350SDL Transmission Woes - PeachParts Mercedes-Benz Forum

MongooseGA · #1 07-23-2016, 11:25 PM

Hi all,

I had a little mishap the other day, in that one of the trans cooler lines burst and drained the transmission of fluid. It started slipping slowly and my goal was to baby it home about 2 miles, then I completely lost all usability. Had it towed home, repaired the line and refilled with Dex3.

The car seemed to pick right back up and drove 90% fine after this. However, it does shift very hard now, and will hold gear much longer than it really should. I've also been having to sort of 'manually regulate' the shifting with the throttle. Seems to like to shift hard from 2nd to 3rd, then immediately into 4th. I know on my W123 300D I can play with a bowden cable to adjust the shift points (or firmness?) but I'm also curious if there may have been serious internal damage to my transmission. I'd say total distance driven while the trans originally started to slip may have been about a mile on the highway.

I have a lead on a good rebuilt unit for $200, but it's from a 1987 W124 300D. Seller says it should be compatible, I'm not sure. Any advice here?

precision32 · #2 07-24-2016, 09:43 PM

Have you checked the vacuum line to the tranny? When mine comes off the tranny shifts like you describe.

MongooseGA · #3 07-24-2016, 10:33 PM

I haven't, I'm not sure what to look for. Which side of the engine would it be on?

jay_bob · #4 07-24-2016, 10:58 PM

The vacuum actuator is down on the passenger side of the transmission.

The gas transmissions of this era used manifold vacuum to modulate the shift along with pedal position. In a Diesel you don't have manifold vacuum, so the injector pump has a vacuum valve, that adjusts the constant vacuum (from the vacuum pump) with rack position to emulate the manifold vacuum action found on gas cars. Pedal position is related to the transmission with a Bowden cable from the linkage.

As to harming the transmission, these were very stout transmissions, and from the limited knowledge I have of how an auto transmission works, as the fluid bled out, you lost hydraulic pressure. When the pressure is lost, the bands and clutches can no longer engage to pass power through the transmission, so you basically just sat there spinning the main shaft. As you bled out more fluid, the torque converter drained out, which prevented any energy from being passed into the transmission. So long story short you are probably fine since the transmission just shut down from lack of fluid.

MongooseGA · #5 07-25-2016, 08:30 AM

Quote:

Originally Posted by jay_bob

The vacuum actuator is down on the passenger side of the transmission.

The gas transmissions of this era used manifold vacuum to modulate the shift along with pedal position. In a Diesel you don't have manifold vacuum, so the injector pump has a vacuum valve, that adjusts the constant vacuum (from the vacuum pump) with rack position to emulate the manifold vacuum action found on gas cars. Pedal position is related to the transmission with a Bowden cable from the linkage.

As to harming the transmission, these were very stout transmissions, and from the limited knowledge I have of how an auto transmission works, as the fluid bled out, you lost hydraulic pressure. When the pressure is lost, the bands and clutches can no longer engage to pass power through the transmission, so you basically just sat there spinning the main shaft. As you bled out more fluid, the torque converter drained out, which prevented any energy from being passed into the transmission. So long story short you are probably fine since the transmission just shut down from lack of fluid.

Thanks!!

compu_85 · #6 07-25-2016, 08:44 AM

Is your car a 90 or 91? In any case the W124 transmission can't be used. A) the shift timing for the 3.0 won't work for the 3.5 as the 3.0's redline is ~1500 rpm higher. B) W124s have a mechanical speedometer, W126s are electronic. This makes the tail part of the transmission different.

You can burn up a trans running low on fluid, but I'd start with the simple things. Check the control systems as people here suggested. A dex/merc fluid will work. My personal recommendation is to use an Allison Transynd rated fluid.

My transmission does similar things... shifting too late. I've never been able to get it "perfect", and would love to know why. I almost wonder if the kickdown solenoid is leaking or something else is making the governor pressure too high.

-J

jay_bob · #7 07-25-2016, 09:12 AM

Agreed that low fluid and extended operation is bad, but bleeding out and immediate shutdown chances are the OP will be fine.

MongooseGA · #8 07-25-2016, 10:39 AM

Quote:

Originally Posted by compu_85

Is your car a 90 or 91? In any case the W124 transmission can't be used. A) the shift timing for the 3.0 won't work for the 3.5 as the 3.0's redline is ~1500 rpm higher. B) W124s have a mechanical speedometer, W126s are electronic. This makes the tail part of the transmission different.

You can burn up a trans running low on fluid, but I'd start with the simple things. Check the control systems as people here suggested. A dex/merc fluid will work. My personal recommendation is to use an Allison Transynd rated fluid.

My transmission does similar things... shifting too late. I've never been able to get it "perfect", and would love to know why. I almost wonder if the kickdown solenoid is leaking or something else is making the governor pressure too high.

-J

Thanks for the clarification on the W124 trans. My car is a 1990.

My W123 shifted similarly the whole time I've owned it, but the W126 350 shifted PEFECTLY when I got it in November so I'd like to get it back to that. It was even nicer to drive than my 560SEL is.

Quote:

Originally Posted by jay_bob

Agreed that low fluid and extended operation is bad, but bleeding out and immediate shutdown chances are the OP will be fine.

Hopefully I was able to shut down early enough. I was holding gear for a short while and thought I'd be able to make it home but once I lost all functionality, there's not much to do besides call a tow!

MongooseGA · #9 07-26-2016, 06:46 PM

All,

The car's been a real pain in the ass to drive around today. She likes to hang in 3rd gear, at high RPM, requiring me to manually put it in neutral then back to drive to push it over the edge into 4th. I looked on the passenger side of the engine and didn't notice any vaccum lines heading down to the transmission. Would anyone happen to have a picture what I need to be looking for?

I fiddled with the bowden cable at the back of the throttle linkage, but didn't seem to get much accomplished. I'd be interested in lowering the shift point from where it is now. The transmission will hold gear under normal to moderate throttle until close to 4000RPM (way beyond the point of making any additional power). It definitely feels like it's holding boost through the shift, causing it to bang into gear (and clunk into 4th immediately after a hard 2-3 shift). I read some things about the blue saucer on near the driver's fender but didn't see anything conclusive.

I did find a vacuum line disconnected near the IP, reconnected it and noticed no difference. Any specific help (with pictures! ) would be hugely appreciated as tomorrow I'm going to take it to an indy to let them sort it out.

Thanks so much!

MongooseGA · #10 08-28-2016, 11:25 PM

All,

Since my last post, I spoke with the tech once or twice over at Sun Valley transmissions in California. His suggestion was emphatically to drain all fluid and replace the filter. Did that, it ran beautifully for about 500 miles. After that time, the symptoms are back albeit not as severe. So I gave the car another fluid change and new filter, this time draining the TV as well. I filled with exactly 7 quarts of dex iii (which is what I read to be accurate, but it does show just a touch over the full mark right now). Again, drove beautifully at first but after a few hundred miles over about two days the problems came back again.

Ive searched just about every thing I could find on this forum, but have found conflicting or incomplete info. Here's what I've got going on now:

When the car is cold, the Trans shifts very nicely. It does double shift over 3rd and into 4th pretty quickly at low speed, but I can live with that. The car pulls nicely through and just out of my neighborhood. By the time I get a few miles down the road to jump on the high way to get to work, I'm at operating temp and the the problems start. IT'S ALWAYS WHEN HOT. 3rd gear will sometimes hang and never go to 4th. Sometimes it'll Rev higher than it should and then shift, sometimes it won't shift at all. I can remedy this by bumping into neutral manually then rev matching and going back to drive.

The biggest problem is a sort of frequent but inconsistent drop from 4th to 3rd at highway speed. I can't duplicate it on command, but I can feel right when it's about to happen. It's generally OK when the car is just at standard temp but if I'm running 85-90 with air on and it's 90°+, it happens. From what I've read some people think bowden cable, some think vacuum. My thought is that since it can shift properly at lower Temps, then the mechanicals are in good shape. Is there possibly some kind of sensor that's going bad with heat?

I'm getting tired of the issues. Would love to be able to figure out what I need to be looking for.

Frank Reiner · #11 08-29-2016, 10:24 AM

Quote:

Originally Posted by MongooseGA

When the car is cold, the Trans shifts very nicely. It does double shift over 3rd and into 4th pretty quickly at low speed, but I can live with that. The car pulls nicely through and just out of my neighborhood. By the time I get a few miles down the road to jump on the high way to get to work, I'm at operating temp and the the problems start. IT'S ALWAYS WHEN HOT. 3rd gear will sometimes hang and never go to 4th. Sometimes it'll Rev higher than it should and then shift, sometimes it won't shift at all. I can remedy this by bumping into neutral manually then rev matching and going back to drive.

The biggest problem is a sort of frequent but inconsistent drop from 4th to 3rd at highway speed. I can't duplicate it on command, but I can feel right when it's about to happen. It's generally OK when the car is just at standard temp but if I'm running 85-90 with air on and it's 90°+, it happens. From what I've read some people think bowden cable, some think vacuum. My thought is that since it can shift properly at lower Temps, then the mechanicals are in good shape. Is there possibly some kind of sensor that's going bad with heat?

I'm getting tired of the issues. Would love to be able to figure out what I need to be looking for.

Inasmuch as the described symptoms are temperature linked, and are also consistent with a low fluid pressure condition, consider that the main (front) pump is very worn, and unable to provide the required pressure when the fluid viscosity decreases with increasing temperature.

redassag00 · #12 08-30-2016, 03:11 AM

Quote:

Originally Posted by Frank Reiner

Inasmuch as the described symptoms are temperature linked, and are also consistent with a low fluid pressure condition, consider that the main (front) pump is very worn, and unable to provide the required pressure when the fluid viscosity decreases with increasing temperature.

I agree with this diagnosis.

Puking the fluid out might have damaged the pump in some way....

cad58y · #13 08-30-2016, 04:23 PM

Several years ago the same thing happened on my wife's 380 SLC. After fussing with it for 6 months we finally resorted to a rebuild transmission. At that time Silver Star Transmissions told me that you rarely recover from running one completely dry. I think that is where you are headed. Just my TCW. Bob Smits

MongooseGA · #14 08-31-2016, 01:38 AM

Quote:

Originally Posted by Frank Reiner

Inasmuch as the described symptoms are temperature linked, and are also consistent with a low fluid pressure condition, consider that the main (front) pump is very worn, and unable to provide the required pressure when the fluid viscosity decreases with increasing temperature.

Quote:

Originally Posted by redassag00

I agree with this diagnosis.

Puking the fluid out might have damaged the pump in some way....

Would this be an external pump or one within the transmission? Would it be repairable or a replacement job? (my transmission knowledge is about zilch),

Quote:

Originally Posted by cad58y

Several years ago the same thing happened on my wife's 380 SLC. After fussing with it for 6 months we finally resorted to a rebuild transmission. At that time Silver Star Transmissions told me that you rarely recover from running one completely dry. I think that is where you are headed. Just my TCW. Bob Smits

Thank you for the feedback. I wouldn't really be surprised, but it sounded like earlier in the thread that it may still be OK. Let me know if I'm off; I'm thinking that if there was permanent damage to the internals from the fluid blowing out, would I still have the ability to get nice, correct shifts now? I'd think the damage would be something mechanical/physical, thus unable to provide good performance at any time.

400Eric · #15 08-31-2016, 04:14 AM

You may have damaged the pump without damaging the bands/clutches. As others have said, as the fluid warms and thins, the already meager line pressure gets even weaker and now the trans can't function properly.

I'm with you. I'm searching threads desperately trying to find solutions to tranny problems in two different cars, hoping against hope that I can avoid having to r & r a trans or two. After all, these are Benzes damn it! Their trannys are supposed to last forever! Plus, I already have a car that needs a trans r & r (Benzer3).

Regards,
Eric