OK, you don't say the model, yr or the mileage?
There were transfer case problems until they were worked out about '91. Start with the basics in the system-sensors. Make sure your 4matic fluid never got low and let any air in...another reason for the light to come on occasionally. It's a very complicated but wonderful system and if the 'basics' don't reveal anything, find someone who is an expert at 4matics....not someone to poke & pry.
History? Ever a problem before with the 4matic system/light? Did the fluid ever get low (way below its dipstick level)?? Anything ever replaced in the 4matic system? Any leaks in the system? You're right on the cusp ('91) for transfer case problems...although you should have had that by now.
Well, if you want to attempt this fix yourself, you need a flow chart for troubleshooting the system. Otherwise, you'll just be substituting parts for a 'fix'. It's a complex system and so you need some substantial documentation to proceed. Do a search here and see if you can turn up some 'fixes' for the light coming on at certain speeds. I'm wondering if they even bled the system properly after the valve control unit was installed. Seems like they were throwing parts at it, as well.
That's rarely necessary unless you live in some extreme salt-belt. A good brake cleaner on the stator ring as you rotate the wheel, followed by a good polishing of the ABS sensor is all that's necessary. And I sure as hell wouldn't change the gap by grinding anything.
Offering parts????? I need the 4matic 'brain' control unit as a substitute test unit.
-well, you do not grind the top if it is "flat" so the sensor can read every step, but if rust have eaten the metal so the top is sharp, then you cant get a good reading. I had the same problem as "boskerrji" who posted the first message. But mine had the problem at 60 km/h, and sometimes when parking, full lock up. I cleaned the rings, and the problem moved to 80/90 km/h. Then I grinded the top of the ring, very careful, and voila, I can do 160km/h and not one problem at any speed. This is 4 years ago, and still no problem, going strong every winter, I live in Norway, and we do have winter. But I could have done what the local dealers said, "just replace the TC and front drive shafts, cost: 100.000,- nok /20.000 dollar, and then you probably have solved it...." so I think it's worth a shot to try this, it costs you a cupple of hours. But if you DONT know how this works, and dont know how to grind the rings without messing up the sensors ability to read the signals, and maybe reduce the gap between the sensor and ring after grinding, well, just dont do it.
Interesting solution, as I've never seen the stator teeth that rusted away. It makes sense though if they can get that disfigured.:thumbsup:
In my case it didn't, seems like the 4matic is more sensible than the abs, I dont have any other expl. This was the main reason it took me quite some time to figure it out too, it SHOULD be a abs warning light, but it wasn't. I even manipulated the abs signal to see if the abs lamp would light up, and it did, so I didn't spend more time with this tread for some time. Then, when I couldn't find any explanation to why this happend, I went back to "rotation sensors", it had to be something "telling" the 4matic system to engage frontaxle and lockup. If there is a crack in a ring, it shuld be a abs warning light, if there is a bad conection or like a wire broken inside the insulation, it shuld be a abs warning light, and the error shuld come at any time, slow speed/high speed, when you turn etc. But if the error is consequent at one speed, Im pretty sure it's problems with counting the tops of the rings. You dont have to "grind" more than giving the tops a little flat head, if they are sharp. Good luck, and the best part is: it cost you only some labour : )
If you pull the fill plug from the side of your TC and you get a mix of hydraulic oil & ATF spilling out, your TC is toast. It will require a rebuild to replace the aluminum piston with a steel piston & seal, bench bleeding and/or a factory rebuilt TC. The TC is overfilling with hydraulic oil and mixing with the ATF. At least this is the problem from the way I'm reading your post.
Sorry...but it's an old story/symptom.
