There is a pretty good albeit confusing procedure for testing this all out using a mulit-tester and the 10 pin connector on the car on the factory repair CD. On my 1980 the connector is located just in front of the blower motor on the firewall above the passenger footwell. I am unsure of the location on your car but it may be the same.

You can use this with the MBZ testing tool (I have no idea who owns one...) or there are measurements that can be taken using a volt/ohm meter but it is not real intuitive.

It may be the that your controller is bad especially since you do not get full on heat with defrost. This is the default setting for a failed system so I'm a little at a loss. Have you thouroughly cheked your vacuum to the ACC circuits? The ACC servo is controlled via electronics and vacuum. Real PITA to troubleshoot.

There is also an acc testing doc that we picked up from a source on this board that may help. Send me a private message with your email and I can forward it via email in either pdf or word format. Let me know which you prefer.

Good luck, these ACC systems are the bane of us older MBZ owners.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique