Quick transmission shifts 4th by 20 mph... - PeachParts Mercedes-Benz Forum

smartin · #1 05-10-2003, 03:31 AM

1985 (CALIFORNIA) 300 TDT. Shifts right up to 4th gear ( by 20 mph) even if I floor it. Bowden cable adjustment does nothing, all the way in or out - NO change. I can even disconnect cable from socket - no difference. I can shift through gears "manually" no problem shifts well. Car gets up to speed well 60, 70, 80 mph not a problem. But before 20 mph - acceleration is hindered by quick up shifting - on a hill (going up) everybody passes me, again until I get to 30, 40 mph. Help...Fluid is okay filter changed, modulator changed, (By indie mechanic) says overhaul. Anybody? I have posted this before - got different responses - most trying to convince me bowden cable not adjusted right. Anything else I should look for? Thanks in advance for any ideas.

engatwork · #2 05-10-2003, 06:36 AM

I had the same problem in a '85 300D until I swapped out the tranny and resolved an issue with the kick down switch. The kick down switch underneath "fuel" pedal was not getting energized to power the kick down solenoid at the passenger side rear of the tranny. Remove the floor mat and look for the switch behind the pedal. You will see two tabs that have to be squeezed in to pull it out. Make sure it has 12 volts on one wire coming to it and insure that it makes up when depressed. When it makes up it is supposed to pull the kick down solenoid in. If it is not getting power it will not work. You will have to troubleshoot why there is no power to it. In a properly working system all you should have to do is adjust the Bowden cable. I have not torn into the tranny I pulled out of the car to try to ascertain what is going on inside of it yet but I had replaced the Bowden cable and that did not do any good.

smartin · #3 05-10-2003, 03:03 PM

engatwork - hey thanks for the response - I've checked voltage with a low voltage tester (light in screwdriver type) - light goes on and off when "clicked' - seems okay - (took switch out to test) - where is wire under car - is it in front of trans pan - central at (near) bottom - does it have a single wire - I can pull out (single wire) with a short metal cylinder (couple inches long) attached to it along with black rubber boot at top where wire connects. Is this it canI just ground the wire OR the metal cylinder with voltage tester to see if there is current going to it? Do I need to check exactly how many volts are going through? I wonder if the kick down solenoid and the quick shift are related - one causing the other to act in certain ways. Thanks

smartin · #4 05-13-2003, 02:15 AM

Thanks to all out there who have responded. I did get response not long ago that I could just run it that way - "manually" shift when I wanted some extra acceleration, and just leave it alone - OR overhaul. Any last comments? Any cleaners for trans flush? Thanks again.

BIGRED · #5 05-13-2003, 03:41 AM

Have you checked the vacuum and the vacuum valve?

smartin · #6 05-13-2003, 01:36 PM

Do I just check to see if it holds vacuum? With Mity Vac? Thanks

BIGRED · #7 05-13-2003, 07:36 PM

read the info here, it should help.

http://www.mercedesdismantlers.com/AutomaticTransmissionInformation.html

smartin · #8 05-14-2003, 02:18 AM

BIGRED - That is the article I read before - only I did not understand it very (months ago) I amd relatively new to this system - I will immediately get out my Mity Vac!! Thanks. Thanks for finding that article.

smartin · #9 05-15-2003, 01:46 AM

Well - checked vac supply at "T" from brake booster - engine running = 19 inches. (However - did not "jump" up to 19 inches - took about 1-1/2 seconds) Then checked line from white valve on IP, = about 19 inches. Then checked with throttle movement - 1/4 throttle - CLICK at switch over (on fender near black piece that feeds vac line to EGR AND recirc valve on turbo) when CLICK happens (1/4) throttle, vac jumps down to about 10 inches, then goes back up to 19 slowly. After that at 1/2 throttle no change in vac. Do you think that this switch over (3 way) valve is to blame? If I take electrical connector off of this valve - vacuum does not drop at all.

BIGRED · #10 05-15-2003, 10:34 AM

Electrical connector? Are you tapping into the line that goes from the manifold, to a valve and to the ALDA? (flatish metal colored part next to the the white valve I posted the picture on). If so that is the turbo line, not the vacuum line.

A simple test would be to find the black vacuum line that leads down to the trans. See iff that holds vacuum. If it does, unplug it and cap the rubber 2 or 3 way that it came out of. Then take the car out for a drive. If it starts to clunk when you come to a stop and also holds the shift longer than it now does, then it is in your adjustments.

If the line to the trans doesn't hold vacuum in your test, then the modulator is leaking, most likley the black cap that covers the tee adjuster.

#11 05-15-2003, 10:54 AM

All my Mercedes diesels with auto have shifted to high at way to low a speed to suit me.. I started just running them with the shift lever in 3rd.. some are marked 2nd but are really third.. and shifting into 4th or drive when out on the road.. My 76 240 shifted into high at 26. 79 300sd at 24, 78 300cd at 24.. and 85TDT at 22 mph.. Living in hill country it was just not some thing that works well, so have been just shifting to drive when I am running on the highway or need it .. Mercedes says that is why the shift lever has no gate between 3rd (Marked 2nd on some cars and Drive..

While the kick down does work on my cars, I seldom ever use it and just make the shift down by hand.. I do notice the biggest difference with my 92 300 CE as it takes some time for the transmission to shift and seems to wait until the cam timing changes.. Making the shift by hand far faster.. I also notice the shift by hand works faster than the kick down on the 300TDT.. However its some thing we do many times every day we drive so we do notice it more..

Ken

smartin · #12 05-15-2003, 11:49 AM

BIGRED - The elect connector I'm referring to is on fender drivers side, tied into a vac transducer (I think) that converts a signal into vacuum for the EGR and RECIRC valve on turbo. Anyway, the click happens here and it definitely causes the vac to the trans line to drop (RAPID JUMP) down to around 10 inches, then it immediately jumps back up - not a gradual release of vacuum as I thought it should be - or am I missing something. I will test modulator for vac, and drive with it plugged. Thanks.

smartin · #13 05-15-2003, 11:52 AM

Ken - Even your upshift by 22 mph is better than my situation, have not really used lever to "down shift" yet was not sure if everything was okay with trans. Thanks. Once I'm past 20 mph - pick up is good to very good (after 30 mph) - freeway at 70, 80 - all good there, very smooth.

Fredmburgess · #14 05-15-2003, 03:51 PM

Smartin,
Have you checked the vacuum modulator adjustment? Follow the black vac line from the white valve (pictured earlier in the thread) to where it ends on the trans (driver's side, just above oil pan flange). Modulator valve has a black rubber cap on it. Pop off the cap and there is a small metal "tee" handle. Pull the little tee handle out about 1/8" so you can rotate it to adjust the pressure in the trans. That thread posted earlier will tell you which way to turn it. I think you want to go CCW to reduce trans pressure. Just try about 1/4 turn or so to see if it makes a difference. Don't go too far or you'll cause the trans to slip and wear out the friction plates (don't ask how I know that!). Anyway, try searching for "vacuum modulator" which should take you to some good threads.

Alternatively, find a good independent Benz shop and have them check the trans fluid pressure with a gauge and adjust the modulator accordingly. What you have is referred to as "stacked shifts" elsewhere in these threads, I believe.

Fmb

lrg · #15 05-15-2003, 04:01 PM

BIGRED,
Remember that the '85 California cars have a unique electrically controlled vacuum system that ties into the emissions system. The whole group of vacuum lines that most 123s have on the top of the cam cover aren't there on the '85 Cali version. Our cars only have a single microswitch on the cam cover than gets tripped at full throttle. All the vacuum lines are attached elsewhere at other controllers. I think Smartin is on the right track.