Why are TRANSMISSIONS so damn hard to diagnose ? - PeachParts Mercedes-Benz Forum

kamil · #1 09-26-2004, 01:11 PM

Hey.

1983 240d 186k

My problem came back again from a silky smooth transmission. I fixed it a few weeks ago but now I'm at the point where I'm about to buy another cream puff but in MANUAL. I've yet to take this car to any type of mechanic so if you know of someone good in New Jersey please post away.

Anyway.... I'm getting rough shift from first to second so I would love to diagnose the problem. (it's not that bad but rather annoying) I've been planning on keeping this car for years since the body is so perfect and shiny (I bought it from the original owner which has garage kept it all its life). I thought about manual conversion but my father is telling me that there is too much involved in such a conversion (what does he know?). I have a 240D manual sitting around my house for 500 bucks and the car has 176k.

Should I try to fix the vacuum somewhere ? Take it to a mechanic ? Or just buy another nice 240 ? (money is not an issue at all)

Thanks

whunter · #2 09-26-2004, 02:04 PM

Why are TRANSMISSIONS so damn hard to diagnose ?
Because each one is different.

Because it is not a steady state machine.
Because it uses multiple command controls.
Because each OEM thinks that reinventing the wheel on every transmission is good.

Because the nut behind every steering wheel is different.

wolf_walker · #3 09-26-2004, 03:54 PM

Quote:

Originally Posted by whunter

Why are TRANSMISSIONS so damn hard to diagnose ?
Because each one is different.

Because it is not a steady state machine.
Because it uses multiple command controls.
Because each OEM thinks that reinventing the wheel on every transmission is good.

Because the nut behind every steering wheel is different.

Ain't that all the truth.

I've had unusualy crapy luck with MB auto's myself. Any old C4 or 6, 727, TH-whatever, even the 700R4's, I can get em to do what they should and they seem to last just fine with practicaly no maintanance, and forever if they do get regular fluid/filter jobs. But this MB piece, jeez....

I mean, the variation I get with mine just from fluid temp ALONE is crazy. I've never seen it to a degree that anyone other than a mechanic would notice with a *normal* auto. Anyway..

#4 09-26-2004, 06:59 PM

As per previous posts, on other forums and clubs,
It seems that a little ruff shifting is better for the
MB tranny.
I was lead to believe that a soft shift is bad
for them.

But then again I could be wrong.

Louis.

tangofox007 · #5 09-26-2004, 08:28 PM

I had the same problem. Fixed a leak in the door lock vacuum system, then my transmission started shifting smoothly.

jfikentscher · #6 09-27-2004, 09:34 AM

Mine is in the shop now. I did not have time to diagnose myself. The trans guy says there is a vacuum distribution pod located on the rear of the IP, and that mine seemd to be leaking. I will be talkinbg to him, later today and will update.

Many MBZ trans problems are due to vacuum problems.

Joe.

kamil · #7 09-27-2004, 12:16 PM

Quote:

Originally Posted by jfikentscher

Mine is in the shop now. I did not have time to diagnose myself. The trans guy says there is a vacuum distribution pod located on the rear of the IP, and that mine seemd to be leaking. I will be talkinbg to him, later today and will update.

Many MBZ trans problems are due to vacuum problems.

Joe.

which shop in New Jersey did you bring it to ?

Gilly · #8 09-27-2004, 08:55 PM

The reason for trans problems on the Diesels is pretty simply. It is taking a "normal" gasser transmission and installing it in a Diesel. What's the problem? Gassers produce vacuum, which varies depending on throttle position, and it uses the amount of vacuum to vary how hard the transmission shifts; low vacuum= hard acceleration=firm shifts, high vacuum=light acceleration=soft shifts. On a Diesel there is none of that going on naturally, the engine uses a vacuum pump to run the brake booster, locks, and some AC controls. The vacuum pump is a pretty steady delivery device, if anything it will produce MORE vacuum at higher rpms, not LESS like a gasser. So you have this screwy linkage and valve setup on the injection pump to "simulate" the proper vacuum to run the vacuum modulator, you're basically trying to fool the transmission into thinking it's in a gasser!
One big problem I've seen is the brake booster line. The little orifice that the vacuum line comes off can plug up, restricting vacuum, giving the result that the vacuum available to the control valve is too low, giving a low vacuum reading to the modulator, and you guessed it, low vacuum=hard acceleration=hard shifting! Even though you're driving it "easy". vacuum leaks can have the same effect, but don't forget about the booster line! Check to see if you can feel suction at the orifice. There is a seperate orifice JUST for this system, the other one id for the locks and A/C. The modulator needs a smaller sized orifice to provide the correct amount of vacuum. If the booster line IS restricted, I recommend a new booster line, unless the car is a real heap and not worth a new booster line.
Gilly

wolf_walker · #9 09-28-2004, 12:34 AM

Quote:

Originally Posted by Gilly

The reason for trans problems on the Diesels is pretty simply. It is taking a "normal" gasser transmission and installing it in a Diesel. What's the problem? Gassers produce vacuum, which varies depending on throttle position, and it uses the amount of vacuum to vary how hard the transmission shifts; low vacuum= hard acceleration=firm shifts, high vacuum=light acceleration=soft shifts. On a Diesel there is none of that going on naturally, the engine uses a vacuum pump to run the brake booster, locks, and some AC controls. The vacuum pump is a pretty steady delivery device, if anything it will produce MORE vacuum at higher rpms, not LESS like a gasser. So you have this screwy linkage and valve setup on the injection pump to "simulate" the proper vacuum to run the vacuum modulator, you're basically trying to fool the transmission into thinking it's in a gasser!
One big problem I've seen is the brake booster line. The little orifice that the vacuum line comes off can plug up, restricting vacuum, giving the result that the vacuum available to the control valve is too low, giving a low vacuum reading to the modulator, and you guessed it, low vacuum=hard acceleration=hard shifting! Even though you're driving it "easy". vacuum leaks can have the same effect, but don't forget about the booster line! Check to see if you can feel suction at the orifice. There is a seperate orifice JUST for this system, the other one id for the locks and A/C. The modulator needs a smaller sized orifice to provide the correct amount of vacuum. If the booster line IS restricted, I recommend a new booster line, unless the car is a real heap and not worth a new booster line.
Gilly

How did other diesel/auto vehicles do this? I know VW made a very few, but I've never seen one. I assume there are others..

Gilly · #10 09-28-2004, 08:07 AM

Don't know. It's easier since they use electronically controlled trannies now. With the newest diesels (the ones MB is putting in the 211 chassis) I wonder if they need a vacuum pump at all since they have EBC (electronic brake control).

Gilly

coldwar · #11 09-28-2004, 08:27 AM

Quote:

Originally Posted by mb123mercedes

As per previous posts, on other forums and clubs,
It seems that a little ruff shifting is better for the
MB tranny.
I was lead to believe that a soft shift is bad
for them.

Louis.

Indeed! I really like that little slam in the back when it shifts up. All such Mercedes sensory inputs are very re-assuring!

Dave