When you turn on your car’s air conditioning (AC), you just expect the cool air to flow, smoothly and quietly. You certainly don’t expect it to affect the way the car is running. It’s not at all unusual for the engine speed to drop momentarily when the AC is switched on, but if the engine starts to surge up and down, or even acts as though it’s going to shut off, there is something that needs attention.
Air conditioning operates by circulating refrigerant through a series of devices throughout the car. The refrigerant is driven by a compressor and that compressor requires a lot of power. Automobile air conditioning compressors draw that power from the engine via a drive belt. The compressor may require as much as three to five horsepower depending on how large the system is. That may not even be noticeable if your car has a five-liter V8 engine with power to spare, but if you drive a car with a 1600cc four cylinder, you can always tell when the AC is operating.
Modern cars have systems that are supposed to compensate for changes in the engine load imposed by the AC and other accessories. If your car doesn’t seem to know what to do when the AC is on, here are a few steps you can go through that can help you identify some of the more common issues.
Part 1 of 1: Troubleshooting a car that surges when the AC is on
- AC thermometer
- Safety glasses
- Workshop Manual
Step 1: Check the belts. This is a good place to start because you may not have to go any further. The air conditioning compressor is the largest belt-driven device on your engine. It has two substantial hoses connected to it. It may be driven by the same belt that runs everything else, or it may have a belt of its own.
Check the belt tension according to the workshop manual. Check the operation of any automatic tensioner as well. Check the surface of the belt for cracks or glazing. If the belt is more than five years old, you should replace it or have it replaced for you.
A belt that is slipping because it is loose or worn can cause the engine to overcompensate for the air conditioning load.
Step 2: Check the air conditioning performance. Simply put, your automobile’s air conditioner should be able to pull the temperature down by 30 degrees or more: Meaning, if it’s 90 degrees outside, you should be able to get 60 degree or colder air out of the vents.
Set the recirculation switch to off, so that outside air is coming in and drive the car with a temperature gauge in the dashboard vent. If the air conditioning is not performing well, the engine might be surging because the system is undercharged or overcharged.
If it is overcharged, the load imposed by the compressor will be excessive. If it is undercharged, the AC pressure switch may be cutting the compressor on and off in rapid succession.
Listen to the engine while it is idling and the AC is on. You should be able to hear a substantial click each time the compressor engages. If these clicks are coming less than about 30 seconds apart, your system is probably undercharged.
Air conditioning charging equipment is expensive and complicated, and there are a lot of environmental laws regarding handling refrigerants. If you suspect your air conditioner is not performing properly, and that is causing your car to surge, its best to have the air conditioning system checked professionally.
Step 3: Check the vacuum lines. With any automotive performance problem, vacuum lines are a great place to start. Many things are controlled via engine vacuum and if any of these lines are misrouted or leaking it can lead to a wide variety of issues. Check the vacuum lines for good condition and correct routing using an appropriate vacuum diagram.
Step 4: Check the throttle body. The throttle body is fixed to the intake and controls the amount of air allowed to enter the engine. Often, over time, there will be an accumulation of carbon around the edges of the throttle valve itself. If this carbon prevents the valve from reaching a full closed position it just throws everything off on the idle control system. Look at the stop screw on the throttle lever, and be sure the lever is contacting the screw when not pushing the accelerator.
If it is not, there is probably a large accumulation of carbon on the throttle valve itself. Sometimes you can simply remove the large air hose and clean the throttle in place with carb cleaner and a rag, but more often, the throttle body will have to be removed to be thoroughly cleaned.
Step 5: Check the idle control valve. The idle air control valve comes in many shapes and sizes. On some cars it’s integral with the throttle body, other cars don’t have one at all. Check your workshop manual to determine its location.
If your car was made after 1996, a malfunctioning idle control valve will be signaled by a Check Engine Light. Before that, you’ll have to examine it yourself. The idle control valve has a few moving parts inside it that often become dirty and lock up over time. Sometimes you can get away with cleaning them out, but most often a valve that has been on the car long enough to get jammed up needs to be replaced.
If your car was made before 1992, you can check the function by disconnecting the electrical connector while the car is idling. A properly working idle control valve will default to open position when disconnected, so the car’s engine speed should go up sharply, returning to normal when you reconnect it. If you do this to later cars, it will cause a check engine light and a trouble code that will have to be reset with an automotive scanner.
If you have an appropriate scanner, later model cars have functions that will enable you to operate the idle control valve via the scanner to check its operation.
If you’ve been through these steps, and your car still persists in misbehaving, the cause may be more subtle. In that case, there is no substitute for experience. Your Mechanic can send you a technician with the experience you need to evaluate the problem and help you decide what to do.