453 AC Relay Swap

Mod Details

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What Is That Clicking Noise Behind Me?

It’s a complaint I’m hearing more and more, always from 453 fortwo owners. I so rarely drive my 453 and I have all around speakers and a sub so I probably don’t hear it. However, it’s the air conditioning compressor relay hidden behind the side boot plastic.

What’s The Ideal Fix?

Ignore it. Turn your stereo up. Put some sound deadening around the relay box.

What Isn’t The Ideal Fix?

This idea originated on a German forum from what I can work out. Someone decided to fit a different relay that made less noise.

A few people have replaced the relay with this one and they could no longer hear the clicking.

This isn’t a great idea since the original relay is rated for 25 amps. However this replacement is rated for 10 amps. That’s why it’s a quieter relay, it has smaller contacts since it’s not handling as much power.

Expecting a 10 amp relay to open and close a higher amperage than its rating is asking for trouble eventually. The arcing could weld the contacts in the closed position. Potentially damaging the AC compressor since it will never turn off, even when the car is off. Your battery will go flat quickly too.

However

Although the original relay is rated for 25 amps, the fuse feeding the relay is 15 amps so I would expect the circuit to normally run just over 10 Amps. This isn’t ideal but the relay should be able to handle it.

But… a 10 amp fuse can handle a spike way over the rated amperage. So, unless you actually test it, there’s no way to know what the voltage spike is during operation. When a relay closes, you get a massive switching voltage spike.

The modification is totally up to you.

Where Is The Relay?

Open the boot and look ovTake out the Torx screws shown below…

…then pull the lower boot trim towards the centre of the car.

You can then pull the top boot trim away. It helps if you pull the rubber boot seal backwards to clear the trim. There’s a larger relay/fuse box a bit further down and just about it, you’ll find the AC relay.

The Relay Clicks All The Time

If you are getting the relay clicking every 10 seconds (ish), your system is just low on AC gas and it should be checked and refilled.