Some claim the present sport utility vehicle (SUV) craze had its birth in 1984 when Jeep introduced the Cherokee as a scaled-down version of the old gas-hog Jeep had introduced 10 years earlier. Dumping the carburetor on the 2.5L power-plant in favor of Throttle Body Injection (TBI) on the 1986 model year gave the four-cylinder platform a few more horses. When the 2.8L V6 was replaced by the powerful 4.0L in 1987, Jeep had the most powerful SUV in its class on the market--at least for a while.

As a service technician at a Jeep dealership, I saw more than a few Cherokees with more than 300,000 miles. When it came time for a new vehicle, it was common for Jeep owners to keep their old Jeep and buy a new one. In 1991, the fuel injection system Jeep used on the 4.0L from its inception in 1987 was replaced by Chrysler's JTEC system.

My personal vehicle is a 2001 Cherokee, and for the past 40,000 miles I've had no complaints, other than the air conditioner that doesn't seem to produce the frigid temperatures I had become accustomed to with my Taurus. However, the air conditioning (A/C) is adequate.

One of the four courses I taught this past spring was Heating and Air Conditioning. After finals, one remaining student in the lab was using the front-end machine to align his pickup truck, and Ed, a student from the Industrial Electronics department came by to ask about the A/C on his own 2001 Cherokee.

He said the A/C would cool just fine in the morning, but as the day wore on, the air coming from the registers would sometimes warm up to the point that the SUV was downright uncomfortable. Standing next to the Jeep, I could feel nice frosty air coming from the registers.

With the ambient temperature hovering near 85[degrees]F at the time and the unit blowing cold, I didn't even connect the gauges. I speculated that maybe the electric cooling fan wasn't working all the time. Over the years, I don't know how many cars I saw in the service bay that had been to other shops for an overheating problem that couldn't be duplicated. But when I would check the electric cooling fan using my method, I found that the fan was playing electrical roulette. We all know how that works: The fan would run just fine in the service bay, but the car would overheat the first time the customer stopped for a light.

First, I showed Ed how to check the integrity of any direct current (DC) fan motor by wiring a right bulb in series with the motor: Disconnecting the fan motor connector and hooking a jumper wire between one fan motor terminal and B+, you then connect a test fight between the other fan motor terminal and ground. Next, slowly turn the fan armature 360 degrees while watching the light. The light may pulse slightly, but it should stay illuminated the whole time. You then watch the light to see if it goes out. If it does, the fan motor won't run when it stops on that dark spot. But Ed's fan motor checked out just fine.

I decided to show Ed how the electrical side of the Cherokee A/C system works. It's good practice, especially when you teach for a living.

SWITCHES, WIRES AND SCHEMATICS

While a scan tool could be used to monitor the A/C request status as well as fan operation, I opted to use a multimeter and a wiring map to give Ed a few pointers on how he could check the system himself the next time the concern reared its ugly head. Printing out the schematic on the Jeep's A/C system, I took a multimeter out to the vehicle so Ed and I could study the schematic, check the voltages and watch the system work.

The system is simple and direct, with the PCM controlling the A/C clutch and the secondary engine cooling fan through a pair of relays. The 4.0L package has a belt driven fan as well as an electric cooling fan to assist in case the A/C head pressure goes too high or the engine gets too hot.

To begin with, the Powertrain Control Module (PCM) provides the A/C control head with a 12-volt no-current signal via Circuit C90 from the PCM. This 12-voh signal travels from PCM pin 22 through the low-pressure switch, then in series through the high-pressure switch to the control head. The 12-voh signal from the PCM is shorted to ground by design as soon as the control head is placed in A/C or Defrost mode, which the PCM interprets as an A/C request signal. If either switch opens, the PCM responds by opening the circuit feeding the compressor clutch relay coil and the compressor clutch disengages.

I explained to Ed that any intermittent connection on Circuit C90 could cause the compressor clutch not to engage. Because the easiest test point on that circuit was at the low-pressure switch, I had Ed start the engine. I instructed him to turn the A/C on "Recirculate" mode and the blower motor on low so the compressor would cycle. The suction line was frosty and cool in less than a minute and began forming beads of condensation.

I back-probed the low-pressure switch connector and showed Ed how the 12-volt signal is neutralized by the ground from the control head while the switch is closed and how the voltage returns when the low-pressure switch opens. As evaporator pressure drops below about 25 psi, the low-pressure switch signals the PCM, which opens the compressor relay and kills the compressor clutch coil's magnetic field. This releases the clutch to prevent evaporator icing and/or compressor damage in the case of a low refrigerant/oil charge. Each time our meter showed that signal voltage had jumped to 12 volts, the compressor would disengage. So far, so good.

I further explained to Ed how high pressure in the other side of the system would trigger the slightly more complicated high-pressure switch, which also opens Circuit C90 to pin 22 on the PCM. At the same time, the switch delivers that same C90 ground to pin 13, a signal that the PCM interprets as a radiator fan request. The secondary fan only runs when the PCM determines it is needed, either in response to engine temperature or a ground from the A/C high pressure switch. As Ed's Jeep heated up and the A/C head pressure climbed, we watched the high-pressure switch open Circuit C90, giving us 12 volts once again, and within a second or so, the PCM triggered the cooling fan relay to energize the secondary fan.

DUPLICATION

We watched the A/C signal voltage drop back to zero as the switch closed once again, but the radiator fan kept running and the compressor didn't re-engage. Ed asked why the fan was still running even though the switch had changed states. I explained that the PCM responds to the radiator fan request signal in its own sweet time to make sure enough air has passed the condenser to properly lower the high-side pressure. I pondered the still-disengaged A/C clutch: Was the PCM waiting for a few moments before re-engaging it? When moments stretched into more than a minute, Ed reached in the window and felt warm air blowing from the register. I knew then this was what Ed had been experiencing during the hottest parts of the day.

Removing the A/C relay from the underhood fuse panel, I substituted the starter relay, but the compressor still didn't engage. I checked the relay socket coil terminals: One is hot with the key on, and the other gets its ground from PCM pin 1. I found good battery voltage available to the relay coil, but then I knew that because the relay clicked when I plugged it in. Fetching the test light again, I found good strong current available to the relay. Some instructors may gasp at my use of a test light, but I just don't trust a voltmeter reading on circuits that have to pull a load like the compressor clutch. Moving the test light clip to the positive battery terminal, I checked the cavity that feeds power out to the compressor clutch and found a good solid ground coming all the way back to the relay, presumably through the compressor clutch coil. All the circuits were complete, but we still had no compressor clutch operation.

Plugging the relay almost all the way back in, I moved my test fight dip back to battery ground and touched the terminal feeding the clutch coil. It was getting good, strong 12-volt power, but the clutch still wasn't engaged. Holding my pocket screwdriver near the clutch hub, I felt a strong magnetic field tug the screwdriver against the hub. Bumping the clutch with a larger screwdriver, I saw it snap in and run until the next time it cycled off, after which it once again failed to re-engage. Ed's clutch had worn to the point that the clutch air gap needed to be reset.

I know what some of you are thinking: I could have bumped the clutch to begin with and saved myself a lot of trouble. But I was in teaching mode, thus, my systematic elimination of each circuit. Besides, in a PCM-controlled A/C system with two pressure switches, a couple of relays, several electrical connectors and more than one ground, it's really easy to stumble around looking for something difficult before discovering that the problem was totally mechanical.