"Are you the new auto mechanics instructor? I have a problem ..."
If you've been reading my articles, you'll know that I recently took a teaching job at a local technical college. As I brought my toolbox home and parked it in the garage, I was concerned that I was going to be out of the field, out of the service bay and out of the loop. While I had been on the cutting edge of servicing some of the more interesting electronics systems at the Ford dealership, my job as a college instructor would be more research, more paperwork and less actual troubleshooting. Well, I haven't been quite as frustrated and disappointed as I had thought; I've actually run into some pretty interesting situations.For instance, there was the "Saab" story. An upholstery student came walking into my transmission lab one day while I had my automotive mechanics students knee-deep in automatic transmissions. Her turbocharged '88 Saab, it seemed, was giving her intermittent no-start problems. I managed to trace the cause of her no-start to a pressure switch that was intermittently open.
The pressure switch was designed to kill the fuel pump and the injectors in the event of a turbo overboost condition. While I had located the switch on the schematic of the Saab's fuel control system, finding it on the actual vehicle was something of a challenge. It turned out to be above the left kick panel (Saab's shop manual calls the kick panel a "fascia," which confused me into thinking it was out near the headlamp somewhere). I finally found the switch on the vehicle by following hoses from the manifold and managed to verify that it was intermittently faulty and was the cause of the no-start.When the student returned, I showed her where the switch was located and where to apply the jumper wire if and when the no- start repeated itself. She assured me that her boyfriend would replace the switch.
Instead of letting the mechanic/ boyfriend replace the switch, her mother decided to take the Saab to an authorized dealer, where they replaced about $2,500 worth of hardware - fuel pump, PCM, etc. - trying to straighten out the no-start, which they still hadn't managed to fix. The student told them via a long distance phone call to replace the overboost switch and even told them where to find it. They still hadn't found the switch the last time I heard from her, which they vehemently claimed was located out near the headlamp somewhere.
The little Geo A/C problem
The school has a policy that only allows me to work on vehicles belonging to students of the school or faculty members, basically (and understandably) to prevent my department from competing with local repair shops. The student brought in her black '96 Geo Tracker complaining that its air conditioning (A/C) system was inopertive. The paperwork was filled out, and the work order was in my hand when we started the job.What refrigerant is in there?
Before I connect the school's $4,500 refrigerant recycler to any vehicle, I hook up my refrigerant analyzer. Having hydrocarbon-based refrigerant or some other nasty blend contaminate the recently purchased machine would be a dreadful mistake. Well, the analyzer failed the Geo; it detected a blend of 92 percent R134a and 8 percent R12. My recycler wasn't coming near this baby, and I don't yet have a dedicated machine set up to recover blends. It had been outfitted with R134a at the factory, as would be commensurate with a '96 model vehicle.Chasing sparks
I fired up the compressor for a minute or two by energizing the A/C clutch field coil directly, and the A/C cooled surprisingly well. Removing my jumper from the field wire, I pulled up the wiring schematic of the Tracker's A/C system and found that the compressor clutch relay coil is controlled by a nifty little electronic amplifier mounted behind the glove box. The relay coil receives its power from the heater fuse through the "dual pressure switch," so first I wanted to know if there was power available.Having found power at the switch, the students and I next checked for power at the A/C relay, which is mounted in the rear passenger side corner of the engine compartment. We got a nice, healthy glow on the test light bulb.
With the test light clip grounded (as it had been in our two previous tests), we found that touching the test light probe to the blue wire on the opposite side of the relay coil provided a ground path through our test light bulb. This energized the relay and fired up the compressor clutch field coil. By the way, the blue wire on the relay coil was supposed to be pink according to the schematic we were using.
The 'Black Box'
It was time to move into the passenger compartment and check the various circuits feeding the "A/C amplifier." This little 'Black Box' is located behind and below the glove box, and it has a 12-wire connector which sports only nine wires.In the schematic, you'll notice some important items such as the Evaporator Thermistor, which from the appearance of a nice new white connector and the dust-free lower evaporator case housing, appeared to have already been replaced. This thermistor signals the amplifier to prevent evaporator icing.
The thermistor resistance measured out at about 1,500 ohms at the time we checked it, and rather than digging for the spec on that sensor right then, we decided to check the rest of the wires to the module. There is an Engine Coolant Temperature sensor which signals the A/C amplifier by breaking a ground if the engine temperature rises above 235°F, but the yellow/blue wire at pin 12 showed a solid ground when we checked it. We also had a solid ground at pin 9, and a good strong system switched ignition voltage at pin 8. When we touched our grounded test light probe to pin 7 (which, by the way, was pink at the amplifier), we heard the A/C clutch click online as the A/C relay contacts snapped together. The circuit from the amplifier to the relay was good, but why wouldn't it turn the relay on? When we checked the A/C request signal, we found the answer. There was no 12-volt signal available at pin 11. When we fed a 12-volt jumper into pin 11, the amplifier energized the relay and the A/C came alive.Now for the mystery ...
I looked at the A/C control panel, expecting to find the A/C button I saw pictured on the schematic. It wasn't there, but a nice plastic factory-style dummy plug was in its spot.Looking around through the glove box opening, we located the three-pin connector which was supposed to make contact with the switch. There was a ground for the indicator light on the black wire and available system power at the pink/black wire. When we jumpered that wire with the blue one, pin 11 received our jumpered signal, and the A/C woke up once more. Thinking we had missed something or had incomplete information, I called a friend of mine who works at a local Chevy dealer. He researched the Tracker A/C system, checking previous and later model years in the books. He and I agreed on one crucial point: The A/C simply could not have worked without that switch. A call to another Chevy dealer produced the same conclusions.
Yet, when I spoke with the student again, she insisted that the A/C had worked just fine the previous summer. Who knows how? I priced the switch at the Chevy dealer, and it was $65 - too rich for a college girl's budget. We installed a nice tight jumper from the pink/black wire to the blue wire feeding pin 11. Now she has frosty, cool A/C any time she turns on the fan.
