When you turn your car’s AC on, it’s natural for the engine to experience a little bit of added stress. This is because the AC’s compressor is powered by the serpentine belt that gets its power from the cycle of the engine. If you turn the AC on and you notice the engine running excessively hot or overheating, then you’ve clearly got a problem that needs to be dealt with quickly.
If the engine is about to overheat, you need to pull over as soon as possible. The last thing you want is some minor AC system or engine cooling system fault causing major engine damage!
There are a few common culprits that could be causing your car to overheat when the AC is on. Usually, this is a sign of a modest fault in the engine cooling system itself. The stress of the AC system is just worsening the symptoms causing the engine to overheat.
To ferret out the problem, we must better understand the intimate relationship between your car’s engine and the air conditioning system.
How Does a Car’s AC and Engine Cooling System Work
The engine’s serpentine belt moves with the cycle of the engine through a series of pulleys. This includes the pulley for the cooling systems water pump and the pulley that runs the air conditioning system’s compressor.
In older vehicles, a “Fan Belt,” which replicates a modern serpentine belt, drives the cooling fan. Though in many modern cars, the radiator’s cooling fan is powered by an electric motor, which turns on and off as the ECU commons. However, the ECU gets its information from sensors to know when the engine is warming and the fan must be turned on.
As the serpentine belt snakes its way through the engine bay, it also drives the water pump, which helps move pressurized coolant through the engine and back to the radiator. Then outside air rushing in through the grill grates and/or the radiator fan help cool the fluid before it cycles back through the engine again.
Something affecting the serpentine belt’s performance can be a big drag on the water pump. This can increase engine heat and make it difficult for the engine’s cooling system to draw sufficient heat energy out of the engine block.
Among the possible malcontents here is the pulley for the air compressor. When operating normally, the compressor pulley mildly strains the engine. You might even feel it when you turn the AC on in the cab. When there’s a major problem with the air conditioning system’s compressor, the strain on the system can affect engine performance in a lot of different ways.
The AC compressor drives coolant through the system, which heats it up. When the coolant reaches the AC condenser, it changes state and releases heat into the atmosphere. It then returns to the compressor, where the cycle starts all over again.
Car Overheats When the AC Is On: What Are the Symptoms?
The overheating when you turn your car’s air conditioning system on, can start out very subtly. You might not even notice them for several minutes after turning on the AC, especially if you were driving down the road with a headwind helping cool the engine naturally.
As time goes on, you’ll notice signs of the AC causing the engine to overheat like:
Five Reasons Why Your Car Overheats When AC Is On
While several faults can cause an engine to overheat on its own. A bad relationship between the air conditioning system and engine temperature usually boils down to one of the following five things.
1. A Bad Radiator Cooling Fan
Modern cars have divorced the direct marriage of the radiator fan and the serpentine fan or “Fan Belt.” In most modern vehicles, the radiator cooling fan comes on when the ECU tells it to. It draws power from the battery rather than a belt and a pulley.
This also means the electric motor is more prone to fail over time. You might not even notice it when you’re driving down the road at highway speed, as the onrush of air through the grill might be enough to keep the engine from overheating while the AC is on.
Then when you stop, to say idle as a stop light, the ECU should receive a signal from the engine coolant temperature sensor to turn the radiator fan on. The ECU sends the message and then goes back to the busy job of maintaining the engine’s timing and the fuel/air ratio.
Like a lot of bad relationships, a bad cooling fan betrays the trust of the ECU and just sits there stone-dead. Without any air coming from the grill or the fan blowing on the radiator the temperature of the cooling system fluid starts to rise. This pushes the engine toward overheating.
A fault like this can happen on its own if you’re doing a lot of stop-and-go driving. Though having the car’s AC system on applies more drag to the system, which naturally stresses the engine, driving up the temperature and causing the overheating to happen faster.
2. A Failing Engine Coolant Temperature Sensor
The ECU puts a lot of trust in the engine coolant temperature sensor (ECTS) when it comes to knowing when to tell the radiator blower fan to turn on. Flipping the script here, a failing ECTS can send bad information or no information to the ECU.
The ECU thinks the engine block is in the right temperature range and simply doesn’t tell the radiator fan to come on. Again, you might not notice this problem until you’re in stop-and-go traffic when the coolant system relies heavily on the fan.
3. A Fault in the AC Compressor
The AC compressor relies on the firm marriage between the compressor pulley and the serpentine belt. A misalignment in the pulley or a seized pulley can strain the serpentine belt beyond what it can tolerate. This stresses the engine, which drives up the heat.
One of the telltale signs of an AC compressor pulley problem or another fault in the compressor stressing the engine is incredibly weak cold air in the cab. If the pulley is completely seized, you might only feel the fan blowing miserable warm air at you.
4. A Bad AC Condenser
A blockage or a failure in the AC condenser can put extreme strain on the system, increasing the risk of overheating. It can also introduce more heat to the engine bay. The problem here is that most of the time, when you have a blockage in the AC condenser, it’s a broken-off piece of the AC compressor.
You’re staring at a double-whammy with a problem like this. Even if the compressor doesn’t immediately look like it’s failing, it’s probably compromised and about to fail. You end up with an overheated engine, a severely clogged condenser, and an AC compressor that needs to be completely replaced soon.
5. A Blockage in the Engine Cooling System or Radiator
Even a partial blockage in the engine cooling system or within the radiator itself can affect performance, increasing the risk of the engine overheating. On a fair weather day, you might not notice the engine is running a tad bit warmer than usual, but nothing to be alarmed about.
On a hot day, you turn on the car’s AC, which adds strain to the engine, which will naturally drive the temperature in the block and the engine bay up. Of course, it’s also a hot day, so there isn’t as much cooling potential coming to the engine from the outside air. The engine cooling system is struggling already, and the temperature starts to creep up on you slowly.
What To Do if a Car Overheats with AC on in Traffic?
When your car is overheating when the AC is on, the top fix priority is to deal with the extreme engine heat. If the temperature needle on the dash is kissing the red zone, you need to turn off the air conditioning and pull over as soon as possible to let it cool down. If possible, park the car with the grill facing into the wind and pop the hood.
Dealing with an overheating car is one of those times when patience is a virtue. The engine’s cooling system will be too hot to touch, and you can severely burn yourself if you rush and try to open it before it cools.
Quick Solutions To Fix the Problem
With the car pulled over and the temperature of the engine block lowering you can start to assess the fault causing the car to overheat when the AC is running. This starts with a visual inspection of the car’s cooling system, the serpentine belt, and the air conditioning system.
1. Fixing a Bad AC Condenser or Compressor
Locate the AC compressor and condenser. The compressor looks like a cylindrical container located next to the engine and will have a pulley attached to the serpentine belt. Hold your hand over it; if it is emanating heat, it could be that the compressor or the condenser is bad. Look over the pulley for signs of wear and tear.
If you’re lucky, it might just be a mouse nest or other debris clogging the cooling fins. This could strain the system causing the car to overheat anytime you turn on the A/C. The quick fix is to clean the cooling fins. Let the car cool all the way down, and try to drive home. Then turn the air conditioning on again when you’re a few miles from home to see if it works without overheating the car.
If the AC compressor or condenser is the problem, the only quick fix is to drive with the windows rolled won and the AC turned off. A professional mechanic will need to complete a proper repair.
The cost to replace a bad AC condenser and compressor range from $850 to $1,200.
2. Fixing a Coolant Leak & Refilling Fluid
Look for signs of a coolant leak during your visual inspection of the engine bay. This might be white residue and steam around the thermostat, the top of the radiator, or the radiator cap. You might also notice drips on the ground under the car.
Once the engine temperature starts to drop, look at the coolant level in the reservoir. If it’s low, it could be the underlying reason why your car is overheating when you turn on the AC. As long as the engine is at a safe temperature, you can add more coolant.
Just make sure to mix antifreeze 50/50 with distilled water. You don’t want to use pure water or pure antifreeze as it could cause important additive ingredients like silicates, phosphates, and nitrates to fall out of the solution.
This will probably be enough to get you back home or to the mechanic. Then you can properly fix the cause of the coolant leak. This might also call for flushing the radiator and coolant system hoses to ensure that any clogs have been blown out.
The cost to flush a radiator is usually around $100 to $150.
The cost to fix an engine coolant system leak varies from $75 to $300.
3. Fixing a Bad Water Pump
There is no quick fix for a bad water pump. If the pump itself is completely dead, the engine coolant won’t be able to circulate to cool the engine. The wisest thing is to have the car towed to the nearest mechanic.
If you’re within a mile or two of home, you can let the engine cool all the way down, and you might be able to limp home before it overheats again.
4. Fixing a Bad Blower Fan or Bad ECTS
If you start the engine again while it’s still relatively hot but not overheating, the ECU should kick on the radiator fan within a minute or two. If it doesn’t come on, then you know that either the blower fan’s electric motor is burned out or you have a bad engine coolant temperature sensor.
If the check engine light flashes or stays on, you can hook a code reader up to it. If it throws a code P0118, you know it’s a bad engine coolant temperature sensor. The sensor will need to be replaced by a certified mechanic.
The cost to replace a bad engine coolant temperature sensor ranges from around $100 to $150.
You can perform a quick test of the radiator fan by disconnecting it and hooking it up to the car’s battery for a second or two using the jumper cables or simple wiring. With direct DC electricity going to it, the fan should spin instantly. If it doesn’t, the radiator fan’s electric motor is dead.
You can let the car cool all the way down, top up the coolant, and you should be able to drive for a few miles before the bad radiator fan starts to overheat again. This should be enough to get you to a mechanic.
The cost to replace a bad radiator fan is around $150 to $300. However, you might be able to shave something off the part cost if you find a used radiator fan at a scrap parts company.
Conclusion
When your car overheats when the AC is on, you’re most often dealing with a fault in the cooling system that’s being emphasized by the stress placed on it by the air conditioner’s compressor. This usually ends up being a bad radiator fan, a failing engine coolant temperature sensor, or a leak that’s leaving the system low on coolant.
It’s also possible for a bad pulley on the air conditioner’s compressor or a clog in the condenser to be at fault. Though with these issues, you’d notice the cab’s poor AC performance before the engine would overheat.
If you catch the problem early, the fix is usually reasonable before the engine suffers damage from overheating. ECTS and replacement radiator fans typically cost $150 to $300. Though a more serious problem with the AC compressor or condenser or damage to the engine caused by severe overheating can easily cost over $1,000 to fix.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.