jpuzas

Hi Guys,
I took and passed my 609 training and got some R12 to recharge the A/C system in my 1987 928 S4. I hooked up the guages a few days ago before I received the R12 to see what was going on and measured 10psi on the low side and about 90psi on the high side with the car at idle and about 60 deg F outside. A/C not blowing cold just seemed to be about ambient but did not measure.

Today I hooked up the guages and it was not showing pressure on low or high side. I figured that during the process of trying to unhook the low side I could not get the fitting off fast enough and leaked some R12. My assumption was that it might have been enough to kick in the low pressure switch and keep the compressor from kicking on. First tried to charge by gas in the low side with engine on at idle and did not seem to be taking any R12. Made sense if low pressure switch was not allowing compressor to kick on. Turned off the engine and opened the manifold to fill liquid in high side and still does not seem to be taking freon. Purged the lines so I know the R12 is getting to the fittings.

Fans come on when I press the a/c switch and the engine changes rpm slightly but I can't see the compressor turning. Can't figure out why it was working fine two days ago but now can't seem to add R12. Haven't checked any voltages on the low pressure switch yet. Can't find the black wire near the dipstick to start the trace for ensuring power to compressor. Pressures are reading around 60 psi on the high side and around 45 psi on the low side as I have the guages hooked up now with high and low manifold closed and just reading system pressures. If I start the engine the pressures do not change.

Any idea what to do next?

Stromius

What guage setup did you have? Pics?

jpuzas

I have included photos. Blue to low side, Red to high side and I also included a photo of the guage readings.

Bill Ball

Without the engine running, if the car has sat for a while, both high and low sides should have equal pressure. At rest it should be somewhere around 80-90 when fully charged. On a cold day, with the compressor running, the low side could be 10-20, high side 175 or so. Pressures would be higher on a hot day. These are just general numbers to give you an idea of what you should see. The first set of numbers you mention (10/90) would be consistent with a partial charge, compressor running. It's pretty hard to lose all of it out the low side even if you have trouble removing the fitting. And even if you did, when you hook up your Freon source and open the low side valve, it should immediately raise the low side pressure to 50 or so, whether the engine and compessor is running or not, and some will bleed over to the high side as well. If you are seeing no pressure at all with a can of Freon hooked up, you don't have the gauge set hooked up properly or have the valve open on the gauge set or the can is empty. Let's start there. After you figure that out, then you can work on the electrical diagnosis tracing 12V at the freeze switch, low pressure switch and compressor wire.

Bill Ball

OK, so now you are seeing 45/60 regardless of engine running. Like I said, the two sides should equalize over time (half hour at most) when the engine/compressor are not running. So, it seems the gauge set is not accurate, for one thing.

So, I'm not sure which number is correct, but either is probably too low for the low pressure switch to close. You can pull the plug on the low pressure switch and see if there is 12V on one side of the plug when the AC is switched on. If so, then put a jumper wire across the plug leads and see if the compressor kicks on and the gauge pressures change appropriately. If you don't see 12V on either side, you trace back through the freeze switch. If you do see 12V and the compressor still does not come on, you need to find that black compressor wire and test there. You still have to resolve why the gauge set is not reading correctly.

jpuzas

So I guess the conclusion is that the compressor is not kicking in then? Would both sides be equal when the engine is running if the compressor is not engaged? Just seems so wierd that it was working two days ago and then all of sudden. I did not touch anything else but does not mean I did not bump something...

Bill Ball

If both the low side and high side are equal, then the compressor is not running. Yours are slightly unequal. With the engine off the pressures should become equal fairly soon. If they don't, the gauges are innacurate. If they do equalize, and then just go to 45/60 once you start the AC, then that would be a compressor working but not working very well and a low charge. To determine if the compressor is running, you should be able to see the clutch engage and start to spin with the pulley. If the clutch on the front of the pulley never spins, then it is not getting the signal and no compression can happen. With the ignition on and the engine off, if you push the AC button you should be able to hear the compressor clutch "click".

928mac

Bill, if the system is that low, would you not want to evacuate and recharge?

I guess it dose not matter if the system has not been open, and with a site glass it is easy to see how much the system needs.
My system is MT so I was going to evacuate it and see if it will hold the vacuum.

Bill Ball

Yes, ideally you evacuate, check for leaks, and charge by weight, but he does not have the evacuation equipment. So, what he's left with if he wants to do this himself, is a "top-off". The sight glass is one way, ambient temperature-pressure charts (in the WSM) are another. I'm just trying to help him get out of the woods. So far, he has not determined if the clutch is engaging or even can engage and whether the gauge set is working right. I suppose he could resort to the sight glass if the gauge set is innacurate. I like to use both in a top-off.

928mac

BTW Bill, I love your Avatar ha ha, god I have to clean my garage. ha ha

jpuzas

I have verified that the clutch is not kicking in and I cannot see the pulley turning. I guess the first place to start is at the black wire at the dipstick which should have 12v with engine running and a/c button pressed?

Bill Ball

Sure. There are a bunch of ways you could do this. Checking for 12V at the black wire and work back. However, with a low charge, or you don't expect 12V at the black wire, but there should be 12V versus ground on one side of the low pressure switch plug. And if you jumper the plug, then there should be 12V at the black wire. Also, you can check the clutch integrity by applying 12V to the black wire from the nearby positive jump post on the inner fender.

blown 87

Fix your leaks before you throw away some freon, look by your low pressure line, it is oil soaked.

jpuzas

Thanks everyone. It is working fine now. All I needed to do was jumper the low pressure switch. The compressor kicked on and started taking R12. It absorbed 14 oz before I started to see clear fluid in the sight glass. It is not a consistent clear, it goes back and forth between milky white and clear especially if I rev to 2000 rpm and then let off. The center vent temp is 37 deg F. It is 73 def F in my garage today so that is 36 def F temperature differential which is not bad. I think I'm going to apply the "better is the enemy of good enough" on this one and see what happens when it is 90 deg F ambient and go from there.

Thanks guys you all are the best,
Jeff.

jpuzas

Here are some explanations for what I think happened... When I measured the pressures the other night the compressor was working and the gauges were working properly. Between then and today I think when I took off the low pressure side hose enough R12 leaked that it put it in low pressure mode and the compressor did not kick on. On top of that the schrader valve pin depressor on the high side hose fell out so it was not depressing the pin today and not letting any liquid in when I tried charging liquid from the high side(with engine off) to get the low pressure switch to turn on the compressor. So it was still in low pressure mode. When I jumpered the low pressure plug the compressor kicked on and started taking gas from the low side. I then noticed the high side was not showing pressure. The high side was getting pressure through the manifold via the R12 can.