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lou's garage

Right on cue with the warmer weather are frantic phone calls from customers who have just turned on their vehicle’s air conditioning for the first time this season, only to find warm air only blowing from their vents. Most don’t care about the details of how their system functions, just that it works. However, having some basic knowledge of the key part names and what function they perform goes a long way when trying to understand the costs associated with an air conditioning repair bill. Over the next couple of weeks I’ll discuss the basics.

Your air conditioning works because the refrigerant running through the system changes state between liquid and gas and then back to liquid again. When changing state from a liquid to a gas it absorbs heat, which then can be moved away from one point to another where it can be dispersed.

The key components of your air conditioning system are the compressor, evaporator, condenser, dryer and expansion valve. The first three are the most commonly repaired so I will focus on those today.

The compressor is the pump that moves the refrigerant through the system. On most gas-powered vehicles, the compressor uses a belt connected to the engine to drive it. The compressor also has a secondary part attached to it called a clutch, which engages and disengages when commanded to do so by the vehicle’s computer. The compressor can fail in two ways, the simplest being the clutch ceasing to operate which results in a compressor that does not turn. Far worse is a compressor failure where internal components break apart and then are showered into the rest of the air conditioner. This is usually caused by lack of lubrication, more on this later. This kind of failure will require the compressor to be replaced along with several other components that may have been damaged by compressor debris or shrapnel. A nasty repair bill awaits if this occurs.

The evaporator resides behind your vehicle’s dashboard. Evaporators are common to leakage on some cars while other manufacturers rarely have an issue. Debris from being parked under a tree can work its way down into the evaporator insulated casings causing many different problems and musty smells. Regularly lift your hood and remove any accumulated debris sitting on the cowling before it migrates down. Servicing the evaporator in most cases requires complete removal of the dashboard and heater box. While the part is generally inexpensive, labour times are typically more than 10 hours.

The condenser is the part that looks like a small thin radiator and sits in front of the actual radiator. It is the device that exchanges heat back to the outside environment. Sitting directly in front of your radiator unfortunately makes it vulnerable to stones and debris thrown up from the road. A direct impact to the condenser may result in a small hole allowing all the refrigerant to leak out.

AC part 2 next week.


Your automotive questions answered

My long-gone 1998 Volvo V70 required mid-grade fuel but could run on Regular.

After experimenting for a year, I concluded mid-grade provided proportionally more mileage – equal to the cost difference – while maintaining maximum reserve power. So, regular fuel did not save money. One could prove this with merely a tank or two with attentive fuel use tracking.

Dwight A.

Octane rating is a measurement of antiknock properties within gasoline. A higher number indicates a fuel that is less likely to pre-ignite. As you have revealed Dwight, all vehicles that are designed to run on premium can indeed function with regular fuel. An engine that requires premium fuel is typically designed with a slightly higher compression ratio and advanced ignition timing properties. The use of the correct fuel merely allows the performance characteristics to be fully realized as engineered. The onboard Powertrain Control Module (PCM) relies on the knock sensor(s) to detect pre-ignition ping or knock and approximate fuel grade. When a lower-grade fuel is used, the PCM will simply limit ignition timing and other performance characteristics.

I previously enjoyed an Audi S4 that required premium fuel. Whenever I did not use the correct fuel, I could feel a very minor difference under moderate to heavy acceleration, but no difference in fuel economy. Perhaps if I had monitored it as thoroughly as you did, I might have observed a difference in fuel economy, but I am doubtful. It had always used excessive amounts of fuel regardless of grade and yes, it also always managed to put a smile on my face. If I still owned it now however, I imagine it would be parked most of the time because of the current fuel prices.


My wife’s car runs way better on premium fuel. It doesn’t matter if I change the rotor, the plugs or the high-tension leads. I switched to the 87-octane thinking the new parts would assist in the transition. Plus, I switched to lower our fuel costs. Wrong. The car ran a little rougher and seemed to lack the punch. Additionally, with a tank of 87-octane gas the “Check Engine” light came on and did so every time I used it. I’d have the car checked via a computer: no issues. So, I switched back to 94-octane gas and the car ran better and the check light went off and stayed off. I’d have the car professionally set-up and the “new set-up” would only accept premium fuel. I just don’t have an explanation for it.

Jeffrey K.

Because your wife’s car features a distributor rotor and high-tension leads, I am going to assume that it is slightly older. To me, the logical explanation lies in the fact that the check engine light is coming on when you drop to a lower-fuel grade. This suggests that the engine’s combustion chamber is carboned up extensively. This carbon is taking up space within the combustion chamber and inadvertently elevating the engine’s compression ratio, making it higher. The lower-grade fuel will pre-ignite easier, especially in an older vehicle that features an early generation knock sensor, which can’t react fast enough to perfectly limit knock or ping. The check engine light would not just come on without storing a trouble code of some sort so I’m going to suggest that whoever checked it previously missed something related to the engine’s knock sensor circuit. That being said, if it runs fine with the 94-octane fuel, don’t fuss with it and stick with what works.

Lou Trottier is owner-operator of All About Imports in Mississauga. Have a question about maintenance and repair? E-mail globedrive@globeandmail.com, placing “Lou’s Garage” in the subject line.

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