Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
do you have any issues with the tacho recording speed?
if you don not then its likely that this is an issue with a wheel speed sensor rather than the main speed sensor
and with ABS systems 99% of the time its a problem with one of the wheel speed sensors and I'd first check the connections at all four corners first for any signs of corrosion or chafed wiring.The sensor is located in the wheel hub so you'll have to remove each wheel in turn to inspect them all.Also check that the toothed wheel that the sensor points to is clean and free of any debris.you can check each sensor's function by plugging the two output wires into an AC voltmeter and spinning the wheel quite quickly, you should get a voltage signal that increases with rev's.
Its possible that the fault is only a bad connection to one of the wheel sensors.This may be remedied by removing each wheel in turn and unplugging each sensor and cleaning the contacts with a squirt of electrical contact cleaner. Then refitting.
thanks for the extra information
if the truck is immobilsed then I'd say its more likely to be the security system? does the engine start and run OK but has no power at all?
Ok I'll opt out and open this up to others