Falker

In the last couple of weeks I've experienced an issue where the keyless start tries to start the car, it does about half a revolution and then stops. If I remove the button and turn the key, it starts fine.

This seems to happen when it's cold, but cold starts aren't new. I've owned the car for 4 years and this is the first year this has happened. It's a 2013 C63.

I haven't checked the battery for current load potential but it starts the car normally when I turn the key. Keyless door lock/unlock all seems to work fine.

Any thoughts on where to look for the problem? I'm stumped and a search of the forum doesn't seem to turn up any answers.

INS1GNIA

Replace the battery in key fob.

Falker

Did that about a month ago when it notified me the battery was dying. It worked at that time.

This is an intermittent problem. If I've warmed the car up, it will probably work fine.

It _seems_ like it detects too much resistance and then stops the starter. But I can't make much sense of that because it didn't do this in any of the previous years.

BalanBro

I'm speculating, but it seems to me that the voltage drop when the starter kicks on is causing the keyless start module to momentarily shut down and stop cranking. A cold engine is harder to crank, and a cold battery has reduced amperage output. Both of these together will cause a significant voltage drop that will not be as pronounced when the engine and or battery are warm (or when the battery has been recently recharged by the alternator.)

Have your battery checked out.

Falker

I went to try and start the car this evening and it wouldn’t crank, with the key or with keyless go.

I ended up replacing the battery and it it works fine now. However this is strange behavior. I’ve been working on cars for two decades, have modified and repaired many issues, including several startup issues and these cars do not display any of the signs of a bad battery.

I assume that there is some software in the car detecting CCA/load on the battery and telling the starting circuit to stop working even when the battery could potentially start the car. It would make no sense otherwise why it wouldn’t start with Keyless Go but start fine by turning the key.

C’est la vie. It works now. It was the battery.