4matic light stays on

[nuru]

Just bought a 1991 300E 4matic looking almost new. Recently I noticed the 4matic light is always on, after checking I discovered that the test lever was raised, I returning it back to normal the light went off, 2 days later the light came back up again, this time the test lever is still in its normal position, but no matter how I place the lever the 4matic light still stays on.

Any assistance will be highly apreciated

 

[joe bloggs]

Complicated system, you can diagnose 'on car' by shorting 2 pins in the diagnostic socket under bonnet and then counting the flashing of the 4matic lamp (not the operating triangle lamp) socket is the long one on the bulkhead, should have a cover clipped to it with about 8 or 9 unoccupied pins on it. cant remember which pin, but can come back with the info on this. (not the round diagnostic socket with the screwtop)
Could be simple stoplamp switch, steering angle sensor open circuit, ABS sensor (yes the signal can be ok for abs, but the 4matic which is more sensitive can pick up a weak signal as a fault) remove (carefully) sensors and clean crap off, may be stuck though. Have a look at the tyres , are they all the same size, even small speed variances can be detected, and show as a fault, is one brand new?. Enough oil in reservoir, pump ok no leaks at the pipes underneath..common with older E classes. is the abs system working ok?

 

[JohnGalloway]

I too have a 4-matic (1989 estate) with a malfunctioning 4WD and light that stays on. In my case I can sometimes re-set the computer by disconnecting the battery for a while. In that case it will run for between 1 second and 2 weeks before malfunctioning again. It might be worth trying this simple measure as a first step.

The fact that your lever was in the "test" position suggests to me that the seller was aware that the 4-matic was malfunctioning(?)

In the past have found that if the lever is set to "test" then the computer always needs reset by myself or the garage before the 4-matic light would go out again.

For myself, I am interested in the comments made by Joe Bloggs about testing the 4-matic system. I see from your profile, JB, that you are connected to Mercedes somehow. I have not been able to find a mechanic with a proper understanding of how to diagnose the 4-matic system. A MB dealer put the diagnostic computer on my car and diagnosed low pressure in the shared 4-matic/self-levelling rear dampers systems. My rear dampers were replaced (£750) but it made no difference.

Once before my 4-matic system malfunctioned due to a faulty front ABS sensor but that does not seem to be the case this time.

Please, Joe Bloggs, if you have any further details about how to perform the shorting test and how to interpret the flashing of the 4-matic light you refer to, then post them here and it will be very much appreciated.

John Galloway