Revs drop when put into gear (Drive or Reverse)

[harrysihra]

Revs drop when put into gear (Drive or Reverse)

Hi everyone,

I recently bought a Mercedes W108 280SE Auto 1971 RHD UK-specification car. It has the M130 engine. It is an unmodified car, with only 2 previous owners.  Most of the last 20 years has been spent parked indoors.  The previous owner brought it back on the road after a couple of decades but sadly passed away before he could sort everything out properly. 

When I got the car, the idling was too high, and it was running extremely very rich.  It has since been to 2 separate Mercedes classic car (Pagoda) experts who managed to improve things a little but it continued to run very rough.  I have therefore been systematically working on it and have made a significant amount of progress:

1.   Replaced the distributor with a new 123 electronic distributor (as the shaft was wobbly and the contact points kept on getting pitted)

2.   Set the timing at 25 degrees BTDC @ 4,500 rpm

3.   Replaced the spark plugs

4.   Replaced the HT spark plug leads

5.   Checked all the linkage settings (in accordance with the linkage tour)

6.   Checked that the CSV is not spraying any fuel (when the starter is not cranking)

7.   Checked the Warm Running Device: this was not functioning properly, and I found the problem to be a blocked hose (which connects the WRD to the nipple on the Cylinder Head); I removed all the hoses and the body of the WRD and cleaned everything, and now the WRD functions perfectly, with a strong air suction when cold and shutting off completely when engine warms up.

8.   Set up the Idle Air/Fuel mixture, which had been very lean (I suspect the experts had tried to make the mixture very lean in trying to compensate for the fact (unbeknown to them) that WRD was not shutting when the engine had warmed up.

9.   The engine starts easily when cold and when warm, and is now running very smooth. It idles well both cold and warm.  The idling is a little high when cold and reduces as it warms up (as I believe it should)

10.   However, there is one problem which existed before and still remains: when you shift from Park to Drive or to Reverse, the revs drop and the engine stalls most of the time. The Pagoda forums put this down to the Constant Running Solenoid (or allied issues). The problem is that my car does not have this solenoid at the top of the engine (on the main linkage bar / crossover rod). I am assuming that my car does have a mechanism to slightly increase the revs when the car is put into gear, and that the mechanism is different to that which the Pagodas have.  Can anyone please help? By the way, my car does not have AirCon but has Power Steering.  One other observation is that my car does not have a venturi switch on the venture housing (is it missing or does this model not need one?).

Any help and input would be gratefully appreciated.

Thank you.

Harry Sihra

[harrysihra]

Sorry I meant to add this photo:

[Tyler S]

It does appear that you have the correct  cross shaft and cross shaft mount that accepts a solenoid. The throttle body butterfly rod end is missing at least a cover but possibly the switch. A cover is used in the position with the 2 holes when no switch was used. Are there any superfluous wires in these areas? Or exiting the harness?
Also did the car have a transistorized ignition setup before you installed the 123?

[kampala]

I don't know if your w108 is like a LATE w113 but if the throttle body is same and if the linkage is same, here is what I would check:

If you have a vacuum pot directly attached to the throttle body and you have a "slip linkage" on the link to the throttle body that operates the flap, like a late w113, than you need to check that the vacuum pot does what it should.  It holds the flap closed when in Park, and opens the air flap slightly when in Drive, using the slip portion of the linkage.  This allows a little more air and hence raises the rpm's slightly when in Drive.   

As I recall, this type of system typically would not have a Constant Speed Solenoid.  Unless the car had A/C, then it would have a CSS.

This is only useful if you have a system as described.   

[twistedtree]

Right, it looks like you are missing the throttle body switch and the idle compensating solenoid, assuming the 108 is the same as the 113.  The missing throttle switch will also negatively impact your transmission shifting.  Are gear downshifts harsh when you foot is off the gas peddle?

[stickandrudderman]

I find it odd that you have been to two classic merc specialists and neither have sorted it. This should be bread and butter stuff to them.
(I hope I wasn't one of them!)

[harrysihra]

Thank you all for your replies.

- Tyler: I can't see any superfluous wires in the area or going into the harness in that region. I don't know if the ignition was transistorised set up before - is there any easy way of checking this please?

- Kampala: There is no vacuum pot on the throttle body (see photo below).

- Twistedtree: From looking at the various photos of SL113s on this forum, it seems clear to me that that the linkage at the top on my car is designed differently and did not come with the idle compensating solenoid. The photos I posted yesterday should show this (but could I be wrong here?)

- Stickandrudderman: yes, I know, it should have been sorted by the experts but the fact is that they both also missed the fact that the WRD was not functioning because the coolant hoses were blocked.

So, is it possible that the W108 (or at least this version of the W108, which is a late 1971 one, among the very last few W108 280SEs to come out of the factory) has a completely different mechanism for compensating for the idle drop when put into gear? Could it be that there is some or other switch which operates a lever or something directly between the transmission and the accelerator pedal / firewall linkages?

Thank you again for your help.

Harry

[harrysihra]

Twistedtree: in reply to your question, gear shifts seem fairly smooth, except when you're in 4th and come to a hill, the car gradually slows down and doesn't easily shift back into 3rd.  thanks. 

[Tyler S]

Harry, Here are 2 photos of what I was referring to in regards to where the solenoid mounts (ignore the red highlight). It is different on later cars, the bracket actually mounts to the cross shaft mount on the passenger side. It appears you have this style cross shaft mount with 2 holes on the back side.
Cars that had the transistorized ignition had a box mounted under the battery as well as possible switchgear mounted on the drivers fender well. It is possible it has all been removed by the PO

[twistedtree]

The parts book for the car should show whether a solenoid should be preset or not.  The work shop manual might say as well.  It's certainly possible that it's different from the 113, but I would be a bit surprised.

Is there any chance the car originally had a manual transmission and someone converted it to an auto?  That might explain the missing wiring harness elements.  I know you said the car was unmodified, but 45 years is a long time and a lot could have happened.  The serial number will tell you how it was originally configured.

Again, assuming the car is set up like a 113, the transmission has a 3 position solenoid on the top side that controls the internal pressure in the trans.  Normally it is in the center position.  When you floor the gas peddle, the kick-down switch engages and the solenoid moves to one of the end positions (I don't recall which direction).  When the gas peddle is completely released, the switch on the throttle body causes the solenoid to shift to the other end position.  Without any throttle body switch, that trans solenoid can't be working properly.  There is good info about it in the tech manual.

[harrysihra]

Tyler S:

Thank you for your input and the photos, which are helpful.  I am attaching an image I have taken from the Owner's Manual which came with my car. It shows the automatic transmission oil filler and dipstick but coincidentally in this image you can see the nearby linkage. A solenoid is absent.  This may or may not be definitive?

Twistedtree:

Thank you.  Do you know where I can get hold of a parts book or a workshop manual for my car?
There is literally no chance that this car has been converted from Manual but I will find a way to verify definitively.
This weekend I will jack the car up and check the set up re the 3 position solenoid, etc.  There is a kick down switch under the accelerator pedal but I have never tried it, so will check that too.

Thanks again

Harry

[Fintail]

Hello Harry
The 108 has a different gearbox, setup and control of gearbox very different to Pagoda
All the John

[harrysihra]

Thank you Fintail/John. That's what I suspected. So please are you able to point me as to where I should look next to try working out why the the car drops revs and mostly stalls when put into gear? Thanks very much.

[Benz Dr.]

Your first mistake was to take it to a '' classic '' mechanic. 


  Your ignition timing is out a country mile. It should be around 38 degrees @ 3,000 RPM.  No wonder it stalls when put in gear - I bet the vacuum reading is about 10 inches at idle right now.

[harrysihra]

Thanks Benz Dr. Yes, agreed it was a mistake taking to classic experts! But where else should I take it? Main dealer doesn't have a clue...

The recommended timing for my car is 30 degrees BTDC @ 4,500 rpm, without vacuum.  See attached, the left column is for non-USA models (mine is 130.980). I will adjust this from the current 25 degrees BTDC. 

Any further input is appreciated.

[Benz Dr.]

Everyone seems to make that mistake. Set it up with vacuum lines connected and run it up until you get max advance which should be in the 38 to 40 degrees. If everything is working right, you should have about 8 degrees BTDC at idle. The 123 system may not give you these figures and still work OK. This is what happens when you make any major changes to established systems and it might be the laws of unintended consequences if things work differently.

[stickandrudderman]

UN unusually sweeping statement from Dan! I would suggest that a classic expert is indeed the place to go but the OP has been unlucky in that the two "experts" have failed to check the basics.

[Benz Dr.]

UN unusually sweeping statement from Dan! I would suggest that a classic expert is indeed the place to go but the OP has been unlucky in that the two "experts" have failed to check the basics.

Not if you know why. I only recognize real Full Classic cars and everything else is either antique, milestone, or just another used car. Too many classic cars today that are far from it.

[Tyler S]

I kind of like the word "Vintage". Even though it originated from wine classification.

[Benz Dr.]

If you bother to look up classic car you will find several definitions but the common one always includes an old car usually 25 years old or older. This works for government or insurance who really get things wrong anyway because they are only interested in taxation or an easy frame of reference.
 
  If people want to join in with all those who lump their stuff with millions of mass produced boring cars let them. Our cars are too well made and rare enough that they deserve better. My problem with the classic car crowd is they never bother to study automotive history. Just like there is a difference between classical music and classic rock, so too is there a difference between Full Classic Cars and everything else. Any serious student of music knows the difference and so should we.

 Vintage is OK and a step above lazy.