach there-- pull the wires off and connect them together, and the freeze switch will be out of the loop. In my limited experience, the relay in the control head fails is the much more likely culprit. For grins, make sure the light in the AC button comes on when you push it. There's a little connector on the back of that panel that's easily pulled off if you are carelss when working around the radio. No light means connector is likely off, and no connection between the button and the control head eather.

Purging the hose: Connect the refrigerant can to the yellow hose. Both manifold valves are closed. Open the valve on the can or the can tap if you are charging with small cans. Turn the can up so liquid is at the valve end, and open the valve on the can, pressurizing the yellow hose. Now slightly loosen the yellow hose at the manifold end, bleeding until you have liquid flashing at that port. All the air is out of the hose now, so close the fitting and you are ready to charge.

It's easily possible to charge the whole system without starting the engine or running the compressor. It is imperative that you not start the engine or compressor when charging this way. Make sure that you can hold the can upside down so liquid is pushed through the hose. Open the can tap, and the high side (red) valve only. Hold the can in your hands to keep it warmer than ambient temp, and the liquid will force its way into the high side port. Continue with the rest of the cans. If your can tapper valve seals tight, you won't need to purge the line for each can. If ambient temps are above 90º when you charge, you can use a little warm water (no greater than 120º -- you can still put your hands in it) to warm the cans. The difference in temperature is enough to push the liquid in easily. It is injected into the system at the condenser and receiver-dryer, where there's sufficient heat to flash it to vapor. This is an area where there is normally liquid anyway. After you've got all the refrigerant in, close the manifold valves, wait a beer, then start the car and the compressor to see how the system pressures settle out. If you need to add more (shouldn't if you counted the can ounces as you put them in...) you'll do it as vapor only, and only through the low-side (blue) port while the system is running. It's extremely important that you not ever connect the can to the high side while the system is running. The pressure will grenade the can.

Standard safety rules apply when doing this. Glasses, gloves, an apron, and bucket of good sense are required when you are handling liquid refrigerants. They will instantly freeze eyeballs and flesh, causing permanent damage at least to the eyeball.

Good luck!

RyanPerrella

Bob,

I see the freeze switch and was unsure of how to bypass it. But now i know, simply remove the two wire connections and connect them together with some alligator clips then i should be able to get the compressor clutch to engage when i jump the low pressure switch with the ignition on (car not running) and the blower on and ac button pressed.

Notes:

The AC button does illuminate

The AC head unit relay was replaced by Docmirror and i sure its good

Questions:

Does the low pressure switch fail? EVER?

Also: Thanks for the idea of how to recharge without the cfar running, but I see allot more write ups for doing this while the car is running with a fast idle. Seems to be the best, easiest way to do this.

RyanPerrella

update.

Ok i reconnected the battery and tried to jump the low pressure switch to hear the clutch engage, nothing

I next pulled the freeze switch electric leads and wired them together thereby bypassing this switch and then again tried to jump the low pressure switch and still no noise from the compressor clutch.

NOTE: when i put power to the clutch itself i heard a distinctive thud noise indicating that it engaged.

Does this now remove the freeze switch from the list of possible problems, or is it just to say that there is still some yet unresolved issue?

I AM STUCK!

Orest

IMHO maybe start checking for continuity on the lines or just use your test light and start tracing from the fuse box to the switches and then to the compressor. It seems the good news is that the clutch engages when you jump it.

RyanPerrella

UPDATE:

MY DUMB *** WAS CHECKING VOLTAGE OF THE SWITCH NOT THE WIRES GOING TO IT LIKE I SHOULD HAVE!

I jumped the wires for the low pressure switch (not the switch contacts themselves which is what i did before) and BINGO, clutch engages so i am good to go with the recharge.

DoubleAgent

Hi Guys,
My name is Curt and I'm a HVAC tech in Florida as well as a new 928 owner. I just completed my first compressor rebuild.

First, your gauges CANNOT measure the degree of vacuum needed to remove moister from the system you are working on. Second, You really should have a Micron Gauge or Vacuum gauge as they are sometimes called. This will allow you to see how deep of a vacuum you are achieving. Moister isn't fully evaporated from the system until you reach 750-1000 microns. Your Lowside gauge measures a vacuum in inches of mercury. Not very accurate. I usually pull a 300 micron vacuum just to be sure. The vacuum gauge needs to be as far away from the pump as you can get it. Usually in the lowside. You may need some extra fittings or hose.
Close the manifold off (both sides) and let it sit for 10 min should be enough.
Vacuum gauge may rise alittle but shouldn't climb too far above the level you pulled. Weigh the charge in. The manual gives you the correct amount. Check the Site Glass.Top off charge as needed with the car running around
1300-1500 rpm. I put a large Fan in front to simulate normal airflow for the condenser. There is no need to mess with the low pressure switch or freeze
switch. Running the compressor in a vacuum may cause damage.

It doesn't matter how long you put the pump on a system, if you cant tell how deep of a vacuum your achieving, you dont know if there is a leak(could be tiny and very likely won't show in your gauges for sometime) or whether moister is truly gone before charging. The filter/dryer will help but if there is excess moister it wont do much.
Non-condensables may cause loss of capacity or worse Freezing of the TXV. I'll have to look into the Freeze-stat componant to see its location and function on the system.

Micron gauges can be had for 100- 140$(good ones) at your local HVAC supply house.
They are priceless, especially in florida , as it is freaking melt your skin hot in the summer.
Oh yeah and another tip: don't use that florescent dye in you AC system. IMHO it isn't good for the compressor. A halogen leak detector
backed up with soap bubble solution is 100% better for finding leaks..
I know you may not have the time or mula to spend on new toys but this is the correct way to do these things.

I would like to thank you guys for the large amount of info i've used from this site. It is invaluable. Hope this helps.

FIRST POST WOOHOO!