Hey guys. I was nudged to make a separate thread which would be more easily searched regarding the ADSII suspension. I couldn't have easily gone through this repair without the guidance of member "Manhunt" with honorable mention to "Fathertime". Thank you.

I hope this thread helps other owners to repair their suspension and not be persuaded to removing the system! It's really not that complicated or hard to work on if you can get a basic understanding.

I recently purchased a 1997 SL600 with a known suspension problem.

Symptoms: The rear suspension was sagged down completely. It would not move up under any condition - engine running, pressing ride height buttons, it didn't matter. Also, the ride height buttons would not light up with cycling the switch. The ADS light was illuminated on the dash as well. No strange noises were heard. The front suspension was raised but also would not move as the system appeared to be locked out permanently.

Diagnostics:
1.) Checked ADS fluid level including in the powersteering reservoir. Level was high, but the suspension was also stuck down so it made sense to me.
2.) Removed return line from reservoir cap and attached a clear rubber hose to the steel line and routed it back into the reservoir. Started engine and observed the fluid was flowing nicely. This confirmed the hydraulic pump was working.
3.) Continued physical inspection of system components by looking under the car and with the wheels off. No visible leaks or oil stains. Also checked to make sure the front and rear level sensor arms were connected. They are located on the (North American) passenger side. Everything appeared normal.
4.) Now that most components have been looked at, we've narrowed down to the hydraulic control unit, the computer, or wiring. I took the easiest next step and began inspecting the hydraulic control unit. It's located on the passenger side engine compartment along the "frame rail". It's close to the alternator on the M120 engine. The easiest way to see it is by removing the right front wheel. It's the block of aluminum with lots of steel hydraulic lines going to it. There are 4 solenoids mounted on top. The electrical connection leads all the way up to the top of the engine compartment near/on the shock tower. You need to remove the beauty cover to access the connector. The first test is to perform an ohm test on each solenoid. There are 5 pins on the connector. The large pin off by itself is the ground. Each other pin is for a solenoid. Test the connection between each solenoid and the ground one at a time. In my case, I found 1 pin had an "open". The rest measured roughly the same, very low resistance (what you want to see). Now I needed to investigate the solenoids and wiring more closely. I found the wires coming from one solenoid to be completely corroded! Unfortunately it was so bad and too close to the end of the wire that I could not repair it. I discovered 1 other solenoid had weak wiring as well. These units are rare to find used, so I ended up having to buy a new unit from a Mercedes Benz dealer (I used an online parts department to save money) for roughly $1,070.
5.) As part of the ADS servicing and repair, I decided the nitrogen spheres should be replaced as well. The car only has 60k miles but the spheres are original which makes them now 22 years old. I'm sure they needed a refresh. I'm not going to cover the details on how to replace them, but it was easy. The only hard one is the (USA) driver's rear because you need to remove the rear muffler for decent access and even then it's a bit tight. If anyone needs details on sphere replacement, let me know, but it's pretty straight forward and self explanatory once you get in there.
6.) Now that everything has been replaced, it was time to refill the system. The ADSII is self bleeding thankfully. First I siphoned out most of the reservoir to maximize the fluid change/flush. Then I refilled it to 3/4 full (disregard the fill marks at this point if the suspension is collapsed). Make sure you use the appropriate fluid! The ADSII used Febi brand ZH-M or OEM Mercedes ZH-M. Do not use CHF-11S in this version. I used roughly 3 quarts for my work, so having 4 on hand for the job is safe. You could probably get away with only 3 quarts if you simply replace the 4 spheres and don't flush any other fluid. Reinstall your clear tube to the steel return line. Either route it to a bucket to flush the remaining old fluid, or route it back to the reservoir. The point is, we want the cap off so we can keep adding new fluid. Now start the engine and monitor the fluid intake. Keep topping the fluid level whenever it nears the "minimum" line on the tank. Once it seems the car has stabilized the ride height, hit the raise button to the "first dot". Now monitor your fluid level again. Make sure to keep it topped. Repeat again with the highest suspension level. Let the car run a bit longer and begin cycling the suspension up and down several times. Now you're ready for a road test. You will probably notice the ride isn't perfect yet. Don't worry. Keep putting on miles and checking the fluid level. It should smooth out and the fluid really shouldn't need much topping off.

Cheers!

 

Very good write up here, I'm sure it will help a lot of folks from dragging their feet to buy a V12. Great to see a DIY do all this. I think many are afraid of having to go to the dealer with one. I know you will find more to do on it as we all do on any of the SL's I hope to follow your restoration efforts and proving these great machines are not the monster that non owners make them out to be.

 

What brand spheres did you buy? Prices are all over the map I wonder if there is a noticeable difference in the main brands Corteco, Febi, and Meyle

 

I ended up with Corteco but bought the fronts from a different source than the rears. I used FCP Euro and a "random" parts website for the others as they were roughly $5 each less expensive and had free shipping. Since I wasn't in a hurry to get anything, I went with the best prices and free shipping where possible.

 

I recently placed an order for Febi rear and Myele front but on second thought canceled it after seeing better prices I was wondering about the Corteco because of a much better price and the WD front price also. Do you find the ride to be good?

 

I mixed Meyle and Corteco last time around and went with all Febi this time.

I have to say the car has the best ride (even with firm tires) that it ever has.

 

I think it rides pretty good but I've never driven an R129 prior to this.

 

Well I bit the bullet and ordered Corteco rear and Meyle front from autohaus AZ so I hope this car does better than it is now. It's 20 years old with around 60k.