leylandp38
Senior Member
Apologies for cross-posting, I have been asking questions about my 4matic in the drivetrain forum but I am currently theorising that it might be an electronic problem, so I present myself here for (hopefully) some wise counsel...
I have a 1988 W124 300TE 4Matic which randomly engages the 4Matic at speed in what I suspect is Mode 1, ie 35% drive to front wheels and 65% to rear. I think this is the case as the /!\ light will come on, and the car slows quite rapidly and pulls to the right. Not pleasant or safe. The symptoms don't exist at all in 2WD mode (ie with Test lever engaged).
From what I have read, and from what I know about electronics, I think that its possible that the transfer box is being randomly flicked into Mode 1 (35% front/65% rear) by the 4Matic ECU. Normally this should apparently happen momentarily when the car first moves off, but if something is resetting the ECU then maybe it can happen at speed.
The reason for this theory (which I will test more extensively on the weekend) is that engaging the hydraulics (ie the Test lever) but unplugging the ECU leads to no problem whatever, if the fault was mechnical then surely it would happen with or without the 4Matic ECU. That suggests that the 4Matic ECU is at fault, either through an internal failure (eg broken connection or component breakdown), or through receiving spurious input signals (eg steering angle sensor).
BUT, I noticed today that the car has another more major electrical fault... the headlights intermittently go quite dim and the heater blower slows down noticeably. When this happened this morning it also caused an ABS fail immediately after. I think the 4Matic ECU controls ABS in 4Matic cars as it automatically fails when the ECU is unplugged, and the 4Matic uses the ABS sensors for input as well.
Anyway this light dimming etc suggests either some random short somewhere or a faulty alternator or regulator to me. Not sure how I'll find it if its not the alt/reg but maybe a random sudden voltage drop will upset or even reset the 4Matic ECU? This would cause the strange behaviour I suspect, if the rest of my theory is right...
Sorry for thinking out loud here but I'd quite like suggestions on this, I am not yet convinced the transfer box has a fault. Incidentally it seems to work flawlessly on bad surfaces like our muddy laneway. :lol:
Please feel free to correct any misconceptions I may have, it would help me in nutting this out :?
I have a 1988 W124 300TE 4Matic which randomly engages the 4Matic at speed in what I suspect is Mode 1, ie 35% drive to front wheels and 65% to rear. I think this is the case as the /!\ light will come on, and the car slows quite rapidly and pulls to the right. Not pleasant or safe. The symptoms don't exist at all in 2WD mode (ie with Test lever engaged).
From what I have read, and from what I know about electronics, I think that its possible that the transfer box is being randomly flicked into Mode 1 (35% front/65% rear) by the 4Matic ECU. Normally this should apparently happen momentarily when the car first moves off, but if something is resetting the ECU then maybe it can happen at speed.
The reason for this theory (which I will test more extensively on the weekend) is that engaging the hydraulics (ie the Test lever) but unplugging the ECU leads to no problem whatever, if the fault was mechnical then surely it would happen with or without the 4Matic ECU. That suggests that the 4Matic ECU is at fault, either through an internal failure (eg broken connection or component breakdown), or through receiving spurious input signals (eg steering angle sensor).
BUT, I noticed today that the car has another more major electrical fault... the headlights intermittently go quite dim and the heater blower slows down noticeably. When this happened this morning it also caused an ABS fail immediately after. I think the 4Matic ECU controls ABS in 4Matic cars as it automatically fails when the ECU is unplugged, and the 4Matic uses the ABS sensors for input as well.
Anyway this light dimming etc suggests either some random short somewhere or a faulty alternator or regulator to me. Not sure how I'll find it if its not the alt/reg but maybe a random sudden voltage drop will upset or even reset the 4Matic ECU? This would cause the strange behaviour I suspect, if the rest of my theory is right...
Sorry for thinking out loud here but I'd quite like suggestions on this, I am not yet convinced the transfer box has a fault. Incidentally it seems to work flawlessly on bad surfaces like our muddy laneway. :lol:
Please feel free to correct any misconceptions I may have, it would help me in nutting this out :?
