124/300TDT 4-matic, Switch on/off from drivers position. - PeachParts Mercedes-Benz Forum

MB124393 · #1 02-06-2008, 07:16 PM

Here's my solution.
WHY:
As I see it, there is a "design-fault" in the way 124/4-matic operating which give the system and the TC unecessary short life-time.
Shiftstage 1 is always activating when start off, and create the 35/65 % front/ rear drive.
This happens regardless you take off with your 124 four-matic in California at summertemp or in Alaska in the middle of winter.
Shiftstage 1 will be active until the vehicle reach 12km/t, above that speed it's automatically deactivated.
If you are stucked in the usual que, driving very slow you are actually driving with 4matic active on shiftstage1 for long periods
The ECU is programmed to not activate the indicator lamp under this circumstances, so you are not aware of this.
Using the brake will not deactivate the 4-matic under these condition.
Driving in traffic jam means that the hydr system put approx 30-35kg pressure to the hydr-cylinders in TC every time your 4-matic is starting off.
After some years, or 100' to 150' km's depending on where the 4-matic is used, (town or country/ higway), you will have the hydr-syst to leak into the TC and it will be a mixture of hydr-oil and ATF which slowly or even quiet fast fill up the TC completely, and of course give malfunction and bad lubrication of the gears and bearings in TC.
The repair cost $$$$
HOW:
Here's my solution, but before we begin, all of the modifications that I describe here you have to do at your very own risk and cost.
Nothing of this description are approved by Mercedes Benz.
The aim of the change is to manipulate the ECU (by the switch in the mid-console ) to recive a signal from the electrical circuit for the oil-pressure valve that indicate low pressure in hydr system and the ECU will deactivate the 4-matic system without release the hydr pressure, and in this way the rear self-leveling system can be operating normal.
My 90 TDT/4-matic are modified in this way:
One pair of 0.75mm wire are installed from under the rear right seat (where there is a 8-pin multiplug) and forward to the dash.
A standard CAT5 networking cable are installed in the same gate.
I had to take out front passenger seat to do this wiring properly into the factory-installed gates under the carpet. In the passenger footrest area the wiring is taken to the right side and up and behind the "gun-compartment" which I had to remove before going on with the last part of taking the wires to nearby the mid-console.
The upper coverplate in mid-console has to be taken away and a extra swich installed, I've used an original switch that normaly serve the lamps in the rear compartment. The 0.75mm cables have to be connected to the switch in a way that give a closed circuit when the switch is in one of its position.
When this is done it's time for the magic under the rear seat.

You have to disconnect and open the male part of the multiplug and locate the pins that are numbered as pin 7 and 8 and mark these two wires.
If the wiring are in original shape this is the connection for the oil-pressure valve at the main hydraulic unit under the car
Connect the 0.75mm wires to each of this two already marked wires in such a way that you dont cut the original wire from operating.
If everything are correct now you shold have a closed circuit when switch at dash are in the on pos. The voltage will now run from OVP relay throug the wiring under the backseat and to the swicth at dash. When switching on, the circuit all the way to the ECU will be closed and the ECU will recive a signal at 12-14v that indicate low-pressure in hydr system and the 4-matic will be deactivated. Swich off will open the circuit and ECU recognise that hydr-oil pressure is present and 4-matic system will be activated.
A few things in the end.
The 4-matic waring lamp will go on when deactivate the system. If you want to avoid this you can install a simple relay behind instrument cluster that open the circuit for the warninglamp when you switch the system off, and of course, close the circuit for the lamp when you activate the 4-matic. The advantage while doing it this way are that you can follow the process when the system runs the check before ready to serve, because the warninglamp is illuminate during "setup".
When activating 4-matic, the system will run a check that take a few seconds before it is ready to operate.
I will not try to activate the system during driving, because the TC may be "dont like" to be activated in this way.
And the networking cable?
You can connect tree of it's 8 cables to pin 2,5 and 7 in the multiplug under the rear seat, just drop them down into the female part and put the male part back, because the core of these wires are so thin it will not damage the plug. Then connect some diode's in the other end at your dash, grounding them properly and in this way you can have fully control of the tree stages in 4matic, because the diodes will illuminate when the solenoidsrelays are activating. In that way you can visally observe shiftstage 1, 2 and 3 going on and off.

JayRash · #2 06-11-2008, 02:23 AM

I have the same setup in my car, switch is in place of the speaker fader, best way to go with those 4-matics, but u gotta love this car and its abilities.

babymog · #3 06-11-2008, 09:16 AM

My 4matic went through two transfer-case re-seals by the time I sold it (around 110,000miles).

The bigger problem to me was the fact that the 4matic system is a reactive system, you have to slip for it to engage systems, and it had to drop out and slip periodically to know it was still on slippery pavement. Not a sure-footed system on snow and ice.

The best thing you can do is sell that thing and get a permanently-engaged AWD system like a quattro, you'll never go back to a reactive system. Even the Subaru systems are better than the 4matic, whether the early Styre-Puch system you have or the later viscous-coupled system.

JMO.