350SDL Transmission Woes - Page 4 - PeachParts Mercedes-Benz Forum

MongooseGA · #46 11-12-2016, 09:48 PM

Hey guys,

So the trans did start to act up again recently. The little downshifting and gear holding quirks came back slowly over the course of a week. I left the car at the airport for a week, came back, and they were still happening. Another fluid/filter swap seems to have quelled the problems again. I'm seeing this is obviously a very temporary fix. Looks like I'm getting 3-4k miles out of it, meaning my oil change costs basically double.

I'll have to get it rebuilt in the future, but for the time being it's just not feasible.

400Eric · #47 11-13-2016, 12:26 AM

Thanks for the update.

Right before you throw in the towel and rebuild that thing, turn that T-bar in the rest of the way! You will quite literally have nothing to lose at that point and everything to gain!

ROLLGUY · #48 11-13-2016, 10:54 AM

I would suggest adding a quart of Lucas transmission additive (not sure exactly what it is called, but it is in a white bottle with red and blue writing). I used it several years ago in a trans that was having similar problems, and it fixed them.

MongooseGA · #49 11-15-2016, 07:56 AM

Quote:

Originally Posted by ROLLGUY

I would suggest adding a quart of Lucas transmission additive (not sure exactly what it is called, but it is in a white bottle with red and blue writing). I used it several years ago in a trans that was having similar problems, and it fixed them.

I tried some of this before the first fulid/filter change. The Lucas ended up looking like honey that mostly settled at the bottom of the pan. I have had good luck with it in other cars, but have also been warned about it by indy MB techs.

babymog · #50 11-15-2016, 08:12 AM

It depends on the problem.
If it's internal seals leaking, you won't be able to maintain control pressure and things will slip, trans-x or even acetone has been said to soften seals and help.
If you're finding lots of silt in the pan it might be too late, friction surfaces have already worn too much to grip properly, but there's not much left to lose except the price of the additive.

tyl604 · #51 11-15-2016, 09:00 AM

Keith at European Imports on Hwy 41 is an expert on transmissions. I have been going there for thirty years. He personally rebuilt the tranny in my 81 300SD after it pooped out with 150K miles. Nice guy; might be able to give you some advice.

MongooseGA · #52 11-15-2016, 08:58 PM

Quote:

Originally Posted by babymog

It depends on the problem.
If it's internal seals leaking, you won't be able to maintain control pressure and things will slip, trans-x or even acetone has been said to soften seals and help.
If you're finding lots of silt in the pan it might be too late, friction surfaces have already worn too much to grip properly, but there's not much left to lose except the price of the additive.

Ya know, I'm not really seeing a ton in the pan after draining. There is a slight shimmer in the fluid, only visible if I hold it up in the sun just right. Once the pan is dumped of all fluid, there's hardly anything clinging to the sides.

I had a thought, and tell me if I'm crazy: Since the transmission does actually have the ability to shift properly, reliably, given the right circumstances (fresh fluid/filter), would that not rule out a 'hard' mechanical issue vs a pressure/fluid related issue? (I'm thinking of it like hardware vs software) If it were more of a mechanical issue, I'd expect it to happen ALL of the time, consistently. Thoughts?

Quote:

Originally Posted by tyl604

Keith at European Imports on Hwy 41 is an expert on transmissions. I have been going there for thirty years. He personally rebuilt the tranny in my 81 300SD after it pooped out with 150K miles. Nice guy; might be able to give you some advice.

Thanks! I'm curious, what should I expect to pay for a rebuild in this car? For some reason I have the number $2K in mind...

400Eric · #53 11-16-2016, 04:06 AM

Like I said before, it seems like the fresh fluid is working for ya because it's providing just enough line pressure to get the job done but just the slightest amount of fluid viscosity degradation leads to your issues rearing up again because your line pressure is already weak and borderline functional because your trans fluid pump is weak from the damage incurred when it was run dry. Remember that all of this trouble began with that single event.

Everybody here is going to think this is nuts but I remember stories about how the cops in the 70s got more life out of their cop car trannys by running straight 10 weight oil in them instead of normal trans fluid. They used to switch them over to the 10 weight at their first trans fluid change, which occurred fairly soon because they were servicing the cars under the "severe service schedule" regime (of course). This might be another option for a fellow who has nothing to lose. So is the option of turning that T-bar in the rest of the way. Anything to get that weak line pressure back up a little higher. It's clear to me low line pressure is the issue here.

babymog · #54 11-16-2016, 10:11 AM

I agree that there's not much to lose trying a little higher control pressure, but since it seems to work some of the time, it appears to have the pressure setting high enough, just can't maintain it.

The most common reason for this is internal leakage past seals, and this is where a solvent-based ATF additive helps, it swells seals. Trans-X is one that is commonly used and many have success, a friend of mine who ran a couple of racing AutoTranny shops told me that even adding acetone will do this. Different brands and types of ATF swell the seals more/less, if your transmission is used to ATF with a solvent (like Trans-X) in it, you change to fresh ATF without the additive, your seals will likely not seal properly (and likely didn't before the PO started the additive).

However, the normal (in my experience) symptom of this is delayed shifts, stacked shifts, and flares when cold, and as the transmission warms up and starts to seal internally it shifts better.

Your transmission working okay cold and getting worse as it warms up can be the filter clogging and some of the contaminants falling off of the filter when parked (should be able to see a lot of sediment if this is the case), one of the valves sticking in the valve body (Trans-X should help this one), a cracked/damaged piston (should happen all of the time though), or maybe it is just so worn out that the transmission with everything warmed up and expanded to normal operating temp can't move the dogbones far enough with the piston to properly tighten clutch bands, ... which is my guess on this one.

Best of luck, but the route that I would take at this point is to add some Trans-X and see if it helps, and if not I'd go to a transmission shop and ask them how much to put gauges on the transmission and check pressures. The pressure checks will tell the technician a great deal about what isn't wrong.

400Eric · #55 11-16-2016, 02:54 PM

It can't maintain pressure because the fluid viscosity is dropping. The viscosity drops over time, and it also drops as the fluid gets warmer/hotter.

Trans-X is a good idea. So is having the line pressure checked.

However, since it seems that Mongoose doesn't want to put any more money into the car (I don't blame him) and it's becoming increasingly clear to all of us that this trans has serious issues that aren't going to be easily chased away, which means he is reaching the point of having nothing to lose, it's time to turn that T-bar in the rest of the way! The beauty of this fix is that it is the only fix that is free! If it doesn't work you're not out anything! I am VERY sure it will work though. Remember, I've done this to two cars now and one of them has gone over 20,000 more miles with no ill effect. The other one still doesn't have 4th so it clearly has something else going on, but it's first three gears are working a lot better now so it's T-bar is staying all the way in now too!

Granted, this isn't something that you would want to do to a fresh trans, but an older looser trans can use the increased line pressure. Both of my trannies that I have done this to are loving it!

If that doesn't work then change out the fluid and put in the straight 10 weight with one quart of Trans-X and one quart of Lucas transmission additive to replace the solvents, detergents, etc. that are missing from the 10 weight oil. The Lucas mucus should also mix better with the 10 weight as well, much better than it did with the thinner trans fluid.

tyl604 · #56 11-16-2016, 04:05 PM

Think you can buy a rebuilt tranny for about $750; it is guaranteed if you also install a new transmission cooler. Seems like installation is about $500. Easy to find reputable rebuilders on line.

MongooseGA · #57 11-18-2016, 10:58 PM

OK guys, the car's acting up again so tomorrow I'll be doing another fluid and filter change. I'll give it a shot with the 10 weight and trans-x, I think I have some Lucas left as well.

For the oil... I was told by John at BRM that the Type IV would have been best because it has NO detergents in it. Am I remembering this backwards? When you say 10 weight, any particular I should be looking for?

MongooseGA · #58 11-19-2016, 11:18 AM

Guys, I have a very stupid question to ask...

When I'm adjusting the modulator, does the car need to be running? I've been playing with it while the engine is off.

babymog · #59 11-19-2016, 09:21 PM

No.

compu_85 · #60 11-20-2016, 02:24 PM

Not unless you are using a gauge on the pressure tap, in which case you do it with the vacuum disconnected and the engine running.

-J