Today's project is a 1999 Ford Escort ZX2 with 2.0 vin 3 engine and with automatic transmission. The complaint is that the A/C is not cold. The blower works. The air flow is good and from the proper vents.
One of the other techs here in the shop has made some initial checks and informs me that there is sufficient refrigerant in the system but the compressor clutch does not engage. With that information I run a simple test on the drive into the shop. I make sure the A/C is turned off before I start the car and on the way into shop turn the A/C on. I'm listening for the radiator fan to run. It does run. The radiator fan is turned on when I turn the A/C on. Why is that important? I'll get to that.
On this car (and on most cars) you aren't actually turning the air conditioning on when you turn the switch on. You are sending a signal to one of the control modules "requesting" the A/C on. Under normal conditions the control module "sees" the request, checks that conditions are met to allow air conditioning to operate and THEN operates the air conditioning system. If the control module sees a condition that isn't within specifications, such as a low refrigerant condition, it won't command the air conditioning on.
The powertrain control module is the module used to control the compressor and fan systems on this car. When the PCM sees a request voltage at its connector pin 41 and it sees no reason not to run the air conditioning it should do two things that are important to this diagnosis. It should command the compressor clutch on and it *SHOULD COMMAND THE COOLING FAN ON*. The fact that the fan came on with the A/C switch turned on means that the control switch works and that the PCM sees no reason not to turn the compressor on. It even verifies the pressure switches are fine because the request voltage from the switch must pass through the low pressure switch and high pressure switch on the way to pin 41 of the PCM.
Since I know the problem is on the PCM output to compressor clutch side of the system the best place to do some tests is at the constant control relay module. The relay module is a true black box containing several relays and mounted directly under the air cleaner housing at the left inner fender. The PCM may be the one making commands but the constant control relay module is doing the work.
One of the other techs here in the shop has made some initial checks and informs me that there is sufficient refrigerant in the system but the compressor clutch does not engage. With that information I run a simple test on the drive into the shop. I make sure the A/C is turned off before I start the car and on the way into shop turn the A/C on. I'm listening for the radiator fan to run. It does run. The radiator fan is turned on when I turn the A/C on. Why is that important? I'll get to that.
On this car (and on most cars) you aren't actually turning the air conditioning on when you turn the switch on. You are sending a signal to one of the control modules "requesting" the A/C on. Under normal conditions the control module "sees" the request, checks that conditions are met to allow air conditioning to operate and THEN operates the air conditioning system. If the control module sees a condition that isn't within specifications, such as a low refrigerant condition, it won't command the air conditioning on.
The powertrain control module is the module used to control the compressor and fan systems on this car. When the PCM sees a request voltage at its connector pin 41 and it sees no reason not to run the air conditioning it should do two things that are important to this diagnosis. It should command the compressor clutch on and it *SHOULD COMMAND THE COOLING FAN ON*. The fact that the fan came on with the A/C switch turned on means that the control switch works and that the PCM sees no reason not to turn the compressor on. It even verifies the pressure switches are fine because the request voltage from the switch must pass through the low pressure switch and high pressure switch on the way to pin 41 of the PCM.
Since I know the problem is on the PCM output to compressor clutch side of the system the best place to do some tests is at the constant control relay module. The relay module is a true black box containing several relays and mounted directly under the air cleaner housing at the left inner fender. The PCM may be the one making commands but the constant control relay module is doing the work.
Pin 21 of the CCRM is the power feed to the clutch relay inside the module. It should be powered with the key on. This is easily checked with a circuit tester and there is power to the relay. When the relay is closed the power feed to pin 21 should be fed to pin 23 and on to the compressor clutch. When I apply power to pin 23 the clutch does work so I know the wiring to the compressor is good. Since I have power to the relay and know that the clutch will work with power then I know that the relay is not closing. To close the relay needs a ground signal at pin 22. This signal should come from the PCM. It is the "command" for the compressor clutch to operate.
The coil side of the relay is provided power by the PCM power relay. This means that with the engine running and the A/C switch off there should be voltage at pin 22 of the CCRM. With the A/C on and the PCM commanding the relay closed there should be a ground at pin 22. With the Power Probe circuit tester I find I have everything I need at the relay to make the relay close. The relay has failed. Since the relay is integral to the CCRM the entire module should be replaced.
