Does anyone have a procdure for testing the 4matic and/or CIS control boxes on a '91 300TE 4matic? About 2 months ago, I had a voltage regulator go south on me and I didn't realize it for a few days. When I finally checked to see why all my bulbs kept burning out, I found that I was putting out 18 volts. I put in a new regulator and it was fixed. I later learned that the OVP relay doesn't kick in until 24 volts, so it never tripped and the fuses were still fine on it. I think I fried both my 4matic and CIS control boxes, but I'm looking for some more test procedures.
The symptons are that the 4matic activation light has not come on since this incident, even in ice and snow. It also never acted like it was on in those conditions. Also, the car is not idling consistantly. Most of time when warm, it idles at 500 rpm. In general, the car runs fine except for the low idle. One other minor point, its not getting good milage (15 mpg).
I had a spare OVP relay, and swapping it had no effect. I finally (about 3 months after ordering it) got my cdrom manual, so I pulled the harness to the 4matic box tonight and it's getting voltage (with ignition on) from the OVP relay on the pins it should be (1 and 19), so I think that's another sign that the OVP is fine.

Does anyone know a way to test the 4matic and CIS boxes themselves? Can the dealer do this?

Thanks,

'91 300TE 4Matic
'84 300D

Dealers DON'T have that type of equirtment. Doubt if anyone has that type of test eqiptment.

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have you tried rebooting the computers by removing battery power for several minutes and reconnecting?

Bosch will check certain controllers for a fee. I suspect they would do the KE controller, but the field way of testing is to view the ouput of the controller.

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Steve Brotherton
Continental Imports
Gainesville FL
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33 years MB technician

Ok, after unplugging the boxes to test the voltage, I must have either jarred something back in place, or cleaned a contact; because the 4 matic is working again. I took some hard turns over the weekend, and the light came on twice. That was good, because I found out that it's a really expensive box. The fuel injection is still messed up, so I'll have to keep troubleshooting that one or looking for a cheap used one.

Dave