Don’t replace a/c compressor — check controls

Hey there, time traveller!
This article was published 15/06/2007 (6730 days ago), so information in it may no longer be current.

QUESTION — Our 2003 Camry’s air conditioner started blinking on and off last year. I took it to the dealership, but by this time the air conditioner began working again. The mechanic told me that a component part was not sending the message for the air conditioner to come on, but they could not check to see which component it was unless it wasn’t working, so I was to bring it back when it quit the next time. It worked fine all the rest of the summer.

This spring, about a month ago, the air conditioner blinked a few times and quit totally. I called my Toyota dealership and after several calls back and forth we were assured it was the compressor and that it had to be replaced for $500 for a reconditioned one. This was diagnosed on the phone. I was in the car yesterday, tried the air conditioner and voila! — it worked and remained working. We feel there are two totally different explanations here and if the compressor is shot, why does it start working again? Is the dealership just guessing? They want to put the compressor in right away, but we are going to postpone this work at least until we get a few more answers. We would appreciate your opinion.

ANSWER — You are wise to delay replacing the compressor and get another opinion. An air-conditioning compressor, just like any other type of compressor, is just a pump. It may fail completely, or it may partially fail and not pump enough volume, but they don’t fail intermittently. I am not sure how they were able to diagnose your system over the phone to be able to narrow it down to a compressor. I suspect you were talking with someone who doesn’t understand air-conditioning operation.

The problem is also not likely a low refrigerant charge, although a low charge will cause the compressor to cycle on and off frequently. If the charge becomes too low, a low-pressure switch on the system prevents compressor clutch activation.

This sounds like a control problem. You don’t mention the model of your Camry or whether the air conditioning is a manual or an automatic system. There are differences in the control wiring for the compressor operation. Manual systems have a switch that provides an input to the engine computer. The engine computer controls a relay that turns on to provide power to the compressor clutch, engaging the compressor.

Automatic systems use an electronic control head. When an a/c mode is selected or the interior temperature sensors indicate the interior is too hot, the electronic control head will send a signal to the engine computer to operate the relay for the compressor. There are also pressure cycling switches and pressure limit switches on the a/c system that can prevent the compressor from operating.

As you can see, there are several components in the a/c system that must work together to engage the compressor. I would look for poor wiring connections first, and then test the compressor relay and pressure cycling switch next, as these components cycle frequently.

QUESTION — We are planning a family vacation and our car will be loaded with camping gear. Should I raise the tire pressure or keep it at what the tire pressure decal says?

ANSWER — The tire pressures listed in the owner’s manual or on the tire pressure decal are a minimum pressure necessary for safe operation of the tires. Some owner’s manuals will also list a higher tire pressure if the vehicle is being driven at higher speeds. Tire pressures are always a compromise between vehicle handling and vehicle ride comfort. With the tire pressure set at recommended levels, you will get the best ride quality. Raise pressures and you will get better handling due to stiffer sidewalls.

Tires do have a maximum inflation pressure listed on the sidewall. Typically, this is 35 psi on many passenger-car tires. As vehicle loads increase, it is wise to increase tire pressures to reduce sidewall flexing. Flexing can cause excessive heat to develop in the tire. However, never exceed the maximum pressure on the tire. If you find the ride is too harsh, or the tire tread is wearing more in the centre than on the shoulders, reduce pressure slightly. I would rather have the pressure in the higher side of the limits to improve handling and stability.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net