A/C gauge readings - GURU needed

[Spittybug]

I need a little reality check here please. I'm still R-12 and less than 5 years ago I had the compressor, dryer, evaporator, orifice tube and at least one of the long hoses replaced at DMCH in what was a horribly expensive ordeal at a time when they were having service manager/mechanic "issues" that have since been resolved.

I've used a sniffer and can't conclusively find any leak. I put some gauges on today and found the following:

• Ambient air temp 80* F
• Vent temp 69* (spec is to be 34 - 38 at this ambient temp)
• Car RPM ~2,000
• Low side pressure cycled between ~24 psi when the compressor was engaged to ~45 when disengaged
• High side pressure cycled between ~125 when compressor was off, to ~170 when engaged
• Compressor was cycling on/off every 4 seconds or so
• With engine off, both side equilibrate to ~62

Reading our manual, I think I understand that at 80* ambient, when the "clutch lets out" (disengages), the low side reading should be 25 - 32 and the the high side 145 - 190. I believe this means that my low side is too high (45 when clutch cuts out, spec is 25 - 32) and my high side is OK (170 when clutch lets out, spec is 145 - 190). Have I got this right?

A little googling and I found this: "High low side, normal high side. Expansion valve stuck open. Hot water valve (heater control valve) trickling hot water into heater core causing AC plenum to get warm. Open seam on evaporator case drawing hot air from engine compartment. Bad condenser fan. Bad fan clutch. Too much oil in system. Possible overcharge and what appears to be a normal high side is actually too high."

Talk about being anything under the sun! I can rule out bad fan, bad clutch and open seam. I'm going to *assume* that DMCH didn't put too much oil in the system, so rule that out too. I'd really hate to hear about the orifice tube being the problem since it was that act of changing that out that resulted in DMCH breaking it and having to totally replace it and the evaporator! System hasn't been open to air since then.

So, what do I do next? I'm assuming that the high frequency of the cycling is a function of these pressures, not vice-versa. Thanks.

[David T]

Without being able to see the car I am going to make a lot of assumptions but my guess is you have air in the system, you do not have enough R-12, and the low pressure switch needs to be adjusted. A "sniffer" is not very effective on cars. I use leak detector which is just soap water with glycerin. "Bubbles mean troubles".
Make sure of the following:
The fan is on high
Both radiator fans are blowing and in the right direction
The evap coil and the cond coil are both clean
Mode switch is on A/C, not MAX.
The fans are cycling on the A/C pressure switch NOT the circuit breaker. If I am correct you have to evacuate the system, vacuum it down and refill. Once you get it running you have to adjust the low pressure switch.
f
David Teitelbaum



QUOTE=Spittybug;52043]I need a little reality check here please. I'm still R-12 and less than 5 years ago I had the compressor, dryer, evaporator, orifice tube and at least one of the long hoses replaced at DMCH in what was a horribly expensive ordeal at a time when they were having service manager/mechanic "issues" that have since been resolved.

I've-versa. Thanks.[/QUOTE]

[Bitsyncmaster]

Your high side seems a little low along with the short cycle. Makes me think your just low on R12.

Spray some leak detector in the service valves. That's the most common leak area I've found.

[Spittybug]

I knew the "guru" word would suck you in David .

Fan is on "3". Radiator fans are indeed both blowing correct direction. The condenser coil is certainly clean, and the evaporator coil "should" be clean since it was new. Independent of this, I had a mud dauber clog my drain line so I flush it by spraying water up the tube and from above by removing the air blower and using a pump sprayer with water (thus getting some on the condenser coil I'm sure). Not much debris at all, basically just a few mud particles. I did have the A/C set to MAX, not norm, but doesn't that simply change the input source from external to internal? How do I confirm that it is cycling with the pressure switch and not the circuit breaker?

On my car, I have a switch attached to the dryer and one attached to the condenser at the front of the car. The one at the dryer is the low pressure one, right? It would appear to be the one causing the cycling as I sense it "clicking" when I touch it. It is not apparent that it can be adjusted however.

"Evacuate, vacuum down and refill" = take to shop if I don't want to release R-12. As a SCIENTIST, I realize that every ounce of R-12 ever produced will ultimately find its way into the atmosphere anyway, but as a law abiding citizen, I would never deliberately vent it............... But, given that I still have significant positive pressure and the A/C has worked well since the last time it was worked on, how can there be air in it?

Would any pictures be helpful?

[DMCMW Dave]

The larger one screwed to the bottle is the low pressure switch. The other one is the high pressure switch. We see the low pressure switched leaking quite often. If it's not relatively new, change it. I have seen a high pressure switch leak as well, but not very common.

Agree on the service valves - we change them (the shrader cores) every time we open a system since we see them leaking so often. Cheap insurance.

To adjust the low pressure switch remove the electrical plug, the adjuster is between the electrical leads. You have to turn it with pliers, it is an inverse head screw. Don't mess with that much, if you go to crazy with it you can get the system trying to pull the low side too low. Counterclockwise is lower pressure.

Personally I don't think having it on max or norm should make a difference, especially if you are doing all this with the doors open. The main difference is that you are pulling air from inside the car rather than outside, but MAX also turns off the hot water to the heater core and that can make a slight difference.

Run the blower on 4.

[David T]

You sucked me in! Anyway the service valves do leak. It is good practice to change them as a regular part of service but I also suggest you throw away those cute plastic caps and get some brass ones that not only will keep the dirt out but will also seal in the refrigerant. I have seen the low pressure switch with a slot to use a screwdriver and a bump to use a plier. If you are cycling on the high pressure switch that is telling you the head pressures are WAY too high and the low pressure switch is WAY out of adjustment. To verify jump the switch with a paperclip but be careful not to let it get away from you and overpressureize the high side. You will pop the relief valve. The only way to get the air out (and measure in a fill) is to take all of the refrigerant out. High head pressures are a sure sign of air assuming you have proper flow across the condenser coil and you are not overfilled with refrigerant. Other less likely possibilities include moisture freezing up and plugging the orifice tube, some other kind of blockage in the orifice tube or high side lines. Very bad to run at such high head pressures. You will eventually burn the oil and that will lead to compressor failure.
David Teitelbaum

[Ron]

$.02

At 80F ambient, it should equilibrate to 84.2 (that's with the vehicle at ambient temp), so, with the other info, I would also guess that the charge is low. I agree there should be no air in the system with the given history. It also seems unlikely that there is moisture in the system for the same reason and that it would normally collect (and ice) at the orifice tube causing the compressor to pull the low side down faster than designed making it too low, not high.

Don't worry about releasing the R12 if you go to a good shop. Their machine can pull it out, tell you how much was in there, whether it was contaminated, put it back in, and top it off, if needed, without loosing any (...well a little to purge the lines...into a tank of coarse ;-) This is your best route IMHO...simply answers a lot of unknowns.

[DMCMW Dave]

One other comment - you can replace the low and high pressure switches without purging the system (they are both over shrader valves, although in my experience the valves tend to be leaky so work fast!). The main service shraders can be changed without dumping the system but you need a special tool made for the purpose.

[Ron]

+1

...The main service shraders can be changed without dumping the system but you need a special tool made for the purpose.

...I swear the dang engineers' relatives are in the tool industry...mine reminds me of the last bite in the arse for a tool to pull 3 Valve Triton spark poles with ;-)

[Spittybug]

Lots of good responses, thanks. Looking at them all, it would appear that the best course of action is to buy new switches and service ports. Take it to a guy with a machine, suck out the R-12, change the switches and the ports, pull a vacuum and recharge the system. Sound right?

Since the pressure switches sit on shrader valves, I could go ahead and replace them (if the service valves don't show leakage) without sucking out the system by the sound of things. Since the sniffer has been less than conclusive, I'll test with soapy water and then see which way I need to go.

Thanks guys.