84 911 AC erratic cooling

[rwb6833]

Our 1984 911 was converted to 134 about 4 years ago. Recently had leaks in the system fixed and it seems to be working 'most of the time.' Had the car out last weekend and the AC worked fine for 20 min then it felt like the cool air stopped. Maybe 20 min later the cool air came back on (blower and cool nobs on full the whole time). I'm not sire if it it just a loose connection somewhere or some other issue. Your guidance in getting to the source of this issue will be greatly appreciated.

[uwanna]

Originally Posted by rwb6833

Our 1984 911 was converted to 134 about 4 years ago. Recently had leaks in the system fixed and it seems to be working 'most of the time.' Had the car out last weekend and the AC worked fine for 20 min then it felt like the cool air stopped. Maybe 20 min later the cool air came back on (blower and cool nobs on full the whole time). I'm not sire if it it just a loose connection somewhere or some other issue. Your guidance in getting to the source of this issue will be greatly appreciated.

Your evaporator was freezing up and then thawing causing the on again
off again problem. Dialing back a bit on the temp knob should solve your problem. Give it a try.

[Quicksilver]

A common "The cooling went away" issue on most any car is the magnetic clutch wearing so the gap is too wide for the magnetic clutch to reliably engage.

The way to test is wait until you feel the AC is starting to blow warm and quickly pull over and look under the hood. If the clutch on the compressor isn't engaged give the corner of the clutch a sharp rap with something solid and see if it instantly engages. If it does it is probably an excessive clutch gap.
When the engine is off check the gap with a feeler gauge. Most compressors seem to want a .020" to .030" clutch gap. It seems that they stop working consistently at .045" or more.

Repair is easy. Lock up the clutch by turning the key on without starting the engine and turn the AC on so the clutch engages. (You might have to tap it again.) Remove the bolt in the center of the pulley and pull it off. There should be a stack of washers under the pulley. Remove washers of the correct thickness to bring the clutch gap back in spec.

Whole thing takes less then 10 minutes, is easy, and it is free.

[rwb6833]

Thanks for the replies. I'll give both a try.

[86 911 Targa]

All of the above suggestions are good.

Tell us more about what was found
during the leak repairs.

G.

[GH85Carrera]

Evaporator freezing usually has a funky old socks odor associated with it. Did you notice a funny of "off" odor. If so, yea, it was the evaporator freezing. Common with low charge pressure, or the fan not at max.

For years I had to run mine temp knob at a 1/8 of a turn or so, backed off to prevent freeze up with a properly charged system. I recently moved the probe to a better location in the evaporator, and now it cycles off and on even on a 95 degree day with high humidity. The air temp gets as cold as possible without freeze up, and it cycles off the compressor. That only happens when I have been driving for a while.

[rwb6833]

I worked thru both of the suggestions and no luck.

Now the issue has progressed to where the compressor clutch does not engage. I'm not sure what voltage should be at the clutch but my multimeter showed 0.334 at the 20V setting with the fan and temp switches at max.

[mtb001]

might still be dealing with refrigerant leak. Low pressure will cause the evap to freeze up, once pressure in system is low enough, the pressure switch will open, no voltage to compressor clutch

[kuehl]

Originally Posted by rwb6833

I'm not sure what voltage should be at the clutch but my multimeter showed 0.334 at the 20V setting with the fan and temp switches at max.

You brought up an good point, if the comp clutch is not pulling in the system probably won't feel cold. Let's check the basics.

Battery in fully charged state. Engine off, car parked on flat ground, E brake on. Front trunk open and rear engine deck lid open. Turn ign key to accessory on position. Turn thermostat to max cold (fully CW). Turn Evap Fan speed switch to your first speed (slow; so you can hear things).

1) What is the battery voltage between its pos and neg posts?
2) The front condenser blower motor should be running. Is it?
3) The compressor clutch plate should be pulled in. Is it?
4) Unplug compressor clutch wire and check voltage between the
supply side and the compressor body. What is it?
5) And, check the resistance of the compressor clutch coil to compressor body.
What is it?

[rwb6833]

Here are some more test results per the following points:

1) What is the battery voltage between its pos and neg posts?
12.35
2) The front condenser blower motor should be running. Is it?
NO
3) The compressor clutch plate should be pulled in. Is it?
Appears to be but not sure
4) Unplug compressor clutch wire and check voltage between the
supply side and the compressor body. What is it?
When I did this the front condenser blower came on full.
The voltage was 11.47
5) And, check the resistance of the compressor clutch coil to compressor body.
What is it?
4.7 @ the 200 setting

[kuehl]

Originally Posted by rwb6833

Here are some more test results per the following points:

1) What is the battery voltage between its pos and neg posts?
12.35
2) The front condenser blower motor should be running. Is it?
NO
3) The compressor clutch plate should be pulled in. Is it?
Appears to be but not sure
4) Unplug compressor clutch wire and check voltage between the
supply side and the compressor body. What is it?
When I did this the front condenser blower came on full.
The voltage was 11.47
5) And, check the resistance of the compressor clutch coil to compressor body.
What is it?
4.7 @ the 200 setting

Battery voltage is okay.
11.47 volts at the clutch feed wire is expected due to voltage
drop for a long run of wire. Okay.
Resistance at the clutch coil is not so bad, depends upon you meter,
2.8 to 3.5 is expected.

The front condenser blower motor should be running. Is it? NO
The front condenser blower motor should come on when the contacts in the thermostat "make" (come together); provided your knob is turned fully clockwise to right and the evap core is not extremely cold. Power to the compressor clutch also runs through the thermostat contacts, hence they see a lot of load and wear over time.
================================================

New tests:

1) Unplug the front condenser blower motor connector. At the motor side of the plug apply 12 volts to the motors male spade red wire, and ground to the brown male spade wire. You can run test leads off your battery. If the motor spins you are okay to do the next check. If the motor does not spin replace the blower motor.

2) If the motor spins check your Front Condenser Blower motor relay in the fuse panel. (if you do not have the schematic card that shows which relays and fuses are for what you might find one in a search here on the forum or google-image it).
You should have power coming out of the relay's white/green wire that leads to the connector you were testing at. You should have power coming into the relay from a red solid wire. When the thermostat contacts 'make' or come together you should have power coming to the solid green wire at the relay to engage it. Run the Tests.
If you don't have power coming from the solid green relay, chances are you have issues at your thermostat, just buy a new one. If you do have power coming from t
the solid green relay, check the spade terminals on your thermostat further below.

3) If you had power at the solid green wire mentioned above, open up the center console. Locate the terminals on the back of the thermostat (be careful working with and around the thermostat, if you put a kink in that aluminum tube or break it then you will need to buy a new thermostat). The female spade connectors are designed to fit rather snug, you will want to use needle-nose pliers to work with them. When the ignition key is on 'accessory' or the engine is running, and you turn on the fan speed switch, power should be coming from green/white wire (colors are gonna look difficult to see) which comes from the fan speed switch. Check your power lead coming into the thermostat. So, at this juncture, if you are seeing power coming into the thermostat and power coming out, it could be you are getting voltage reading, however, the contacts in the thermostat are corroded or shot, or even the contacts in the fan speed switch are corroded or shot as well; both thermostat and the fan speed switch contacts handle a lot of amperages; you are seeing voltage but not enough current flow. If this section of the testing grammar or directions seems vague it is because I need a liquid lunch.

[rwb6833]

Before I undertake these tests, a short question: If the compressor is not engaged, will the front condenser blower be running?

I know the front blower works because when I did test #4 above (disconnect the power to the compressor) the blower ran.

I'm inclined to think the issue is the simplest one - there is still a leak in the system and there is not sufficient refrigerant pressure to allow the compressor to operate.

[kuehl]

Originally Posted by rwb6833

I'm inclined to think the issue is the simplest one - there is still a leak in the system and there is not sufficient refrigerant pressure to allow the compressor to operate.

Oh, you have a pressure switch. Have you checked for continuity across the switch?
Power to the switch and power coming out of the switch?

And, "If the compressor is not engaged, will the front condenser blower be running?". Both compressor clutch and front condenser blower motor relay are turned on and off by AC Thermostat. And, if you have had a refrigerant pressure switch added to the system, usually it will only control the compressor clutch; depends upon the circuit design.