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Old 08-17-2006, 02:13 PM
Registered User
Join Date: Jun 2004
Location: Toronto, Canada
Posts: 27
300 E A\C MAs Control Unit

Many thanks for all who have helped me with this problem over the last couple of months. My inoperative A\C compressor operation was restored by the replacement of the MAS Control Unit. All worked as it should for 2 weeks until yesterday when the A\C quit again. The same problem is back.

The compressor fails to engage and the "check engine light" stays on. There is something that causes the failure of the MAS Unit within the wiring of the A\C. Before I replace yet another, is there a procedure to follow for diagnosis or has anyone had this type of problem and repaired it effectively?

Walter Sawczak
1990 300 E
1993 Lincoln Mark VIII
1983 Jaguar XJ6
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Old 08-17-2006, 09:17 PM
Senior Member
Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
I would suggest getting a simple code reader and getting codes from the ECU at pin #3 at the box by the battery. Then if you don't want to spend about a grand on a brake out box and harness, you can back probe pin #14 on the MAS and check for codes in that unit.
Auto Zentral Ltd.
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Old 08-18-2006, 10:31 AM
david s poole
Join Date: Mar 2005
Location: dallas
Posts: 1,822

walter,a few thoughts.......if you jump right in it would seem that the coil on the compressor clutch must be drawing too much power[amps] is rare for a coil in that condition to pull in the clutch.......hmmmm,are you sure that everything else that the mas relay controls is working? got to start somewhere suggest that you use a long pce wire to jump the two sockets for a/c under the mas relay and use a clamp on ammeter to read current.5-6amps should be it but if it's pulling 10-15 then you could look for burnt track in mas relay.david poole

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