1990 SE Trans Rebuild Question - PeachParts Mercedes-Benz Forum

robert_stoever@ · #1 08-28-2006, 10:32 PM

Hi All,
I just rebuilt my beloved 1990 300SE trans (722.351 - details to be posted at later date). It had 215k miles and lost reverse. While replacing B2 band, I decided to replace all friction surfaces + both bands + all seals. Installed in car and went for a test drive. I now have reverse but lost 4th gear. It moves through all forward gears well, gets to 4th and revs - can't find 4th at speed. I have read this may be due to, among other things, improperly adjusted modulator.
Would someone describe:
1. How to measure and adjust modulator pressure (is it pressure or vacuum), and most important,
2. What pressure values I should see? For example, what pressure at idle, what pressure at 30mph, etc.
3. What gauge range do I need for testing?
4. Anything else I should try?
The trans rebuild book describes this procedure not very well, and does not list the pressure values.
Cant wait to get back on the road.
Much appreciated,
Rob

mbdoc · #2 08-29-2006, 08:15 AM

There is a "blind plug" just below the modulator to connect a guage. Incorrect modulator pressure CAN'T cause a loss of 4th gear.

You measure modulator pressure with engine running & NO vacuum at modulator. That trans should have a black modulator & have 4 BAR of pressure.

No 4th gear is ALMOST always a problem with the 2 "teflon" seal rings on the K2 clutch guide flange at the inner rear of the main trans case. Can also be leakage from the K2 "lip" seals in the clutch pack.

david s poole · #3 08-29-2006, 10:01 AM

mb doc is absolutely correct.assuming that you had 4th gear before you started this rebuild,most probably if you replaced the two white teflon seals on the short stubby shaft in the rear of the case[the rear band fits in the same location with the rear clutch drum in the middle on the short shaft.unless you are very careful and use generous amounts of grease on the two white teflon seals,you will pinch one of them resulting in fluid loss to the case and no 4th gear[4th requires both the front and rear clutch drums to be pressurised with fluid] ps did you take apart the two clutch drums and replace both lip seals and then air check to make sure no leaks?

robert_stoever@ · #4 08-30-2006, 09:38 AM

Gentlemen,

Thanks for your replies re 4th gear. All forward gears including this one worked fine before the rebuild. I did not replace the clutch pack lip seals - only the friction material and checked the clearance from last steel to circlip. I did replace the teflon seals and used grease to install - I remember paying particular attention to these and removing the shaft just to be sure they stayed put.
Question: What initiates shift into 4th gear ? Might this be related to shifting/valve body?
The symptom I describes and your replies make the most sense - I want to explore all options before I crawl under to pull the trans for the 2nd time...

Does anyone know of a reputable trans shop in NJ who will work with me? I would like to fix myself but am running out of time.

Thanks,
Rob