Transmission Flaring - PeachParts Mercedes-Benz Forum

diesel don · #1 05-19-2002, 11:59 PM

I could use some advice on my transmission problem on my "new" 300D-T.

The problem resides with the transmission flaring in the early stages of acceleration. When I purchased the car, the PO was straight forward with this problem stating that it always happened on the first shift each morning but only then.

When I drove it away for the first time, I could tell there were some transmission inconsistencies. It seemed to shift hard and too early. On occasion, it would just seem off in the slow stages of putting around town. On the highway and up to speed, it always performed well. This continued through the 1400 mile trip home.

Tonight, I began to track down the problem hoping it was in the vacuum modulator. I kept tracking down vacuum leaks and comparing the system to my 84 (same car that functions perfectly). I discovered that several vacuum lines were misplaced and therefore, the white valve was getting NO vacuum. I rearranged all lines according to my other car and once again had vacuum in the right places. Upon test driving it, my fears began to mount.

The little, erratic shifts were gone which is good news.

But the flaring was must worse and completely consistent in the initial acceleration (I can't tell if its from 1st to 2nd or 2nd to 3rd?). Even after the car seemed to warm up, it still did the same thing.

Is my tranny dead? I might also mention that when you shift it to reverse, it takes a second or two to engage. This isn't so however with any of the forward gears.

Should I rearrange the vacuum lines in the wrong places to minimize the problem?

Thanks for any advice.

Don

#2 05-20-2002, 12:17 AM

I am no tramsission person, but I know what problems I've had in the past and so can only base on that. My Ford truck did that hesitation thing then I replaced the torque converter and it went into gear Now. But, I had also changed the filter and fluid, so it may have beena combination or one or the other. I have no idea if a MB has a filter or not.
Does the fluid smell burnt or is it a brownish color? If it is a dark color, almost black - it is rebuild time!
If it is a brownish color, you may be able to save the day by changing the fluid and filter if it has one.
If the fluid is a nice red color, I haven't a clue! Other than maybe a torque converter.

leathermang · #3 05-20-2002, 12:58 AM

Here is what I would do... suck out a little more than two quarts of trans fluid... and put in two quarts of TRANSX...then drive it 50 to 100 miles and change out the trans fluid and filters.. then see if that helped...

the hesitation changing to reverse I think is a pecularity of these cars... my 81 wagon does the same thing... and I have read others posts saying theirs does it...

Really go over the vacuum system... there are tricky deals in there... and it needs to be right to be able to sort out whether it is a vacuum problem or trans problem, there may be one way valves, calibrated orifices, etc missing from your system... that are needed... do a search and read everything you can find on it.. Greg

Holson Adi · #4 05-20-2002, 01:42 AM

I don't know if this is going to help. However my car shifted sooo hard three days ago and it smoked A LOT!!! It was good to piss off losers on the road.. (by "smoking" them)

I decided to pull over, checked the engine oil level, trans fluid, etc.
They all checked out fine! I then looked at the EGR and thought "...go to h**l..* then all my shifting problems went away. It now shifts like it should - smooth, firm, positive shifts.

You could also try cleaning out the banjo fitting....
again, they may not help... :p I'm still learning

good luck!

per eriksson · #5 05-20-2002, 05:27 AM

But have you adjusted the modulator itself?

If it has been tampered with by a PO it might be way out of adjustment by now and should be adjusted back to spec before doing anything else.

Ken300D · #6 05-20-2002, 07:11 AM

To reinforce what has already been said, a good transmission can be made to shift improperly by misadjustment (or misassembly) of the following:

Vacuum Lines (you need a diagram to be sure its right - my car was completely misconfigured when I got it. Plus, there are test procedures to make sure the vacuum "bleed off" works correctly for different accelerator pedal positions.)
Bowden Cable (generally should be slightly slack at zero pedal depression, and start pulling the cable as the pedal is pushed in - there is a more technical way to set this I'm sure.)
Modulator Pressure (this is adjusted on the side of the transmission itself. To do it right, you need a pressure gauge.)

While I am no expert myself, these are the things you should search on and read about here - lots of information.

Ken300D

diesel don · #7 05-20-2002, 09:28 AM

Thanks for the overwhelming support of ideas.

To date, I've cleaned the banjo, checked the tranny fluid (1.5 inches over the full mark, could this be causing a flare?), re-configured all the vacuum lines to spec (they were really messed up).

I'm greatly encouraged that the tranny smoothed right out and is acting near perfect EXCEPT for the flare in going from 2-3.

TXBill: don't feel bad about this as I'm making good progress. In searching old threads, it appreared as if you received help on your flaring issue awhile back. What is the method for the mechanical control device?

I'm planning on another filter change with synthetic but still think there will be a flaring issue. Due to the different way the transmission shifts (when I go from no vacuum to vacuum), I think it is not a bad tranny, but a simpler issue.

Ken:

What is this bowden cable thing?

Thanks to all

Don

DieselHead · #8 05-20-2002, 10:54 AM

I have two questions relating to transmission vacuum and fluid change but I didn't want to start a new thread so here they go:

1) If I put the front of the car on ramps and just open the trans. fluid drain hole and let the fluid drain out, about how much ATF will I have to add? (no filter change)

2) If there is a vacuum leak somwhere, wouldn't the transmission shift harder and not softer?

Thanks so much!

Alex

nellotare · #9 05-20-2002, 12:07 PM

Just thought I'd throw in my .02.

Mine "flares" when cold, then runs fine after warming up. In searching this site and others I found that "a clogged xmission filter" has been known to cause flaring. The remedy of course is to change fluid and filter.

just lettin ya know

Robert W. Roe · #10 05-20-2002, 12:35 PM

An inch and a half over the full mark is way high. I had good results switching to Mobil 1 synthetic ATF in my 1984 SD. The shifting was very rough and the synthetic has smoothed it out considerably. $4.99 a quart at AutoZone last time I checked.

diesel don · #11 05-20-2002, 02:22 PM

New data:

I've been searching and studying on the topic and decided to drive it without the vacuum hooked to the modulator. So I popped off the vacuum line atop the "white bleed valve" which sits on the back of the IP. I went out and drove it hard and alas, I couldn't make it flare. In fact, the car drives somewhat normal now with the slight exception of "little jerks" in the shifting here and there. But under good acceleration, it shifts fine and will not flare.

Am I correct in assuming I have a vacuum problem and not a transmission problem? My next move is to swap "white bleed valves" on my cars and see if that is the culprit. I suspect that it is not bleeding off enough vacuum under acceleration.

Bob: thanks for the info on tranny fluid. I ordered a filter last night and will be going with synthetic.

Any other thoughts would be appreciated.

Don

Ken300D · #12 05-21-2002, 07:54 AM

The way vacuum seems to modulate transmission shifting, taken from the measuring of vacuum on the line going directly to the transmission is:

- Less vacuum, harder shifting

- Too much vacuum, shifting is too soft and can flare

Too little vacuum is obvious - a leak

What is a little more hard to diagnose is a condition of too much vacuum. You really need a gauge to tell for sure. What appears to happen is that relatively constant vacuum coming from the vacuum pump is "modulated" by a controlled vacuum leak device that is mounted on or near the injection pump. The amount of the leak is determined by the position of the accelerator pedal (and a calibrating adjustment). The vacuum leak device "leaks" more vacuum as you press down on the accelerator pedal. The transmission is supplied this modulated signal. To make this work properly without killing the vacuum source to the rest of the car's systems, there is a flow restriction called an "orifice" that is the same thing as a resistor would be in an electrical sense.

The vacuum leak device on the injection pump can and does go bad, which can greatly affect transmission shifting. The typical failure mode is probably "leaking" too little (blocked), resulting in soft shifting.

There is a spec for how much vacuum you should have on the line going to the transmission at a certain position of the vacuum leak device on the injection pump. I don't have the CD with me but can provide more information later.

I'll bet dealers have made a TON of money rebuilding Mercedes transmissions that had nothing wrong with them except a bad vacuum modulation signal going into them.

Ken300D

DieselHead · #13 05-21-2002, 08:28 AM

Ken,
What would this vacuum leak device on the IP be called in FastLane. I have mildly soft shifts and flaring from 2nd to 3rd, and it sounds like it would be a good idea to replace this vacuum part that you're talking about. Thanks.

Alex

dieseldaddy · #14 05-21-2002, 08:43 AM

I know I am not Ken...
I found one on ************************. It is listed (graphically) as a "Vacuum Valve" and costs $129.00 there. It HAS to be less expensive on FastLane.

DieselHead · #15 05-21-2002, 08:50 AM

There's something called the injection pump vacuum valve on FastLane (and it asks for your trans. # so I guess that might be it) listed for $148. I guess that's an awful lot to spend on something that I don't even know is broken. How would I go about testing this valve to determine if it's bad? Thanks.

Alex

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