1979 300SD Transmission drops out of gear.
 

I have been using this forum for years to diagnose and resolve a myriad of issues with the diesel Mercedes I have owned throughout the years.

I have usually been able to resolve issues without posting however this one is a complete mystery to me and I would be thrilled if anyone had any insight into any of this.

I recently purchased the car and I am having it shipped to me. So as of right now all of this is armchair troubleshooting however the previous owner has been very pro-active with documenting the exact symptoms.

Car: 1979 300SD W116 Chassis with 722.120 transmission. Unknown mileage, however car is in astounding condition... someone loved the car cosmetically, not sure about mechanically.

Symptom: Vehicle runs and drives beautifully, however while cruising ONLY at partial throttle ( such as a country road or flat highway) the speedometer will drop about 15MPH and dance around a bit which is then followed by the vehicle dropping into neutral. Apparently this happens at nearly any speed, however not during acceleration. I can only surmise it is a neutral indication and not a "flare". This symptom only manifests randomly ( previous owner says maybe 4 times in 70 miles). Sounds like a pressure issue?

I am suspecting several issues here. Previous owner can not hear an audible click from the transmission servo when the ignition is on and the switch under the pedal is depressed.

Modulator valve on side of trans disconnected from vacuum source and shifting does not change ( shifts well according to him).

Pressure control rod has been adjusted with no change. ( he had a flaring issue but adjusting this rod corrected it)

When I receive the car I will do a thorough run through checking all vacuum systems and put a gauge on the test port and report back.

Meanwhile I have one question: Why on earth would the speedometer become erratic before dropping out of gear? ( Car goes back into gear if you lift throttle).

Where is the speedometer pickup inside the transmission? To me this sounds like a mechanical decoupling of some sort. Almost like the shifter linkage is bumping the car into neutral...

It has been a few years since I have messed with one of these cars and I am not familiar with the 116 Chassis.

Thank you in advance!

The gear drive for the speedo is in the tail cone of the tranmission. I would check the big nut on the tail of the tranmission that holds the yoke to the output shaft. They have been known to loosen off and cause issues.

Note: I don't believe your tranmission has a vacuum modulator so I question how it was disconnected. Yours should have the two rods as you mentioned.

Thanks Dude99,

That is a definite possibility I can't wait to investigate once the car arrives. It is the only plausible thing that makes sense to me in concert with the speedo issue.

Good to know regarding the modulator. For some reason I thought these were sort of a hybrid setup with the pressure rod, and vacuum off the VCV but I could be wrong.

I ordered the mercedes source transmission manual so I hope that sheds some light on things.

I believe your transmission does have a modulator. My guess is the transmission linkage bushings are worn or missing and are allowing the transmission to slip into neutral.

To my knowledge the only setups used with the OM617 were:

1. Control rods only (no vacuum control of an kind)

2. Vacuum only (used for one year only, I can't remember which, I think 1981)

3. Vacuum+Bowden cable

His car being a 1979 will have the first setup using control rods and no vac modulator.

This wouldn't cause the speedo to go wonky and the same time.

Those kinds of changes go by VIN number.. not ' year model' ...

You know what guys, I am 99% sure we are dealing with a classic issue of the output shaft retaining nut becoming loose.

It is the only plausible issue considering the symptoms.

I found a few threads where folks had the erratic speedo then the transmission "failing" or "dropping to neutral" and either recovering or not.

Some folks pulled over and had a tow, others continued on after the splines re-engaged the output shaft.

The driveline lash and movement is just enough to cause the disengagement which explains the jump speedo followed by the abrupt loss in power perceived, but is actually a decoupling of the drive shaft from transmission.

I hope I am right... will not be an easy fix but I would rather have a smoking gun issue than some vague gremlin that can surface at anytime on any road.

It has not even arrived on your side of the Continent and you have solved the problem... this is a new record...

Well hmm, I attributed my loose transmission yolk to a stupid PO. Interesting it is a common issue.

Leathermang don't take the wind out of my sails!

:)

I told you we are armchair diagnosing this one so I stand by my 99%.

I will give you the 1% doubt though.

Regardless of the issue I am excited to get this beauty in my garage and start tinkering with it.

It would be a new record though right?

I declared you had the record for pre arrival distance problem solving... no wind out of your sails there...

722.120 has both a control rod to control shift points and a modulator to control shift quality
i surmise that when you take possession of the car you'll find it not shifting too great if that's been disconnected, regardless of what the seller says.
ask me how i know :D

I don't think that wonky speedo and in/out of gear symptoms could be caused by a loose output flange.

If the trans is functioning, the output shaft that drives the speedo is turning whether or not the driveshaft is turning.

Plus I don't see how a stripped spline/loose nut on the flange could be an intermittent problem. Reports I have read is that the flange finally and suddenly strips and that's it.

I would guess an internal pressure problem.

I have suggested to him in a PM that he might try Trans-x first to see if built up varnish is affecting any important internal pressures....

I would agree with this. I would also change the fluid and filter before adding the trans-X, just as a starting point for future diagnosis.

I can't buy trans-X where I am, however have used Gunk transmission fix (virtually the same product as trans-x) successfully before. It's one of the few "miracle in a bottle products") that I actually believe in and have seen work.