300CD first gear... slip? - PeachParts Mercedes-Benz Forum

CDTurbo001 · #1 12-06-2017, 09:49 PM

Since adjusting the throttle linkage and Bowden cable on my '82 300CD the other day, shift behavior is much improved. No more harsh 1-2 and slipping 2-3 - under normal acceleration the shifts are firm
but not violent. However, after having pulled up to a stop in traffic, etc., when it comes time to move again I have to tip just a bit into the gas pedal, wait 1-2 seconds until the transmission seems to fully re-engage first gear, then I can move on and it behaves fine until the next stop. My initial setting for the Bowden cable was where all slack in the cable was out at idle, but no further. Tried 1.5 turns out and that doesn't correct the problem - haven't tested where the WOT shifts are as a gauge of cable adjustment, don't really want to run the engine that hard, that much until I can adjust the valves and check timing chain stretch.

Also, I checked the modulator line for vacuum leaks - held pressure fine. Also replaced the hard lines from the switchover valve to vacuum supply, and to the coupler leading to the transmission; it seemed the multi-connector on the SOV had become hard and was allowing a small leak. Removed the line to the ported vacuum switch/EGR as well. Did not test the vacuum signal from the SOV - as I understand it's supposed to vary with throttle position?

Any insight on what's causing first to partially disengage/slip until the pedal is advanced slightly?

DeliveryValve · #2 12-07-2017, 08:14 PM

Good chance your B2 piston is slightly sticking. You may want to consider upgrading the piston and a newer nylon sleeve before it ruins the piston bore.... Unless it's been upgraded already...

.

CDTurbo001 · #3 12-07-2017, 08:51 PM

Okay. Does the valve body have to be removed to get to it?

What, exactly, is the B2's function(s)?

DeliveryValve · #4 12-08-2017, 02:41 AM

The B2 piston/band controls the 1st gear. When the piston and sleeve is bad, it starts to bind. With so much hydraulic pressure on the piston, in time, the piston can get stuck and no forward gear will work.

You don't need to remove the valve body. It's located behind a round cover on the right side of the trans.

Here is a write up about how to replace it.
b2piston

B2 piston replacement

.

CDTurbo001 · #5 12-08-2017, 09:02 AM

Cool, thanks. I've also narrowed down that it doesn't do this when the shifter is in "L", but can make a REALLY nasty noise on the 1-2 shift if you attempt it manually. Does that further support a sticky B2?

Diseasel300 · #6 12-08-2017, 09:46 AM

Since the shifting was harsh before you fiddled with anything and now it's slipping, I'd go back and find out what you adjusted to invoke the current behavior. If you have too much vacuum going to the modulator, or if the modulator is misadjusted to compensate for the lack of vacuum previously, you'll have problems.

Get a copy of the FSM and follow the procedure TO THE LETTER on how to set the modulator pressure (it will involve connecting a pressure gauge to a test port on the tranny), then fiddle with vacuum to tune it "just so".

DeliveryValve · #7 12-08-2017, 09:59 AM

Quote:

Originally Posted by CDTurbo001

Cool, thanks. I've also narrowed down that it doesn't do this when the shifter is in "L", but can make a REALLY nasty noise on the 1-2 shift if you attempt it manually. Does that further support a sticky B2?

B2 issues are for all First gear, so it should also do it on "L".

Not sure on the nasty noise on the 1-2 while manually doing it. But the B1 controls the second gear.

Recheck your previous work, but I still wouldn't rule out the B2.

Frank Reiner · #8 12-08-2017, 04:14 PM

Quote:

Originally Posted by CDTurbo001

Since adjusting the throttle linkage and Bowden cable on my '82 300CD the other day, shift behavior is much improved. [1] No more harsh 1-2 and slipping 2-3 - under normal acceleration the shifts are firm
but not violent. However, after having pulled up to a stop in traffic, etc., when it comes time to move again I have to tip just a bit into the gas pedal, wait 1-2 seconds until the transmission seems to fully re-engage first gear, then I can move on and it behaves fine until the next stop. My initial setting for the Bowden cable was where all slack in the cable was out at idle, but no further. Tried 1.5 turns out and that doesn't correct the problem - haven't tested where the WOT shifts are as a gauge of cable adjustment, don't really want to run the engine that hard, that much until I can adjust the valves and check timing chain stretch.

Also, I checked the modulator line for vacuum leaks - held pressure fine. Also replaced the hard lines from the switchover valve to vacuum supply, and to the coupler leading to the transmission; it seemed the multi-connector on the SOV had become hard and was allowing a small leak. Removed the line to the ported vacuum switch/EGR as well. Did not test the vacuum signal from the SOV - as I understand it's supposed to vary with throttle position?

Any insight on what's causing first to partially disengage/slip until the pedal is advanced slightly?

Quote:

Originally Posted by CDTurbo001

Okay. Does the valve body have to be removed to get to it?

[2]What, exactly, is the B2's function(s)?

Quote:

Originally Posted by CDTurbo001

Cool, thanks. I've also narrowed down that it doesn't do this when the shifter is in "L", but can make [3] a REALLY nasty noise on the 1-2 shift if you attempt it manually. Does that further support a sticky B2?

[1] For the record: the transmission is a 4-speed unit that normally starts in 2nd. CDT, since you have not mentioned a 3-4 shift, it just may be possible that you believe the trans to be a 3-speed, since it starts in 2nd.

[2] The B2 piston applies the #2 brake band (hence "B2"). B2 is applied in 1st, 2nd, & 3rd, and released in 4th.

[3] When the selector is placed in L, the trans is placed in 1st. On the actual 1-2 shift, two things occur: a one-way clutch (aka, "freewheel" or "sprag") is overridden, and band B1 is applied. The most likely cause of the noise is a chattering band due to a slow/weak applying B1 piston. A weak B1 apply (low hydraulic pressure at idle) would also be consistent with the "slipping" complaint when starting in 2nd from a stop.
Band B1 is applied only in 2nd, released in 1st, 3rd, & 4th.

CDTurbo001 · #9 12-08-2017, 08:22 PM

Thank you for the details, Frank. However, I'm aware the transmission is a 4-speed but I've never known mine to start in second. I've heard mixed opinions whether the diesels are supposed to do it or not, but it seems it's normal as no one else has mentioned it. I've not mentioned a 3-4 shift because it's never been a problem nor has its behavior changed greatly through my adjustments.

I actually believe the transmission is operating more the way it should now than before my adjustments - engagement of the forward gears when it's cold isn't as much of a gamble as it was before, and each shift is more consistent. Will too muchvacuum with the Bowden cable at "0" (accelerator at rest) cause the 1st gear glitch? If higher vacuum = lower trans. pressure, might excess vacuum drop fluid pressure below the point that the B2 stays engaged? Maybe I should disconnect the switchover valve (supposed to just make all shifts hard?) to see if it still does it? Not saying that's a substitute for checking vacuum and line pressure, but a preliminary test.

Edit: upon more reading in other forums, some owners report a 2nd gear start with a drop to first upon even the slightest accelerator movement. This could be what I'm feeling, but it seems Mercedes calibrated the transmissions differently for gas vs. diesel, and even for which rear end gears were in the car.

I agree a sluggish B1 could cause what I'm feeling, because if 2nd gear is in play from a dead stop (which I hadn't yet considered) the problem isn't slipping in first, but indecision or delay between first and second.

DeliveryValve · #10 12-10-2017, 09:25 AM

Regarding 1st gear starts.
I am in the camp that believes all Turbo Diesel 722.303 and 722.315 start in the 1st gear.

N/A diesels and gassers start in the 2nd gear.

.

bigsky · #11 12-10-2017, 01:19 PM

My 78CD, 79CD, and 79 280CE all start in 2nd. The 81CD NA , and 83CD both start out in 1st.

CDTurbo001 · #12 12-10-2017, 01:23 PM

It seems mine should start in first, then. Was this because the turbo cars were geared faster, so the give and take was lower highway RPM's with 1st gear starts to retain off-the-line pep?

bigsky · #13 12-10-2017, 11:01 PM

Believe that all the turbo coupes came with 1st gear start tranni's, and longer legged diffs. Was surprised when the 81CD NA started out in 1st gear. The 81CD is in really poor shape, and I have only put about 100 miles on it. Front end is in need of complete rebuild/bushings. Tires shake bad enough to make it unsafe to test further. Will probably not do anything with it until spring, letting the $$ situation build up.

CDTurbo001 · #14 01-13-2018, 08:50 PM

The saga continues...

Changed the fluid about 3 weeks ago, including the filter and torque converter. I have confidence the Valvoline Maxlife fluid is only going to help the transmission, but first gear only seemed to get more puny. The last week or so I had to be extremely gentle with it until I got past about 5 MPH or it would slip.

But, today I put it up in the air and checked transmission pressure... a full 5 bar! It appears to have the updated modulator on it with the plastic, slightly-adjustable cap - I wonder if the pressure wasn't checked and set correctly when that was swapped out some time in its past. Anyway, I corrected the pressure to approx. 3 bar (spec is 2.9) and it behaves much differently - including engagement of first. I only had time to take it down the street and back to make sure it would go through the gears, but I'm optimistic that it will improve first gear's behavior.

I noted that when the modulator is connected to vacuum pressure at the check port goes to near zero - why is this? Also, any theories why excess line pressure could cause apparent first gear slippage?

ROLLGUY · #15 01-13-2018, 11:31 PM

Quote:

Originally Posted by bigsky

My 78CD, 79CD, and 79 280CE all start in 2nd. The 81CD NA , and 83CD both start out in 1st.

The 80 and older cars had the 722.1XX, and 81 and newer had 722.3XX. I find it hard to believe that any N/A engine would have a trans that started in second. I believe the 300SD (W126) did start in second, but can't believe any of the others did, especially the W123. I have had many W123 coupes and sedans, and I don't remember any of them starting in second.