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Old 10-01-2003, 12:13 AM
Posts: n/a
Climate Control

On my 1978 300SD, I have problems with the climate control adjusting temperature.

Over the past 6 months, I have changed the regulator with a new aluminum one, the ambient temperature sensor and the in-car temperature sensor and an auxillary water pump. The climate control amplifer was replaced with a used one.

Recently, I have not been able to switch from cool air to warm or hot air without trouble. The other morning, I was driving at 48 degrees in the morning and the air was through the center vents and cool.

After turning the system off and on once or twice, the system started to warm the air and move the vents to the floor.

After driving for 50+ miles the vehicle become quite warm and the air never moved back to a more neutral warm or cooler to compensate for the inside temperature.

Also, when I turn on the defroster they move to the defroster vents but the air never turns warm. When I shut the system off and on, the next defroster cycle will be warm.

I am thinking potentiometer or the amplifer not working properly? How do I inexpensively test?

Thanks for all the help,

1978 MB 300SD
1991 MB 350SD
2002 MB CLK320 Cabriolet

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Old 10-01-2003, 08:32 AM
Registered User
Join Date: May 2002
Location: Central Oregon
Posts: 1,004
testing the ACC

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.
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique
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Old 10-01-2003, 10:21 AM
jcd jcd is offline
village idiot
Join Date: Jun 2001
Location: Northern New Jersey
Posts: 1,102
If you would like

Send me your e-mail address in a private message and I'll e-mail you a diagnostic bulletin that may prove helpful in diagnosing your problem.

Jim Dowden
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Old 10-02-2003, 10:07 AM
Posts: n/a
10 Pin Test Connection

I received one of the documents, but a couple of the pin numbers were very hard to read in the "early" test process.

Could someone let me know what the pin layout is of the connector.

I know pin 4 and 5 are the in car sensor
I know pin 2 and 3 are the ambient sensor
I know pin 9 and 10 are the tempurature dial

What pins do I test for the resistance circuit?
What pins do I test for checking the servo? I know it is Pin 1, but can not read the document to determine if the other is Pin 6 ?


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