Shift Flare 2-3, 3-4 '83 300D - PeachParts Mercedes-Benz Forum

hobberslobbers · #1 05-31-2010, 08:50 AM

After replacing both the K1 and K2 accumulator springs, I am still getting a medium to severe 2-3 and 3-4 shift flare, especially when the engine/trans are cold. The problem with the flares almost entirely go away once the engine/tranny are completely warmed up. The flares occur with vacuum to the modulator BUT NOT with the line disconnected. I confirmed that the modulator holds vacuum but it does leak down slowly. I have considered the problem being the B2 piston, however the car starts off beautifully under acceleration with the 1-2 shift nice and firm, and almost unnoticeable. I really want to get this sorted out. When I did the K1/K2 springs, I refilled with Dex III/Merc fluid after draining the TC and putting on a new filter.

tangofox007 · #2 05-31-2010, 08:53 AM

Quote:

Originally Posted by hobberslobbers

After replacing both the K1 and K2 accumulator springs....

What did you replace them with?

hobberslobbers · #3 05-31-2010, 10:17 AM

thx tango. 2x K1/K2 spring kits from phil. additional information. i just checked the vac on the modulator line in the engine compartment and it leaked down slowly (approx. 1 psi per second). so, i jacked the car up and checked the vac right at the modulator and i had the same rate of leakage. i compared these results with the vac on my '82 (which has no shifting probs) and the '82 vac was rock steady. i reconnected the vac line to the modulator and took the car out for a test drive with the vac line to the modulator detached in the engine compartment. different results this time. the car shifted fine (though rougher) through all gears at any rate of acceleration with a clunky 2-1 downshift (expected with no vacuum) when coming to a stop. so, am i correct in my assessment that the vac modulator needs replaced? if so, is there a step-by-step posted anywhere which will show me how to do this?

tangofox007 · #4 05-31-2010, 11:22 AM

Quote:

Originally Posted by hobberslobbers

the car shifted fine through all gears at any rate of acceleration with a clunky 2-1 downshift (expected with no vacuum) when coming to a stop.

When coming to a stop in "D", the final downshift should be 3-2. The 2-1 downshift should occur when leaving the stop under load.

Quote:

Originally Posted by hobberslobbers

so, am i correct in my assessment that the vac modulator needs replaced?

The leak could be solely due to the modulator cap. However, there probably aren't many 25+ year old modulators that could not stand to be replaced. (The spring inside the modulator is prone to rusting.)

Quote:

Originally Posted by hobberslobbers

is there a step-by-step posted anywhere which will show me how to do this?

There are plenty of past discussions on the job. My advice is to buy a new pin with the modulator. It's tough to avoid breaking the old one.

hobberslobbers · #5 05-31-2010, 11:49 AM

tango, you are correct sir! i pulled the flimsy old-style rubber cap off and it is cracked around the margin of the base. i have a new modulator and pin, and wouldn't you know it, the cap is different. anyways, i took the vac with the cap off and it leaked down very quickly (3 to 4 psi per second). with the cap on, only about 1 psi per second. so, anyone know of a quick fix until i can figure out how to replace the modulator?

Biodiesel300TD · #6 05-31-2010, 12:04 PM

A bad modulator usually is leaking, and a leaking vacuum system causes hard shifting not flaring. Took much vacuum can cause flaring. Also, if your shifting problem occurs with the vac modulator hooked up and unhooked then the modulator is not the problem. Usually when you unhook the vacuum modulator you get really hard shifts. If you still get flaring with it unhooked I'd venture a guess that you issues are a little deeper.

hobberslobbers · #7 05-31-2010, 01:04 PM

yes, with the line disconnected it does shift harder, but no flaring. and, it does have an abrupt downshift when slowing down. i checked vac at all other points in the engine compartment and they hold. since i have the vac modulator in-hand, i would like to go ahead and change it. at this point, what i need is a step-by-step link on how to do this. i see the retaining ring on the vac modulator with the two 10mm bolts that have to come off, but my 1/4" 10mm socket is too fat and won't go down far enough to get over the bolt head. hmmm...

Phil · #8 05-31-2010, 02:16 PM

There is a rather lengthy post/ write up about setting up the transmission. I don't know what else you have done other than the k-1 and k-2 but all the other adjustments are part of dialing it in. Now that you have change out those two items you can go through the rest of the set up. The writeup is probably in the DIY section at the top of the page.
Here it is.
It's CRITICAL... how you set your transmission's vacuum system on your diesel MBZ...

dkveuro · #9 05-31-2010, 02:44 PM

Try making a constant 2 inches of vacuum to the trans and drive it again. If the shift is okay, firm'ish but no flare then you need to adjust the vacuum control valve on the i/p to mimic 2 inches at part throttle.

If the restrictors in the supply of vacuum to the shift circuit are missing or too big you'll never get the shifts right because the vacuum valve can only bleed off so much vacuum signal . High vacuum volumn makes for soft shifts and often flare.

Steve Brotherton wrote the definitive paper on transmission vac' control and adjustment..read it all, it's very enlightening. Go to this page and click on link 'MAGAZINE Articles'..........

http://www.continentalimports.com/ser_index.html

.

hobberslobbers · #10 05-31-2010, 05:33 PM

thx for the tips. for curiosity i went ahead and put a "T" on vac line going to the tranny. to my surprise, it showed 15" of vac at idle. i have read elsewhere that this value should be nearer to 10". so, i went for a drive with my mighty vac under the wiper and, under acceleration, the lowest the vac went to was 5". it should be at 0" (no vacuum) for a normal shift, right? how do i get the vac to be at 10" at idle? i have read about orifices to put on the vac line going to the main vac line, and there is a yellow one on there already.

dkveuro · #11 05-31-2010, 09:17 PM

Quote:

Originally Posted by hobberslobbers

thx for the tips.......... so, i went for a drive with my mighty vac under the wiper and, under acceleration, the lowest the vac went to was 5". it should be at 0" (no vacuum) for a normal shift, right? how do i get the vac to be at 10" at idle? i have read about orifices to put on the vac line going to the main vac line, and there is a yellow one on there already.

Refer my last post and the link I posted and all will be revealed.

The DOME on the vac' valve mounted on the i/p must be pried off and the nut/bolt clamp on the shaft can now be adjusted.....at half throttle aim for 5 inches.

hobberslobbers · #12 05-31-2010, 09:53 PM

dkveuro, thx. yes, this is a great article and i will try the adjustment in Figure 2. i also read a DIY on making sure the vac control modulator is set properly. i will assume for now that it is set properly and try adjusting the vac valve on the IP tomorrow afternoon. i will update this post with the results.

dkveuro · #13 05-31-2010, 10:18 PM

You have to call Mercedes with your transmission number 722.3**** or 722.4 ***? They will/should have the modulator line pressure for that particular transmission.

Trouble is, you'll have to measure it with a pressure gauge.

tangofox007 · #14 05-31-2010, 11:00 PM

Quote:

Originally Posted by dkveuro

Trouble is, you'll have to measure it with a pressure gauge.

The other "trouble" is that those pressure specs don't mean a lot with an "experienced" transmission.

Cr from Texas · #15 05-31-2010, 11:28 PM

Quote:

Originally Posted by dkveuro

Trouble is, you'll have to measure it with a pressure gauge.

There's a thread on how to connect a banjo bolt and clear tube to measure the pressure by the height of the fluid in the tube. I'm not sure what search term to use - I didn't find it yet.