Cherokee Heat Wave

By Richard McCuistian

Even on a simple system, it’s easy to suspect the electronics when the source of the problem is mechanical.

 

2001 Jeep Cherokee

4.0L Engine

AW4 Transmission

79,524 miles

A/C cools fine until vehicle goes through a hot soak.

When An SUV Isn’t Cool

            Some claim the present Sport Utility Vehicle craze had its birth in 1984 when Jeep introduced the Cherokee as a scaled down version of the old gas hog Jeep had introduced ten years earlier.  Dumping the carburetor on the 2.5L  powerplant in favor of Throttle Body Injection on the1986 model year gave the 4 cylinder platform a few more horses, and when the gutless 2.8L V6 was replaced by the powerful 4.0L in 1987 (the early 4.0L engines were produced for Jeep at a John Deere engine plant), 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 Jeep Cherokees with over three hundred thousand miles showing, and when it came time for a new vehicle, it was the norm for Jeep owners to keep the old Jeep and buy a new one.  The Bendix fuel injection system Jeep used on the 4.0L from its inception in 1987 was replaced in 1991 by Chrysler’s JTEC system.

     My personal vehicle is a 2001 Cherokee, and in the past 130,000 miles I’ve had no complaints, other than the fact that I had to replace a leaking radiator at just over 125,000 and the air conditioner doesn’t seem to produce the frigid temperatures to which I had become accustomed on my ‘95 Taurus.

Why?possibly because of the extra cubic footage in the passenger compartment and all that extra glass, factory tint notwithstanding.  Still, the A/C is adequate.

            One of the four courses I taught this past Spring was Heating and Air Conditioning.  All the students were done with their finals and the one remaining student in the lab was using the front end machine to align the front end on his pickup truck when Ed, a student from the Industrial Electronics Department came by to ask about the air conditioner on his own 2001 Cherokee.  He said the A/C would cool just fine in the morning but as the day wore on, there were times when the air coming from the registers would warm up to the point that the SUV was anything but cool. 

                Standing next to the Jeep, I could feel nice frosty air coming from the registers.  With the ambient temperature hovering near eighty five degrees at the time, and with 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. (The 2.5L Jeep doesn’t have a secondary fan)

First I showed Ed how to check the integrity of any DC fan motor by wiring a light bulb in series with the motor.  Disconnecting the fan motor connector, hooking a jumper wire between one fan motor terminal and B+, then connecting a test light between the other fan motor terminal and ground, I slowly turned the fan armature through 360 degrees while watching the light (it may pulse slightly, but it should stay illuminated the whole time), I watch to see if the light goes dark.  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.

Incidentally, 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, and when I checked the electric cooling fan using my method I found that the fan was playing electrical roulette and 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.

 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 water pump-driven fan as well as an electric cooling fan to assist in cooling if the A/C head pressure goes too high or the engine gets too hot.

To begin with, the PCM provides the A/C control head with a 12 volt no-current signal via Circuit C90 from the PCM.  This12-volt signal travels from PCM pin 22 through the low pressure switch, on through the high pressure switch to the control head (the two switches are wired in series). The 12 volt signal from the PCM is shorted to ground by design as soon as the control head is placed in A/C or Defrost, 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.  Since the easiest test point on that circuit was at the low pressure switch, I had Ed start the engine, put the A/C on Recirculate and the blower motor on low so the compressor would cycle.  The suction line was frosty and cool within a less than a minute and began forming beads of condensation.

  I back probed the Low Pressure Switch connector, touched the voltmeter to my back probe, 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, releasing 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. 

Note: This is a low current 12 volt signal, so you can't read it with a test light.You have to use a digital multimeter (<$30 at the parts store). So far, so good. 

further explained to Ed how high pressure in the other side of the system would trigger the slightly more complicated A/C high pressure switch (see schematic), which also opens Circuit C90 to pin 22 on the PCM but at the same time it delivers that same C90 ground to pin 13, a signal which 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) and 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.

Problem Duplication: Compressor Clutch Offline

Ed and I 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 reengage. 

Ed asked why the fan was still running even though the switch had changed states and 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 high side pressure. But as I pondered the still-disengaged A/C clutch, I wondered at first if the PCM was waiting for a few moments before reengaging it, but when moments stretched into more than a minute, and Ed reached in the window to feel warm air blowing from the register, I knew 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.  Checking 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, but then, I knew that because the relay clicked when I plugged it in. 

Fetching the test light again, (Some instructors may gasp at this point, but I just don’t trust a voltmeter reading on circuits that have to pull a load like the compressor clutch) I found good strong current available to the relay. 

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.

Plugging the relay almost all the way back in, I moved my test light clip 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 (be VERY careful when doing this, else you could lose some health - that compressor pulley is spinning fast enough to throw a screwdriver all the way across the shop if you do it wrong or carelessly), I saw it snap in and run until the next time it cycled off.   Ed’s clutch had worn to the point that the air gap needed to be reset.

Yeah, yeah, I know what some of you guys 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, and several electrical connectors and more than one ground, it’s really easy to stumble around Robin Hood’s barn looking for something difficult before discovering that the problem was totally mechanical. 

The Jeep book calls for an air gap somewhere between 0.016 and 0.031 inch, and the clutch hub has to be removed with a puller.  I called an auto A/C specialty shop and made arrangements for Ed to have the air gap set, and when I saw him later that day he reported that his A/C was working fine.   R.W.M.