722.3 harsh downshift / clunking into 2nd gear

[phoenix217]

Hello everyone,

I'm new here and recently sold my last 124. This forum has been such a blessing to help me get my W140 S500 back on the road it's amazing. Thank you so much for this!

I have another particular problem with my gearbox.

A little background:

My 1994 W140 came into life as an S420 and was rebuilt into an S500 15 years ago. It was in terrible shape when I bought it (broken chain guides, transmission pops, differential makes noise, bent driveshaft, etc).

Now I've finally got it running almost perfectly.
The current mileage is 341,000KM.

What I have done in the 9 months of ownership (not driving of course ):

-Used but partially rebuilt transmission (~190,000KM)
-new differential with very little play
-new used drive shafts
-Used, but very good cardan shaft
-basically everything made of rubber (engine and diff mounts, flex disks etc.)

The problem is this, and I can't figure it out:

When I slowly roll to a stop (low rpm) and shift into 2nd gear (basically under no load), the differential makes a clunking noise. It‘s barely noticeable to the body, but to the ear it is very much.

Also, the upshifts are quite hard and I have to put my foot down quite low (3/4?) to get the engine above 4000 rpm. But that may also be normal (I usually only drive 722.6).

This clunking has improved with the new differential.
I also had this problem with the old gearbox. Even more so with the old differential, but that had a lot of play.

Tried things out:

-partially overhauled gearbox
-Whole rubber
-Modulating pressure valve (set to standard)

Things on my list to check next:

-Check bowden cable tension
-Adjust modulating pressure valve to soft (might eliminate my hard upshifts under normal/slightly increased acceleration)

The transmission tech's explanation (he used to work for MB, so should know a bit about it), told me it's the play in the differential. Although I replaced it yesterday and the problem is still there, but got a bit better.

Are there any known issues related to this? I've searched for hours and only came across one guy who had the same problem 15 years ago but never got it fixed.

Something else I should perhaps mention:

I haven't measured the vacuum system yet and haven't really had the opportunity to do so. If anyone can tell me the best way to do this, I'd be delighted.

The EZS and LH module are still from the 420, but I have both here from a 1992 S500. Could that be the cause? Probably not.

Many thanks in advance!

 

[nocfn]

Assuming the flex disks and carrier bushing are in good shape, fronts and rear engine mount (AKA transmission mount) are not crushed or deformed.... valve body issue with flow pressures... check the line from the firewall engine connection back to the modulator, sometimes that line sits on the body of the trans, gets a friction or heat burn in that line. At a minimum, be sure the rubber connections are solid and not leaking. You can also pump up a vacuum in the modulator box to see what you get, does it hold etc. Not sure what the vac holding pressure should be....

 

[phoenix217]

Assuming the flex disks and carrier bushing are in good shape, fronts and rear engine mount (AKA transmission mount) are not crushed or deformed.... valve body issue with flow pressures... check the line from the firewall engine connection back to the modulator, sometimes that line sits on the body of the trans, gets a friction or heat burn in that line. At a minimum, be sure the rubber connections are solid and not leaking. You can also pump up a vacuum in the modulator box to see what you get, does it hold etc. Not sure what the vac holding pressure should be....

Thanks for the reply!

Yes, I replaced all 3 engine mounts, flex disks, differential mounts and the center bearing of the cardan shaft recently.
The modulator valve is brand new from MB (new style, black).

All lines are in great shape and I have replaced all modulator rubber connections. The lines must have been replaced a few years ago, but I will double check.

Modulator spec for my transmission is 4.0 bar.
I‘m assuming that’s measured when the engine is idling?
Already ordered the necessary tools to measure.

Since this issue occurred on both transmissions , I’m assuming the modulator pressure isn’t right.
Either I have a leak or I need to adjust the pressure and make it softer.
Noticing that I can hear and slightly feel the 4-3 downshift as well.

I checked the bowden cable and adjusted a bit. Didn’t make much of a difference if at all.

 

[nocfn]

There is a member who has a w126 and the trans are similar. He had a writeup on his owner thread about his axle rebuild and differential. HE did all the work. Name is Roy, handle LIVIU165 He may have it on this forum, not sure.

 

[phoenix217]

@nocfn

Thank you for reminding me of checking the passenger side again! I found one vacuum hose to be unstuck. Replaced the rubber as well.

Immediately the clunking is improved by 70% at least.

Need to adjust shift points and turn down the modulator pressure. Shifts are all over the place and way to uncomfortable.

Noticing there is a slight play in the differential itself. But that’s fine for now.

 

[gsxr]

It sounds like you are assuming the noise/clunking is from the differential, but could be related to the transmission. If the differential has normal play, the diff is likely a red herring and not the cause of the issue.

How are the upshifts at both part throttle and full throttle (6000rpm)? The vacuum modulator should be adjusted to provide acceptable part-throttle shifts with zero slip/flare at WOT upshifts. If you aren't testing WOT upshifts, you need to do this (it is not damaging to either the engine nor transmission).

Bowden cable adjusts shift RPM, not shift firmness.

Sounds like you located a vacuum leak that helped the issue significantly - check the entire vacuum system.

Why didn't you use the 5.0L LH + EZL from the beginning? At a minimum you need to install the 5.0L LH module to get the proper fuel mapping. As you stated, this won't have any effect on the shifting. Since you have the EZL, install that too (with fresh thermal compound).

BTW - welcome to the forum!

 

[phoenix217]

Thank you very much for taking the time to help me with this problem.

Let's start at the beginning:

The car was partially converted from S420 to S500 about 15 years ago by the previous owner (I've only owned it for a good 9 months). Specifically, only the engine and gearbox were replaced. The differential, cardan shaft, drive shafts, LH control unit and EZL were from the S420.

I have spent the last few months rebuilding the broken chain guides, timing chain, badly worn differential, bent drive shaft, missing catalytic converter and everything else mentioned above but probably way more. The engine runs great and I don't think it draws any secondary air.

Last night I converted the LH and EZS to 5.0L. The engine continues to run very well and feels like it has the same power. But more on that in a moment.

Neither the gearbox nor the differential (as I bought the car) have ever had an oil change. The gearbox shifted terribly and the differential was clunking and grinding.

Then I only replaced the differential without knowing that the cardan shaft was 2 cm too long. The S420 and S500 differential cases are different lengths and widths. The almost 1000KM must have damaged the differential.

I had the gearbox fitted under warranty a good 6 weeks ago. Former Mercedes transmission builder. He also noticed that the cardan shaft was too long. This was then replaced with the correct one.

However, the engine was missing the sleeves that bring the gearbox 100% correctly into position. This was not noticed during installation. The result was a broken ring in the front of the oil pump and massive oil loss.

So back to the workshop and the oil pump replaced and the sleeves fitted.

I then replaced the differential. Used, mileage unknown. It's a bit noisy, but not as bad as the previous one.

Now comes the exciting part:

I just drove for a good two hours and played with the shift point.

Yesterday, before I found the vacuum leak, I had set the shift point to very late (i.e. soft). In combination with fixing the leak, I was naturally pleased.

But no kickdown was possible and there was not enough power available. So I raised the shift point significantly again, and lo and behold: the differential clunks again when shifting down. But not only when shifting down, but also - and this is where I need help to understand the matter - load changes. You can imagine it as if I'm cruising without standing on the gas (the car just rolls), I tap the gas pedal very lightly and hear a pop in the differential. The same happens when I cruise with my foot on the gas pedal and then let go. I can also provoke the whole thing by abruptly stepping on or off the accelerator pedal.

So I set the shift point to later again, but so that the kickdown works and a rapid downshift occurs when I press the gas pedal 3/4 down.

Now I'm at a point where the gearbox works well as mentioned and the clunking in the differential is not too strong.

However - and I've tried a lot - I get to a maximum of ~5500 revs.

About the shifting behavior:

Grotty. The upshifts are almost imperceptible at 10% throttle position, beyond that they are hard. Clearly noticeable and abrupt. The description can also be applied to downshifts.

At WOT (up to the kickdown switch), the transmission shifts very noticeably. There is room to make the gear changes smoother.

Plan for tomorrow:

-Check the vacuum system for leaks (this time with proper tools)
-Check rear axle mounts
-Set the transmission shift point a little earlier (or stock?)
-Adjust modulating pressure significantly (probably 0.8 bar / 2 turns)

Unrelated question: Can I combine a 4.2L EZL with a 5.0L LH module? What are the differences?

 

[gsxr]

Put gearshift in "B" position, it should pull to 6300rpm before the rev limiter engages. If the engine will not pull to 6000rpm, check fault codes, in particular on the E-GAS module (or, T/LLR if your car does not have ASR). If the module is not receiving a valid signal from the NSS / gear position indicator on the transmission, this can cause the 5500rpm limit. The WOT (WOT means including kickdown activated) upshifts should look like the video below, although your 140 will accelerate a bit slower due to added weight and taller gearing.

I still believe what you think is a "differential clunk" is actually from the transmission. I've heard a similar driveline clunk in the 124 chassis and it's from the transmission / driveline, not the differential.

You can usually use a 4.2L EZL with a 5.0L engine and 5.0LH LH module. The ignition advance curves may not be optimal, but most combinations will work fine without negative effects or fault codes.

 

[phoenix217]

Put gearshift in "B" position, it should pull to 6300rpm before the rev limiter engages. If the engine will not pull to 6000rpm, check fault codes, in particular on the E-GAS module (or, T/LLR if your car does not have ASR). If the module is not receiving a valid signal from the NSS / gear position indicator on the transmission, this can cause the 5500rpm limit. The WOT (WOT means including kickdown activated) upshifts should look like the video below, although your 140 will accelerate a bit slower due to added weight and taller gearing.

I still believe what you think is a "differential clunk" is actually from the transmission. I've heard a similar driveline clunk in the 124 chassis and it's from the transmission / driveline, not the differential.

You are (unfortunately) right.

The problem comes from the gearbox.

I have checked the vacuum. This also changes accordingly.

I have turned back the modulating pressure by 3 turns. Result: Every 5th gearshift is still clearly noticeable, the rest almost not at all. But I have lost the kickdown and generally downshifting at 3/4 throttle position.

I have made a video of the gearbox noises during and after shifting. Can you make any sense of it? The “knocking” clearly comes from the gearbox and is transmitted to the propshaft center bearing and then ends up in the differential.

I converted the E-GAS module to fit the S500 dispenser. Still no kickdown. In fact, they are far from it. Read out the fault memory beforehand. This was empty. Switching to “B” doesn't change anything either. Any ideas?

 

[TerryA]

Food for thought. Rough shifting can be coming from the valve body. I would check it for broken springs or just give it a good cleaning and replace the springs.

 

[phoenix217]

Food for thought. Rough shifting can be coming from the valve body. I would check it for broken springs or just give it a good cleaning and replace the springs.

True! But what I haven’t mentioned is the noise of the transmission itself. When I have it in P or N, I can hear it howl slightly. I can also hear the up- and downshifts directly below me.

I think it‘s gone. And luckily under warranty. Will be interesting to know if they overhaul it or not.

I‘m debating about converting to a 722.6. They are the better transmissions anyway.