Ac clutch won't engauge what am I missing?

[racebird1 0]

I have searched hear and can't find what I am looking for so hear is the question. I have pressure tested the ac system and it held 80 lbs pressure. I evacuated the system to -30" for 1 1/2 hours and put one 12 oz can of r-12 in system and the gauge is showing 80 psi and I can't seem to get the compressor clutch to come on. I have 12 volts to the pressure switch on the receiver / drier. Does this power come in from the micro switch and thermostat or does it go there after the pressure switch. I tried to jump out the PS pins at the connector but still did not come on. Does it need a certain minimum pressure in the system before it will come on and if so what tells it what the min pressure is. I know it still does not have the reccomended amount of r-12 in the system yet but I don't want to put the rest in if something else is wrong. Thanks AC is not my specialty.

[d240zx2 2]

I had the same issue. A connector on the t-stat knob had loosened, hence no power connection. Check all your connections.

[Kurbycar32 41]

If you just told me that your AC clutch wasn't engaging i would say that you didn't have enough refrigerant in the system and your low pressure cutoff switch was disabling the system. You were right to attempt to bypass the cutoff switch for testing ,only leave it engaged for a moment. If after bypassing the pressure switch the clutch doesnt engage you should start by checking voltage at the AC clutch with the switch bypassed. If you have voltage i would assume bad clutch. If you dont have voltage i would move back to the relay and test that. If the relay tests good move back to the AC switch inside the car. you could also go for broke and just fully charge the system, it could just start working without all the diagnostics but you would be betting the value of R12 on that

[racebird1 0]

I got it working Just in time for the current heat wave. It turned out to be a bad relay. I still have a question about the low pressure. Is there a low pressure switch because the manual states that the pressure switch on the drier is to protect the system from high pressure. That switch only has two wires so I don't understand how it can see too high and too low of a pressure. There is no other pressure switch only the thermostat and a micro switch which is only a switch connected to the lever when you turn on the AC. I only have one 14 oz can in the system and the sight glass is full of bubles but the air is ice cold. It's probably the first time anybody was glad to see bubles in there sight glass but at least that means the compressor was working.

[Wade Nelson 2]

The ELECTRICAL switch on the dryer is a low pressure switch to prevent the compressor from engaging if loss of R12 occurs. the brass doo-dad on the dryer is a physical pressure relief switch which will vent R-12 if pressure gets too high.

Most newer cars have both a low AND a high pressure electrical switch.

[280~Master 0]

I miss my R-12 system so much colder than R134.

[racebird1 0]

Even though it is working I am still curious about the pressure switch. The factory manual I have specifically states that the switch is to protect the system from abnormal buildup of high pressure. I can't test the switch to see if it is open or closed at low pressure now because I have freon in the system. According to the wiring it is in series with the Thermostst and the micro on / off switch and all three needs to be closed to send the curent to the relay which then sends power to the Compressor clutch. I know it shouldn't matter now that it is working but I just like knowing exactly how things work.

[Wade Nelson 2]

A good R-134 conversion should provide plenty of cold air. I would suspect you have either air or water contamination in your system. Replace the dryer, pump it down to 10 microns or so, and THEN recharge with the PROPER amount of R134 and I'll bet I never hear you complaining again. Overcharging is worse than undercharging. YOu could start by simply bleeding off some R134 while watching a thermometer stuck in the vent to see if you're overcharged. Let it settle for 3 minutes or so between each bleeding, and put a big floor fan in front of the condensor so you're not just battling an ever hotter engine bay.

Think about it this way --- new cars get plenty of cold out of R134, why shouldn't yours? There's no black magic even if the expansion valve is slightly different.

[hr369 113]

A good R-134 conversion should provide plenty of cold air. I would suspect you have either air or water contamination in your system. Replace the dryer, pump it down to 10 microns or so, and THEN recharge with the PROPER amount of R134 and I'll bet I never hear you complaining again. Overcharging is worse than undercharging. YOu could start by simply bleeding off some R134 while watching a thermometer stuck in the vent to see if you're overcharged. Let it settle for 3 minutes or so between each bleeding, and put a big floor fan in front of the condensor so you're not just battling an ever hotter engine bay.

Think about it this way --- new cars get plenty of cold out of R134, why shouldn't yours? There's no black magic even if the expansion valve is slightly different.

I might add that lots of r134 conversions were done on old worn compressors that still work, but their pressures at idle are not enough to keep the a/c cool. Summer is coming up and so does interest in a/c. If and when you retrofit to r134 with the original 30+ year old parts, do not expect new car performance from the ole a/c parts.

[racebird1 0]

Well the AC stopped working I pressure tested the system before evacuating and charging. I charged the system and after a few days I believe the freon is low. The clutch still engauges but no cool air now. I did not replace the cap after I charged the system because I knew I was going add more freon. The question is, will it leak from the low side valve if you don't put the cap back on? Is the cap there to seal the valve?

[Willoughby Z 1]

Well the AC stopped working I pressure tested the system before evacuating and charging. I charged the system and after a few days I believe the freon is low. The clutch still engauges but no cool air now. I did not replace the cap after I charged the system because I knew I was going add more freon. The question is, will it leak from the low side valve if you don't put the cap back on? Is the cap there to seal the valve?

The Shrader valve holds pressure (if it's any good). The cap just prevents dirt from gumming up the valve. I've removed a cap and heard a little "pssst", indicating a valve leak. The cap, if snug, would hold a little pressure but not much.

[ZCurves 4]

Racebird - since the A/C is not functioning, get yourself a replacement set of Shrader Valve's, evacuate the system and install them. Check the pressure switch on the Drier and make sure that it is functioning. With the gauges attached and both sides open, repeat your previous evac to -30 kPa and let it sit for 10 mins. Evac again and watch the needle for 30 mins to an hour. Recharge to 10:1 ratio - usually 2 1/2 cans. If the gauges are reading crazy high on the HIGH (red) side like over 250, then you have a blockage in the system (e.g. bad drier or expansion valve). Conversely, the LOW side (Blue/Suction) should be pulling hard (watch the low side of the gauge drop). If not - then again there is a blockage somewhere. If you get a charge, use soapy water to check the Shrader Valve's, pressure switch and connections for leaks (bubbles).

[sblake01 23]

The Shrader valve holds pressure (if it's any good). The cap just prevents dirt from gumming up the valve. I've removed a cap and heard a little "pssst", indicating a valve leak. The cap, if snug, would hold a little pressure but not much.

I've actually had caps with good o rings hold the pressure that would have otherwise escaped through a leaky valve. ZCurves, how did you arrive at the 10 to 1 ratio? Generally, low side should be about 40°+/- 5, and 10 times that for the high side would be way too high.

[ZCurves 4]

SBlake01,

I agree that 400lbs Highside would be crazy. When I got my certification, we were taught that a 10:1 Compression Ratio is a good "rule of thumb" - but then the FSM contradicts that with the 40lbs Lowside instruction. The lower the CR, the more efficient a compressor will be. The '78 FSM provides an excellent chart of ratios on AC-56 - none are exact 10:1 CR's. Looking at my notes for my Z, I was actually running 235/35 or 7:1.

[sblake01 23]

I believe that would be the 'Performance Chart' on AC-57. The reason I say that is beacuse I have that page printed out and on the wall above my workbench. It along with the chart for the 810 I used to own weren't in the database I use. Anyway, pressures are only a part of the story, however, and should only be used as a general guideline. That chart also takes humidity and ambient temperature into consideration; factors which are as important if not more so than suction/discharge pressures. I use a charging machine which has a thermometer and a hygrometer on it and I charge by the recommmended system refrigerant weight rather than pressure. When I do put gauges on the system after charging it with the machine, the pressures are within the figures shown on that chart or the chart for the particular vehicle's sytem I'm working on. Using the machine it's 'one and done' while charging with cans or even refrigerant cylinders and gauges can be somewhat trial and error or hit and miss if you don't have a way to factor in temperature and humidity.

Edited July 12, 2012 by sblake01

[ZCurves 4]

That is good information, now if I only had a charging machine....