Air Conditioning works but..

ackuric said:

When traveling at highway speeds (55+), the a/c clutch will not engage, but once I slow down below 45 mph the clutch kicks in and cold air begins to blow, and it will work just fine even while increasing speed back to 55+. Anyone know whats up? Pressure switch? I've tested the r-134a and the pressure is good (40psi)

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40psi measured how? if that is the static pressure, engine off, etc, that is too low. 70 on a cool morning, 100+ if it is really hot out. You might be screwing up the superheat level as too much air thru the condenser can lower pressure to the point that the low-pressure cutout will prevent the thing from kicking on...

ackuric said:

40 psi by standard testing of the low pressure side, engine off, any higher and on the meter I would be 'in danger'.

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No, you are misinterpreting the meter. 30-40 psi is OK when the compressor is _running_. Not when engine is off. You are undercharged. The low side should oscillate between around 22psi and 35 psi when the motor is running, car is cool inside, AC is on recirculate, fan on low. If you run it with the interior hot, or the fan on high, it will likely settle in around 25psi or so and run continuously.

40 psi with motor off is way too low, unless perhaps if you live in alaska and it is winter...

like bob has said... 40psi on the low side with the compressor off is too low... with the compressor running i generally here in florida try to keep them about 32-35ish with a high side pressure anywhere from 200-225ish. when the compressor is off both the high and low sides should equalize to round 75-100 as what bob said... and from the problem you have stated.. i would safely assume from here that.....

1. your system has a leak somewhere..
2. when ur stationary your compressor kicks on and off frequently due to the low side pressure switch detecting too little pressure in the line. but still putting out some cold air.
3. when your driving the compressor kicks on and off so frequently it does have time to work as intended..
4. sounds like you need a new mechanic.

Agree with Bob and Will... around here with the car being sitting in the sun all the time and hot days I got 110 PSI. When it gets below 70 PSI there is a leak...

Bob and Will are right.

Your neighbor may be a "certified" mechanic, but he apparently doesn't know enough about A/C. Don't take this a calling you or him stupid, it may simply be that he specializes in something and has forgotten what he has learned about A/C in the past.

I like to keep my low side in the low 30s/ high 20s when running. If you have 40 when it's not running, the system is low. You need to have an A/C tech check it out and fix the leak before recharging it.

This sounds strange. The only things that will turn off the compressor are high discharge pressure, low suction pressure, WOT... Am I leaving something out? Is there a thermostatic switch on the evaporator (like '68s have) or elsewhere? Is the WOT sensor malfunctioning thus telling the CCRM A/C WOT relay to open?

Hardtop68 said:

This sounds strange. The only things that will turn off the compressor are high discharge pressure, low suction pressure, WOT... Am I leaving something out? Is there a thermostatic switch on the evaporator (like '68s have) or elsewhere? Is the WOT sensor malfunctioning thus telling the CCRM A/C WOT relay to open?

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No, you're pretty much right on with it. High pressure switch, low pressure switch, WOT cutout. I don't recall a thermostatic switch.

The question is why is it cutting out above 55 MPH...I would initially suspect whatever signals WOT to whatever module controls the A/C.

Is it the PCM that controls the compressor, the CCRM or another module?

What tells the module that the engine is at WOT? Logically, the TPS would give that signal. Is the TPS voltage within specs?

It could be that there is a restriction in the high side before the orifice tube causing the high side pressure to be too high and having the compressor turn off by way of the high pressure switch. In order to consider this, you really need the pressures from both the high and low sides. The pressures at idle may reveal if this is the problem.

Just a few thoughts to maybe help you out. If you can get the pressures, both high and low sides, when the issue occurs, it would be easier to determine if it is a problem with the A/C itself or a sensor/electrical issue.

BTW, do you have a P1460 set?

The attachment is from a Ford service manual. It wouldn't let me print to a pdf, so I had to do a screenshot. If you have a WAC circuit fault, this is tell you.

HOOCBB said:

No, you're pretty much right on with it. High pressure switch, low pressure switch, WOT cutout. I don't recall a thermostatic switch.

The question is why is it cutting out above 55 MPH...I would initially suspect whatever signals WOT to whatever module controls the A/C.

Is it the PCM that controls the compressor, the CCRM or another module?

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PCM makes the decision, but uses a relay in the CCRM to turn the compressor off if it chooses. The relay is normally closed, but the PCM can energize the coil and open the circuit.

I still believe it is a charge problem. More air thru the condensor makes it more efficient, and as it gets more efficient, the pressure will drop. You can always just jumper the cycling switch connector and drive it to see if it still cycles on and off as you change speed. If not, then the either the cycling switch is bad, or the charge is low. If it still cycles as you described, can't be the switch so something else is amiss...

What tells the module that the engine is at WOT? Logically, the TPS would give that signal. Is the TPS voltage within specs?

It could be that there is a restriction in the high side before the orifice tube causing the high side pressure to be too high and having the compressor turn off by way of the high pressure switch. In order to consider this, you really need the pressures from both the high and low sides. The pressures at idle may reveal if this is the problem.

Just a few thoughts to maybe help you out. If you can get the pressures, both high and low sides, when the issue occurs, it would be easier to determine if it is a problem with the A/C itself or a sensor/electrical issue.

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Without both pressure readings, this is just guesswork...

My Brother gave me a 30 lb container of 134a and i thought that I could use it to recharge my cars, but it only has about 20-25 PSI. So how do I get the134a into my system. Will I have to buy some kind of Vehicle charging system?

lamothe1 said:

My Brother gave me a 30 lb container of 134a and i thought that I could use it to recharge my cars, but it only has about 20-25 PSI. So how do I get the134a into my system. Will I have to buy some kind of Vehicle charging system?

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You charge thru the low-pressure port on your car's A/C system, but the compressor needs to be running. Often, I have jumpered the cycling switch connector so that the compressor will run long enough to drop the low-pressure side enough to take freon...

25psi sounds mighty low for a 30 pound canister unless the canister is _very_ cold. That is a good pressure for the low-pressure side of your A/C system, the canister should be closer to 70psi normally and up to 100 or so if it is hot. Are you sure your gauge is not screwed up?

BobHyatt said:

You charge thru the low-pressure port on your car's A/C system, but the compressor needs to be running. Often, I have jumpered the cycling switch connector so that the compressor will run long enough to drop the low-pressure side enough to take freon...

25psi sounds mighty low for a 30 pound canister unless the canister is _very_ cold. That is a good pressure for the low-pressure side of your A/C system, the canister should be closer to 70psi normally and up to 100 or so if it is hot. Are you sure your gauge is not screwed up?

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The system will not pull anything from a can that only has 25 PSI unless the system is under vacuum. If the system is running 40 PSI on the low side when running, he will only push some refrigerant out of the system.

I still don't think it is a low charge problem.

HOOCBB said:

The system will not pull anything from a can that only has 25 PSI unless the system is under vacuum. If the system is running 40 PSI on the low side when running, he will only push some refrigerant out of the system.

I still don't think it is a low charge problem.

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First, the normal low pressure side on a mustang is 22-35 psi. 22 when the compressor is on, interior cool, air on recirc, fan on low. Once it reaches around 22 psi the compressor will kick off (the cycling switch does this).

No normal can of freon has 25psi of pressure. If it is _extremely_ cold that might happen, but just put the can in a bowl of hot water and the pressure will ramp right on up to 80-100 quickly. You can charge the system until almost the entire system is liquid if you try hard enough... but it won't be cooling and compressor damage is at hand...

If the low side won't get below 40psi, the compressor is on its last legs and is a replacement candidate. If you lock it on with a jumper 20psi is easy to reach.