SQLGuy

How powerful a compressor did you try it with? I've got the older version, and it takes quite a lot of air to run it.

Ed Scherer

Never tried it myself; I remember being warned off of them.

SQLGuy

We're about 11psi ambient up here at 6000'. The older HF red box vacuum pump I have will pull the system down to about 4psi. That's with a cheap 4HP compressor. A bigger one would probably do better. It's way better than no vacuum pump, though.

dprantl

For A/C work you want a vacuum pump that can pull at least down to 25 microns. You want to pull a high enough vacuum so that all the water in the system will boil off in under one hour. That H/F pump is woefully inadequate for A/C work. 4psi IMO is not even close to good enough. The weakest pump I would ever consider using would be something like this:

http://www.harborfreight.com/two-sta...112&zmap=66466

Dan
'91 928GT S/C 475hp/460lb.ft

SQLGuy

I'm not familiar with that. I thought a micron was a thousandth of a millimeter. What is it in terms of Pascals, psi, or inches of mercury?

Edit: Looking around, I think you meant 25 millibars. That would reduce the boiling point of water to about 32 degrees F.

Thanks,
Paul

dprantl

No, a micron in this case is a micrometer of mercury (µmHg), which is a unit of pressure/vacuum. It is very similar to a millitorr.

Dan
'91 928GT S/C 475hp/460lb.ft

SQLGuy

Cool. Hadn't heard that meaning of it before.

I would have thought, though, that, even with the 25 millibars, which is about 800 times more pressure, you'd be OK. It's still well below the boiling pressure of water at typical room temperature.

Mads

I got a refill a couple of weeks ago i don't drive the car much, but this weekend i took it for a drive. After the drive i noticed that that AC compressor cut on/off every 2 sec. is the AC overfilled or is that normal ?
the outside temp was 30c/86f.
how much is the pressure supposed to be ?

Regards

Mads

WallyP

That is called "short-cycling", and usually indicates that the refrigerant level is low - so you probably have a leak. It would be a good idea to have your friend check the system for leaks.

The oil in the system circulates with the refrigerant, and if there is no refrigerant to circulate, the oil will be pumped out of the compressor, which will quickly fail. To help prevent this, there is a low-pressure switch on the receiver/dryer that cuts the compressor off when the system pressure gets too low. The compressor runs and pumps refrigerant from the low side. If the level is low, the pressure in the low side will be lower than the switch setting, so it opens, cutting off the compressor. The pressure equalizes on the low side and high side, the pressure goes up enough on the low side for the compressor to run. The compressor runs and pumps refrigerant from the low side. The pressure in the low side will be again become lower than the switch setting, so it opens, cutting off the compressor. The pressure equalizes on the low side and high side, the pressure goes up enough on the low side for the compressor to run. The cycle repeats.

dcrasta

I got one for 7.99 with a coupon at HF. And I was able to pull down to 30hg of vacuum and it held that for 1 hour, then I refilled my system till my low side pressure was 40psi . My rear AC is very cold, my front not so much. I ziptied my heat valve closed, however I still have troubleshooting of the HVAC head controller to finish.

If you are just doing a home job (cheapest way) this worked for me. My small compressor was able to do the job with this pump, but it is not the 'best'.

(Pro tip. Do your AC service on a cool-low humidity night).

Pcplod

This is something that I need to check on my car because I have hot air from my vents no matter what the temp setting is. Where would this valve be on my 86.5??

dprantl

On all 928's this valve is under the air box just slightly to the passenger side. It is connected with a very short coolant hose directly to the engine block. It is black and white and has a pod with a vacuum line attached to it.

Dan
'91 928GT S/C 475hp/460lb.ft

Pcplod

[QUOTE=dprantl;7713120]On all 928's this valve is under the air box just slightly to the passenger side. It is connected with a very short coolant hose directly to the engine block. It is black and white and has a pod with a vacuum line attached to it.

I know this defaults to the "hot" setting, and others have talked about Zip-tying it in the cold position. If it's in the hot position and you force it to the cold to tie it back, will it damage it? If the valve isn't moving in the first place, does that mean that it is damaged and I should just go ahead and replace it instead of using something to tie it back??

dr bob

The vacuum actuator won't be damaged by tying the valve closed.

If it isn't moving in the first place, the cause is most likely a vacuum problem in one of the other circuits, possibly a vacuum problem with the valve itself. You can quickly diagnose a vacuum problem with the valve itself by using a MitiVac tool directly on the valve. If applying vacuum causes the valve to close, and the valve stays closed without bleding down, at least the vacuum part of the valve is OK.

Jfrahm

Be aware that these valves can be closed but still leak hot water into the heater core.