Only Using Half The Springs In 722.3 Transmission Reverse Piston - PeachParts Mercedes-Benz Forum

Squiggle Dog · #1 03-24-2016, 07:07 PM

I heard from Sun Valley Transmissions that one way they correct the early wear of the reverse clutch discs on the 722.3 transmissions is to use only half of the springs in the reverse piston (up by the front pump) so that the reverse piston doesn't slam so hard against the discs. Has anyone else done this? I heard that the early transmissions had weaker springs and there were complaints by customers that reverse was too "harsh", so starting in 1990 Mercedes compensated by using stronger springs, which corrected the harsh reverse engagement but at the cost of early wear of the discs.

Squiggle Dog · #2 04-03-2016, 01:31 AM

I just pulled apart a 1987 722.317 transmission and noticed that the reverse piston springs are much lighter than the ones in the 1991 350SDL 722.361. I had to use two large clamps to compress the springs in the 1991. In the 1987, I was able to compress them using my hands alone. So I will indeed only use half the springs with the 1991 reverse piston for firmer reverse engagement and less slipping of the discs.

BillGrissom · #3 04-03-2016, 05:16 PM

Sounds wise. I doubt the extra wear is from "slam so hard". In all auto trannys I know of, excessive clutch wear comes from a slow, slippy engagement. Think of a new driver operating a standard transmission, burning up the clutch. To avoid wear, you want firm, quick shifts. That is what "shift kits" are designed for. The manufacturers tend to make the shift slow and slippy because that is what clueless customers think means "good transmission". The shift kits make it more like an educated owner wants (though some are geared to racers only, w/ full manual control, reverse shift pattern, etc).

When I used a shift kit in some 1960's Chrysler trannys, the instructions had you remove some of springs in the actuating pistons and/or replace with a weaker spring. You also removed some balls from the valve body and drilled a few channel walls away. I recall it was the accumulator piston springs you buggered with, but the reverse piston springs in your M-B tranny must also be to slow the shifts, so same idea. Afterwards, the shifts were firm, but not bothersome. Indeed, not sure I could tell a difference.

Squiggle Dog · #4 04-03-2016, 05:35 PM

Right. It took me a while to figure out how the setup works. I initially thought that the spring plate hit the discs, but what actually happens is the spring plate is already up against the K1 clutch and the piston works against the springs in order to press on the discs.

So, a harder spring means reverse engages more softly that if it were to have lighter springs.

The discs engaging hard will prevent wear because otherwise they would slip for a brief moment before engaging. By only using half the springs, reverse will slam harder when engaging, but the discs will last longer as they won't be slipping.

The initial problem with the early transmissions was reverse slamming into gear and customers didn't like it, so Mercedes went with heavier springs so that it would softly feather into reverse, but the slipping of the discs wore them down too quickly.

So, a person has to choose between harsh reverse engagement and short reverse life.

When I replaced the transmission in my W116 300SD (722.120), I set the shifts on the firm side to save the friction material, but no so harsh as to jolt the driveshaft and rear axle.

You want to experience a transmission with a firm shift? Try riding in a 1960s W111 220S with automatic transmission featuring a fluid clutch instead of a torque converter. It about bucks you out of your seat shifting up and down.