Why Does My Car Not Start When the AC Is ON?

The automobile can develop many kinds of mechanical faults that can prevent it from starting. Sometimes, your car won’t start when the AC is on. It might be an indication of a faulty battery. The alternator could be damaged, even the AC system itself.

A car that has developed any of these failures would show some distinct signs. This article is to help you, the driver, identify with ease the source component from which the fault originates.

Here’s why the car won’t start when AC is ON

The main cause why your car won’t start if the AC is ON is due to an excessive load that your battery can’t handle when turning on the engine. Other less frequent causes include a bad alternator, a seized AC compressor, or a faulty Idle Air Control (IAC) Valve.

man pushing car

Let’s discuss the possible faults in more detail, and see what you can do to fix the issue.

01. The battery can’t handle the excessive load

If you’re the type that forgets to turn off your headlights or interior lights as you park your car, you risk draining your battery’s resting charge. The charge is restored to the battery only when the vehicle is in motion.

Depending on the health condition of your battery, you could wake to a drained or failing battery. Now, you might be able to start your car with a weak battery, but turning on the AC would weigh on the car battery. The additional load can cause an increase in net voltage loss over net gain.

In this instance, restarting your car might be an issue since the battery is very well drained.

Again, too many electrical components in a car simultaneously, like the AC, a DVD player, headlights, and interior lights, increase battery load. You love cold soda, so you install a small car refrigerator. All these appliances consume power from your battery faster than can be recharged.

It is advisable not to turn on the AC immediately after you start your vehicle. Now, a car with a bad battery and one with a terrible alternator show similar signs. We’ll get to that in a bit.

Related content: Car Is Slow to Start, but Battery Is Fine – (Here are 7 Causes)

02. A bad alternator

The function of an alternator, put simply, is to keep your car battery charged.

How does it do this?

The alternator consists of an electromagnetic rotor and stationery set of wire coils, and a rectifier. Its shaft is connected to the car’s engine via a belt and pulley. As the car moves, the shaft and rotor spin. The motion of the electromagnetic rotor generates electricity.

A rectifier inside the alternator helps convert the direct current to alternating current, which can charge the battery. The size of the output voltage varies directly with the car’s speed. However, the alternator has a regulator that helps keep the output near stable. This is how your car battery is recharged.

The battery of a parked car has a resting charge which allows you to start the vehicle. The battery loses considerable resting charge when you turn on the ignition key and start your vehicle. As you start moving, the alternator recharges your battery and restores power.

A good alternator does this and also produces the electrical energy needed to power other electrical components of a car, like the AC, the headlights, the wiper, and even your phone charger. The car engine only produces mechanical energy.

However, a bad alternator could produce a high irregular voltage that can overheat the battery from overcharging and damage electrical components or little to no charge. Thus, dumping all electrical load on the battery. Your battery drains faster, causing a hard start.

In rare cases, a faulty alternator can drain power from the battery itself. This is called a parasitic draw and usually stems from the rectifier or wiring connection.

A faulty alternator and a drained car battery pose similar threats and show similar symptoms. How would you know if either of them is at fault?

Turn the ignition key. If the car doesn’t start or there was trouble starting with no cranking noise, you’d probably have to deal with a damaged alternator. Now, if the vehicle starts but later stalls during motion, the car battery isn’t adequately recharged. It is an indication of a faulty alternator.

If other accessories using electrical energy like the car wipers, radio, and AC are slow and malfunctioning, or your headlights are overly bright or significantly dimmed, then you check out your alternator. It has most likely developed a fault.

On the other hand, if your engine cranks or starts only after multiple attempts, you have a battery problem. If you experience this, you should physically examine the battery. Check for corrosion, plumps, or swelling. For any of this, seek a replacement. You can jumpstart the car with another battery if the battery is drained and keep the engine running to charge the drained one.

Sometimes the car engine light on the driver display turns on, indicating an issue with the charging system.

However, if you’re still unsure which of the two is faulty, you can take your car to a mechanic or use a multimeter to check the charging system. You can also run load and conductance tests to check the battery’s health. In most cases, with a faulty alternator, all you need to do is replace damaged components like the rectifier or the regulator. It costs only a few hundred bucks.

A drained battery can be recharged a couple of times till the primary substance inside the battery is incapable of storing any charge. At this point, it is safer to change and recycle the battery. Depending on the size and power, a new battery should cost you between 45$ to 250$. Refilling the battery is also possible, but you must take extreme care due to the acidic nature of the primary substance.

03. A seized AC compressor

A bad AC compressor can cause a hard start or prevent the car from starting at all. To understand why this can happen, check out my other article on why a bad AC compressor might cause a no start.

The AC compressor is part of your car’s cooling system. It functions by pressurizing the refrigerant and pumping it to the condenser coil, where it is cooled by outside air from the air blower. It serves as a means of heat exchange between the atmospheric and inside temperature of the car.

It is powered using a pulley connected to the serpentine belt. The serpentine belt runs over the alternator and other accessories in the car. This way, the accessories, including the compressor, draw power from the engine.

A compressor can seize up due to several reasons.

1. Bad quality refrigerants

2. Low lubricants

Leaking lubricants would allow the movable metal parts of the compressor to grind each other. The compressor eventually locks up. You’ll hear clunking noise from under the hood if you’re trying to start your car.

3. Leaking and poor performance of the cooling system

Without the cooling system, the compressor clutches can heat to a high temperature, up to 1000 degrees Fahrenheit. The temperature melts down the bearing seals and can stiffen up the compressor. A good indication of this is when you turn on the AC, which blows warm air instead of cold air.

4. Debris, specks of dirt, or soot can get in the compressor and clog it

If you have a seized AC compressor, you can always check it out and replace the damaged part. You can also replace the whole unit. That would cost about $1000, and labor costs sit between $150 to $250. You can detect problems with your compressor by using a multimeter.

Related content: Will A Bad A/C Compressor Throw A Code? Answered

04. Sticking AC Compressor Clutch

A sticking AC compressor clutch can indirectly prevent your car from starting. The clutch is a component of the AC compressor. It allows the free movement of compressor belts over the compressor pulley. As you start your car, the clutch gets triggered by electricity, thus, engaging the compressor belts.

The inability to turn the air on and off indicates a clutch problem. You might also perceive a smell like that of burnt rubber.

The AC compressor clutch can be replaced. The cost of replacement circles around $700, and labor costs range from $150 to $250.

05. A faulty idle air control (IAC) valve

The role of the IAC valve is to regulate the air flow into the engine and is usually found on the throttle body. It permits airflow into the intake manifold. A faulty valve might not allow enough air for the fuel/air mixture. In this case, the car won’t turn over. There wouldn’t be any clunking noise. To test for this fault, you can use an OEM-level scan.

In most cases, the valve only needs to be cleaned. Carry out a physical examination and check the valve for holes or burns. Depending on the model of the vehicle, the cost for replacement can be between $150 to $500.

In Summary

A car which is hard to start or doesn’t start at all is probably every driver’s nightmare. There can be multiple faults that might cause this issue, but if the no start happens only when the AC is running, it is probably one of the causes discussed above.

One thing is for sure, to fix this issue will make a considerable dent in your budget!