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Old 08-20-2003, 03:48 AM
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Join Date: Jul 2003
Posts: 10
Angry 1991 500SL computer problems :(

Hi Guys,

Does anyone have any ideas on how I can check if the Multi function relay (MAS unit?) on my 500sl is working?

My car is running really rough (not adjusting for cold weather starts)

Any ideas much appreciated

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Old 08-20-2003, 09:08 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
A 91 should be KE. All the techniques for evaluating KE cars apply; EHA current, duty-cycle codes and evaluation, and digital impulse codes.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 08-20-2003, 04:49 PM
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Location: Evansville WI
Posts: 9,616
Pretty sure the 91 SL has an OVP also. Check the OVP fuse.

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Old 08-20-2003, 05:53 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The MAS relay has 5 functions & shouldn't have any effect on HOW the engine runs.
MAS controls; fuel pump, a/c compressor, air injection pump, O2 sensor heating, & kickdown.

If the MAS relay has a problem then the car shouldn't start OR the check engine lamp should come for the O2 sensor.

The CIS computer must get power from OVERVOLTAGE relay & must apply current to the EHA for the car to have a good cold start. See above posts!!
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Old 08-21-2003, 03:20 AM
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Join Date: Jul 2003
Posts: 10
Thanks for your advice Steve, Gilly and M.B.DOC

This problem started a while ago when the KE unit was not working. The car would start rough in the cold, backfire, and not run well until it had warmed up.

I sent it off to an electronics agent who said a transistor was blown and that the battery polarity had been reversed at some stage ??

The overvolt relay has been repaired by him, and now apparently so has the KE unit.

Now that he has apparently fixed the KE unit, it runs even worse!

I think it is running rich and sounds like the timing is retarded. Yet he insists the KE unit is ok (although he said he does not have the wiring or pin diagram to test it )

Would you have any ideas where I should be looking now?

Thanks guys, I appreciate it


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Old 08-21-2003, 05:06 AM
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Location: Evansville WI
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Check the FUSE in the overvoltage protection unit!!!

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Old 08-21-2003, 05:47 AM
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Join Date: Jul 2003
Posts: 10
Cheers Gilly,

That was the first thing I checked when it happened,

It was blown but i have replaced it now. Little unit in the top right corner on the engine bay with the clear top on it

That is working now as the ABS & SRS fault lights have now turned off.

That's why I'm still left scratching my head
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