The most common problems with this system is the over voltage protection relay or rust on the front wheel sensors, i've seen one or two failed front wheel speed sensors. Front wheel speed sensors are all the same for the two wheel drive cars made after 1987, as can the plug for the rear sensor fail it's under the rear seat on the right side next to the central locking pump.

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600SEL '91
300E 4Matic '88
240D '83
280SE '77
350SE '73
The most complex systems can fail in the simplest way.
Contra verbosus noli contendere verbis, sermo datur cunctis, animi sapientia paucis.

i don't believe in the lord! He's never bought me a Mercedes Benz.