Gauge rises when air conditioner on

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June 27, 2009

Auto Scanner The Salem News Sat Jun 27, 2009, 06:00 AM EDT

Q: I have a problem with my 1999 Mercury Sable that my mechanic can't seem to figure out, as it only happens in the summer. When the outside temperature is above 75 degrees and the air conditioning is on, the car will overheat. The gauge goes three-quarters of the way toward "hot," taking about 15 minutes in city driving and 45 minutes on the highway. Once the car exits the highway, the thermostat will start to rise. When this happens, I would shut the AC off and the temperature gauge would return to normal. Oftentimes, when the AC is turned back on, there will be no cool air and just the fan.

I had the car at the mechanic on a hot day to show him this and let the car idle with AC. The mechanic purposely allowed the car temperature to rise and gauge to go to three-quarters toward hot. The car did not overheat, but the AC shuts itself off.

All the standard maintenance items have been done, such as coolant and transmission flushed, water pump replaced, freon level checked, thermostat replaced, spark plugs and wires replaced.

A: My first thoughts go to the radiator electric fan. Your mechanic should be checking the electric fan on, when the AC head pressure reaches 225 psi. The failure of the fan circuit is the most common problem for the situation you refer to.

The circuit is a little tricky but nothing too crazy. The system consists of the following components: power control module, power control module relay, high speed fan relay, low speed fan relay, 40 amp fuse #9 and a pair of fans. When your computer recognizes an overheat condition, or your AC high pressure sensor goes up to 225, it sends a signal to the relays and gets the fan spinning. The fan main ground wire is located on the right front fender near the coolant overflow bottle. Like I said, there are a lot of components in the system, but it really isn't that complicated.

Q: I have a 1998 Chevy Tahoe LT 2d 4X4 5.7L "350R." I have been the primary mechanic, and it is well-maintained. The air bag Idiot light is on. It has been on constantly for a year or so. Prior to that it was on intermittently, and Chevy could not repair something that was not broken, although they charged for the attempt. What do you think?

A: The air bag system on the Tahoe is about as basic as it gets. There is a module under your seat, an instrument panel switch and module, a clock spring under the steering wheel, and a discriminating sensor under and forward to the radiator. Because of legal liabilities, I cannot tell you any testing procedures in this column. Should the air bag deploy because you probe the wrong wire, there would almost certainly be injuries. I know you are your own mechanic, but this is a sensitive system, and a test light on the wrong wire will deploy either the passenger side or driver side air bag. I really want you to see a professional on this job. Now that the light is on steady, it should not take more than a few hours for diagnostics.

Car Care Tip: Even though the weather is warm, it is still very important to turn off all accessories such as air conditioning, wipers, etc. before turning off your car.

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Larry Rubenstein is a master technician who owns a North Shore service station. His column appears every Saturday. Write to Larry at Salem News, c/o Auto Scanner, 32 Dunham Road, Beverly, MA 01915, or send e-mail to scanauto@aol.com.

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