Lousy Shifting - Where to Start? - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 01-29-2017, 09:09 PM

The weather is improving and days getting longer and the time has come to tackle the problems of truly crappy shifting in my SDL. Spent days reading this forum, watching videos on Youtube, reading other forums and repair articles, visiting a certain "source" website, etc, and I've come to the conclusion that I have no idea where to even start with this car.

Car is an '86 300SDL with the 722.3 transmission. For all intents and purposes, let's assume all the hardware on the transmission is what it came from the factory with (the rest of the car has been that way), and that someone has taken the time to change fluid and filters at some point in the history of the car (no I don't have any records to prove it). ODO says 173K, but had the famous gear failure, so I suspect more like 225K at least.

What it's doing:

1-2 shift is late (~4200 RPM with light throttle) and VERY firm and abrupt *WHUMP*.
2-3 shift is late (same RPM as above) and acts lazy in engagement. Kind of like it makes the shift, then firmly engages. RPM doesn't flare and this sort of 2-stage shift happens regardless of load or acceleration style.
3-4 shift is late (>4000 RPM) and varies in firmness, but usually on the firm side.

When the accelerator pedal is hard down (floored, but not engaging the kickdown switch), the engine will accelerate to ~4600 RPM (governed - more on that in a minute) and have a delay before shifting gears. It does this for all upshifts in a WOT condition or when the pedal is more than 1/2 way down.

Downshifts are simply ridiculous. The transmission will not downshift under load unless the kickdown switch is pressed and held, then will not shift up again until the kickdown switch is released (the SL doesn't do this, if the KD switch is engaged, it'll shift up anyway if the RPM is high enough). The quality of downshift is extremely abrupt (it almost feels like someone's ran into the car). On occasion, the downshift occurs, then IMMEDIATELY re-upshifts.

I suspect I have several issues going on here. I have fluid and filter change on the list, but want to hold off on it until I know if there's something I need to be servicing inside the transmission while the pan is off.

The fluid is bright red and doesn't have a burnt smell. When stone-cold in the morning, shifting into reverse takes less than 1 second, with a firm "whump" on engagement. Same when shifting from R to D. Once the transmission has warmed up (5 minutes driving or so), the shifts into R and D are nearly instantaneous. This leads me to suspect that the mechanical health of the transmission is probably OK.

My car has the injection pump from a 350SDL in it. As a result, the redline RPM is lower than stock. I have a feeling this has a lot to do with the ridiculously high shiftpoints, basically at the rev limiter of the IP (~4700 RPM).

I assume the bowden cable needs adjustment for one thing. Would turning it in help lower the shift points to a more acceptable level? If I give the engine very little throttle (like resting my foot on the pedal to take off), it seems to shift nice and early, which is how I'd expect it to do with light throttle. The 2-3 shift is still crappy and slow though.

Additionally, I suspect that the modulator may not be working. I've played with the VCV on the side of the injection pump and no position made any difference. I even unplugged the vacuum line to the transmission and it made no difference.

I've read a lot about piston and spring replacements in the valve body as well. Are the symptoms above indicative of a piston with worn spring?

Issues with the "flying saucer"? Switchover valve? VCV?

It's important to note that the transmission doesn't slip and I've never experienced a "flare" in RPM during a gear change. 2-3 is a bit weird, but it doesn't ever "flare". I also have no leaks or drips. The pan gasket has some "sweat" around it, but nothing on the floor under the car (surprisingly the engine doesn't drip oil either).

Help!

sixto · #2 01-29-2017, 09:43 PM

To start, I'd back off the control pressure (Bowden) cable until it shifts at a reasonable rpm given throttle input. Also make sure the throttle link lengths are to spec. Next, I'd take the BFS out of the equation. Run the line out of the VCV through a green damper to the transmission modulator. I forget the spec but it's something like 12-15" Hg at idle and light throttle input. Shifts might be harsher until the transmission warms up. The right way to do this is monitoring transmission pressure.

Oh, make sure the lockdown switch isn't stuck on. Pull the wire off the lockdown solenoid at the tail end of the transmission if necessary.

Sixto
83 300SD
98 E320 wagon

Diseasel300 · #3 01-29-2017, 10:27 PM

I've run off copies of the throttle rod lengths. I'll make sure they're properly adjusted tomorrow. Can also bypass the BFS.

Doubt the KD switch is stuck on since pushing it until it clicks will cause a downshift.

Mxfrank · #4 01-29-2017, 10:39 PM

First thing to check is vacuum. A leak anywhere in the engine compartment or HVAC system will cause harsh shifting. Change the fluid and filter. Next, bowden cable adjustment. An easy check is to temporarily plug a vacuum gauge into the modulator line and see what the readings are when you are driving. If you have a funky 2-3 shift, the K1 accumulator spring would be worth checking/replacing. That's the easy stuff, reassess when that's done.

tyl604 · #5 01-29-2017, 10:44 PM

I would do something different - check to see if it will shift properly with vac. You need zero vac for up shifts and vac for downshifts.

You can test this by unplugging the line from the vav to the trans modulator. Hook that line into a Mityvac and run it inside the cabin. Then drive around without pumping up any vac to the trans modulator. The upshift should be smooth. Before coming to a stop, pump up the Mityvac and your downshifts should be smooth.

If it does not work like this, you probably have a vac problem. Of course there could be other things but this might identify one.

Diseasel300 · #6 01-30-2017, 10:44 AM

I guess I should clarify the downshift I'm talking about:

When needing to speed up and you're in a high gear - say you're stuck behind someone doing 30 in a 60 and they turn off the road (you're now doing 10 while they "square-turn" out of the driving lane instead of pulling into the turn lane) - you push the pedal firmly to drop out of 4th and into 3rd or 2nd to actually build some speed THIS century.

When doing this, simply pressing the pedal firmly will not cause a downshift. Ever. The ONLY way to get a downshift is to press the downshift button. When the downshift occurs, it is violent and sudden (feels like someone plowing into the back of the car!) and may or may not IMMEDIATELY shift back up into the higher gear. You simply never know.

Now the downshifting when coming to a stop or coasting is less noticeable. 4-3 is never noticed, nor is 3-2. 2-1 is a bit abrupt, sort of a "whump" when it happens.

Having read way more than I want to know last night, I'm suspecting that my modulator isn't working and I'm likely having issues with the K1 and K2 accumulator springs (the latter possibly not being an issue).

Today's task is to check the linkages and make sure they're the right lengths, then check the amount of vacuum going to the modulator to make sure it's actually getting some. I have a brand new vacuum pump and redid ALL of the vacuum lines in the car, so I doubt I have vacuum issues (the gauge will tell the story). Everything vacuum based works like a champ - shutoff lever, HVAC controls, door locks, brake booster, etc.

86-300sdl · #7 01-30-2017, 11:30 AM

1st triple check the points OP's are noting.

For reference my '86 SDL gets "clunky" on the downshift when there is a vacuum leak / modulator issue. Up shifts fine.

Delayed upshift points have been cleared by adjusting the Bowden cable.

You might want to drain a pint or two (trans pan hex bolt drain plug) and run a pint or so of Trans-X for a few hundred miles to clean things up. Then service the fluid and filter. This was recommended to me for my first trans swapped out at 320,000 or so miles.

I've also found that if I used non spec AFT, shifts were squirrely. Found this when I refilled the old trans (front pump seal leak) with non spec fluid (I think it was Dex4) that was available while out on the road.

Mxfrank · #8 01-30-2017, 11:32 AM

Downshift is controlled by an electrical switch located under the gas pedal. You'll probably have to lift the carpet to get to it. Sometime, debris builds up around the switch and prevents downshifting. Sometimes the carpet bunches up. Sometime the switch gets funky and has to be cleaned or replaced. Power for this switch comes from the Klima relay, which may not sound logical, but that's where it comes from. So a bad Klima will cause the downshift circuit to fail. When the switch engages, power goes to a solenoid on the back of the transmission. I've had the harness connector fail at the solenoid. It's a weird terminal, and needs to be sourced from Mercedes. Where I would start is to lift the carpet to expose the switch. I'd temporarily clip a momentary contact switch in parallel with the downshift switch. Then I'd take it on the road and test. If the temporary switch does the trick, replace the downshift switch. Otherwise, trace the power and replace the Klima, solenoid, or connector as appropriate.

freesoul · #9 01-30-2017, 11:50 AM

Look at the driveshaft flex discs, my car was driving terrible and the front flex disc was all torn up almost to the point of failure. When I replaced it, drove like a new car.

Diseasel300 · #10 01-30-2017, 07:35 PM

Houston - We have MULTIPLE problems.

Step 1: rig up ye olde redneck vacuum gauge out of an old manifold gauge from a Waukesha engine

Step 2: Connect said redneck vacuum gauge to a T fitting between the output of the blue UFO and vacuum line to the tranny modulator. Result: 8" Hg. Went for a drive. Reading would fluctuate, but not really move. Under WOT, it would actually RISE to ~12" Hg.

Seriously not right.

Plugged into the distribution T right above vacuum pump. Only got about 9" Hg.

Pulled off the fitting and measured directly on vacuum pump. 23" Hg.

Connected everything back up and started isolating circuits:

Disconnect HVAC: No change.
Disconnect Vac Accumulator: No change.
Disconnect VCV: Vacuum rose up to 21" Hg.

Isolated the output side of the VCV: Held at 21" Hg.
Isolated the line to the switchover valve: Held at 21" Hg.
Reconnect UFO: Drops to 9" Hg.

At this point, I bypassed the UFO and went straight from the VCV to the modulator through the green restrictor. Straight out of the VCV I get about 15" Hg. With the modulator hooked up, I can't get past about 10" Hg. I suspect the modulator is leaking vacuum.

That said, I went for a drive with the gauge plumbed in and it was behaving as I'd expect. Vacuum dropped with position of throttle. The 1-2 shift was MASSIVELY improved. 2-3 is still really crappy. 3-4 didn't really change. Shift points are still really high.

At this point it looks like I need a modulator and some vacuum lines (broke 2 messing with them). Is it worth replacing the blue UFO? What exactly is its purpose?

On the positive side, with the UFO bypassed, the downshifting was working properly. With the transmission shifted all the way up into 4th and going slowly (15mph), putting my foot down got it to drop to 3rd without having to lay on the kickdown switch. It also didn't feel like I was being rear-ended. Definitely onto something with the vacuum and control pressure.

Mxfrank · #11 01-30-2017, 08:19 PM

The Bufo is a sort of analog transistor for vacuum, it's properly called the vacuum amplifier. It takes the following feeds: full unmodulated vacuum, modulated vacuum from the VCV, and a boost sensor feed off the intake manifold. The output is an amplified vacuum signal, corrected for boost and throttle position. It helps the transmission shift more smoothly. Bridging the VCV signal directly into the transmission vacuum modulator line gives you a modulated but unamplified vacuum signal. That's most of the benefit.

The high shift points are probably caused by a misadjusted bowden cable. I don't know how the linkage on this particular car is set up, but the cable should be just exactly slack with the linkage in the idle position. If there's tension in it, shifts will be higher, and you may have a problem freewheeling as you stop. Beyond that, crappy engagement in 2-3, 3-4 may indicate problems with the K1 and K2 accumulator springs respectively.

my123ca · #12 01-30-2017, 08:29 PM

You could also check the switchover valve(s). Ive seen them get stuck .

Diseasel300 · #13 01-31-2017, 09:50 PM

Today's task was to check the connecting rod lengths and play with the Bowden cable.

Connecting rod lengths were definitely wrong. FSM says the main accelerator rod should be 120+/-1mm, cruise control is 101+/-1mm and IP rod is 154+/-1mm. I had 126, 101, and 168mm rods. Corrected them to what the FSM says and it's remarkable how much better the accelerator pedal feels.

The bowden cable was definitely too tight. In idle, it had a fair bit of tension on it so I turned it in just enough to release tension at idle. This seemed to drop the shift points at WOT down to ~4300 RPM, and speed up the shifting in town. I can now go into 4th gear in 30mph traffic, and the transmission rapidly shifts up under gentle acceleration instead of revving the nuts off between gear changes.

The additional vacuum to the modulator from the VCV bypassing the blue UFO seems to have an adverse effect. I suspect the modulator is way out of adjustment. With the shift points lowered and the VCV feeding directly into the modulator, I'm now getting about a 100 RPM flare during the 2-3 shift, and on occasion after shifting to 3rd, will drop back to 2nd for a second, then back up to 3rd. It hasn't done this before today's adjustments.

I borrowed a hand vacuum from my neighbor (I have one somewhere, I just can't find it when I need it!) and confirmed that both the modulator and the blue UFO leak. I couldn't even pump up the blue UFO. The modulator would build vacuum, but quickly bleed down.

At this point, I'm thinking the following:

- New vacuum lines (a bit crispy)
- New blue UFO (if I can find one)
- New modulator
- K1 and K2 accumulator springs
And of course, fluid and filter

sixto · #14 01-31-2017, 09:56 PM

The modulator is adjustable so figure it out before you get a new one or you could be back where you started. The proper way to adjust it is with a fluid pressure gauge. For now turn it a notch or two CW and see if shift crispness improves. Heck, you probably won't make things worse turning it a full turn or two. Keep track of where you started.

Sixto
83 300SD
98 E320 wagon

Diseasel300 · #15 01-31-2017, 10:01 PM

Given the vacuum leaks in both modulator and blue UFO, I'm not gonna waste my time with adjusting the current modulator. I fully intend to set the fluid pressure when it is replaced and the vacuum system is working properly. For right now, I'm mostly trying to figure out what I need to do to it when I'm under the car and have the pan off.

For what it's worth, if I pull the vacuum line off the modulator, it's back to relatively jerky shifts.