Replaced Alternator And AC Not Working [Why + How To Fix]

The most common reason behind a replaced alternator and AC not working issue is leaving the AC compressor unplugged. If you leave the AC compressor that way, your AC won’t come on. Plug the AC compressor back to solve the issue.

replaced alternator and ac not working

However, you won’t always be so lucky and there are other issues that can also lead to your AC not working after an alternator replacement. Can’t wait to find out what they are? Let’s dive in!

Replaced Alternator And AC Not Working [How To Fix]

It’s unfortunate to find out that your AC has stopped working after changing the alternator. We’ll look at the main reasons behind it in this section.

1. Leaving The AC Compressor Unplugged

The number one reason why the AC doesn’t work after replacing the alternator is leaving the AC compressor unplugged. You’d often have to disconnect the AC compressor plug when you’re replacing the alternator.

Ac compressor connector

Many people forget to reconnect it after they’re done with the replacement. Even if you don’t remember disconnecting the AC compressor connecter, remember that it’s in close proximity to the alternator. So, it may have gotten accidentally disconnected when you were working on the alternator.

Regardless of whether you replaced the alternator yourself or hired a mechanic, anyone can make this mistake. So, the AC compressor doesn’t get any power and the AC doesn’t work.


Luckily, this problem is the easiest to fix. First, look at the connection of the AC compressor. If it was unplugged, plug it back in and check if the AC works now. If the AC functions normally, your work here is done. Otherwise, keep on diagnosing to find the root cause of the AC issue.

2. Blown Fuses

There are many fuses in your car that are directly responsible for the smooth functioning of your AC. If any of them are damaged, your AC may not work or even blow hot air.

car fuse

So, you’d have to inspect the fuses one by one to find out if anything’s wrong with them. Look at the interior fuse box as well as many AC components are connected to that.

The easiest way to check the fuses is to use a tester. If the tester lights up when you connect it to the fuse, it means the fuse has power.

If the light doesn’t come on, you have a faulty fuse. If you’ve found one or more faulty fuses, you’ve found the problem.


Once you’ve found the damaged fuse, replace it with a new one. Make sure the new fuse is of the same amperage as the old one. Fuses are there for a reason – to prevent the main components from getting damaged.

If you get a fuse with a higher amperage rating, enough current can go through to damage the main part. So, don’t make that mistake. If your AC works after replacing the faulty fuses, then you’ve solved the issue.

Replacing the fuses is an easy fix by itself. But sometimes, the fuses can keep blowing again and again. That indicates a more serious issue and you have to identify the main reason why that happens. Check out our guide on 10 amp AC fuse keeps blowing to find out why it happens and how you can fix it.

3. Faulty Relay

Just like fuses, the relays going bad will also keep the AC from working properly. If you weren’t careful during the alternator replacement, the relays can go bad when disconnecting the alternator or the battery.

car relay

And if those faulty relays were connected to the AC, it won’t work properly. So, check the relays one by one and see if they’re working properly. You can swap the relays with relays of the same sizes and keep tabs on each one to determine which of them has gone bad.


Once you’ve identified the bad relays, you need to replace them with new ones. Car relays are not expensive by any means and if replacing them solves the issue, you should absolutely do it.

Sometimes, the AC issues are solved simply by unplugging the fuses, and relays, and plugging them back in. That’s because a connection was loose somewhere. So, make sure that all the connections are nice and tight to rule out any issues with the fuses and relays.

4. Disconnected AC Pressure Switch

Another common mistake when replacing the alternator is to disconnect the AC pressure switch by mistake. If the connection becomes loose, the AC won’t work properly. The switch can also break if you rushed through the alternator replacement job.

ac pressure switch


First, check if the switch connection is tight. If it’s not, clean the connector and tighten the switch. If the switch is broken, you’d have to replace it.

However, the switch won’t break in most cases and just tightening it would do the job and get the AC working again.

5. Low Freon Level

This issue also goes back to the alternators where you’d have to remove the AC compressor to replace the alternator. It’s possible that the mechanic forgot to refill the AC system after replacing the alternator.

checking freon level

The AC compressor won’t come on if there’s insufficient freon in the system. So, that’s why you have AC issues. If you had the work done by a mechanic, take your car to the garage and ask whether this could be the issue.


Check the AC compressor. If it doesn’t work, the lack of freon could be the issue here. So, you’d have to refill the AC system with freon so that the AC compressor can work.

When you have sufficient freon in the system, your AC should work just fine.

6. Uncalibrated Blend Door

When replacing the battery, you must have disconnected the battery at some point. If the blend door was not in the correct position, your AC won’t work properly after the replacement.

blend door actuator

The AC can even blow hot air due to the incorrect position of the blend door.


You’d have to recalibrate the blend door if has gone to the wrong position. It’s a simple process. Locate the fuse for the blend door in the instrument panel of the passenger’s side and remove it.

Then, plug the fuse back in, turn on the AC, and let it run for at least ten minutes without any interruption. The position of the blend door should be automatically corrected during that time and your AC should work fine again.

7. AC Issues

Even though it seems unlikely, it’s also possible that replacing the alternator and your AC not working is just a coincidence. Maybe one of them has nothing to do with the other.

car ac Issues

One or more of your AC components could have already been bad and started acting up after replacing the alternator. The most common types of problems that can cause AC issues are:

  • Refrigerant leak.
  • Bad AC compressor.
  • Faulty AC condenser.
  • Clogged cabin air filter.
  • Faulty cooling fan.


The right solution to fix your AC issues depends on the problem behind it. As stated above, there are many potential problems here. Lucky for you, we have a detailed article on this topic.

Check out our guide on bad alternators causing AC problems and scroll down to the “What Else Causes AC Problems?” section to know all about it. Hopefully, you’ll find the answer about what’s causing the issue and how you can fix it for good.

Replaced Car Battery Now AC Doesn’t Work [How To Fix]

Yes, AC problems are not only limited to alternator replacement. Although it’s not too common, many people face issues with the AC even after replacing the battery. We don’t want you to suffer from this AC not working issue.

car battery repllacement

The first thing you have to understand here is that it’s normal for the AC to not work for a while after replacing the battery. It’s because the ECM needs some time to adjust to the new battery.

But this problem should only last for a short time. If your AC doesn’t work even after an hour or two since you’ve replaced the battery, it’s time to start diagnosing the issue. Your AC may even blow hot at times after replacing the battery at times.

Just like you did with the alternator – check the fuses and relays first. Make sure all of them have power. Any of them being loosely connected can also cause this issue. So, tighten all the fuses and relays to get your AC to work.

If that doesn’t work, maybe you made a mistake when installing the battery. Make sure the terminals aren’t corroded and that you’ve actually installed a good battery. Do a load test on the battery and see if it’s okay.

If the battery isn’t faulty and it’s connected to the car properly, your AC should work fine. If you’ve done these tests and your AC still won’t work, you should take your car to an expert mechanic.


Can routing the belt incorrectly also cause AC issues?

Your power steering won’t work if you have routed the belt incorrectly when replacing the alternator. So, if your power steering works, you have routed the belt correctly. The AC issues are the result of some other mistake.

Why is my alternator not working after replacement?

There can be many installation mistakes that can lead to this issue. The most common one is to overtighten the serpentine belt or use a defective alternator for replacement.

Can a bad alternator cause air conditioning problems?

The alternator impacts all the electrical components in your car. As the AC is an electrical component, issues with the alternator can easily lead to AC problems.

Does the AC compressor affect the alternator?

The AC compressor isn’t likely to damage the alternator in any way. Except for being run by the same serpentine belt in most cars, there’s not any other common connection between them.


Replaced alternator and AC not working? Now, you know the main causes behind this AC issue and how you can fix it. In most cases, it’ll be an unplugged compressor. Plugging it back in would solve the issue.

However, if that’s not the case, be patient and go through the diagnosis one by one. Hopefully, you’ll find the solution soon. If you still have any questions about these AC issues, leave a comment below.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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