Throttle Switch Adjustmnt

[Pawel66]

I know this topic popped up here and there, but I am missing a wrap up a bit. Also the Linkage tour is silent about it, it just says Throttle Switch has to be properly adjusted.
So please kindly take a look at the below and tell me if I am right or please put me on the right track.

1. I press the brake pedal and select a gear with the selector. The Constant Speed Solenoid pushes rods, throttle opens slightly to compensate for the gear box load. The throttle switch keeps providing voltage.

2. I touch slightly accelerator pedal. Rods at the throttle move by a milimeter or so. Now throttle switch stops providing voltage and gear is engaged.

Is my understanding correct? Please kindly confirm or deny.

I had it adjusted for the idle throttle position and I felt car jump a bit on gear engagement and I heard clunk sometimes on downshifts. Now I have the switch cut off moment on the edge of where solenoid pushes rods - I have less jump and fewer clunks. I probably need to move it a bit further behind the moment when solenoid pushes rods.

Pawel

[Benz Dr.]

Yes,
you need to allow for the CSS movement for the switch to work right.

[Pawel66]

Thank you, Dan!

[wwheeler]

Ditto what Dan said. The only thing that makes this adjustment sort of tricky is the amount of slop in your linkage. You don't want the switch to open at the precise position the solenoid pushes the throttle to, rather a bit after that. Hard to do on your own. Best is to have a helper in the car and put in a drive gear with the engine warm. Then with the wires disconnected at the throttle switch, use a OHM meter to determine exactly when the switch opens. The switch should open just when the helper slightly raises the throttle (foot firmly on the brake!) above full idle. 

I believe the BBB has a RPM spec for this but that would be for a "perfect" car in perfect condition. Who has that? This procedure puts it in the right spot for your particular car.

[Pawel66]

Wallace, thanks for a steer here!

Yes, it is tricky, already experienced this. Switch turn is very fine. Sensations when driving significant. I need to further fine-tune it over the weekend, as you say.

[wwheeler]

While I am on this subject, I found that my switch would "burn out" after about 6 months of operation. I would clean the points and 6 months later, same deal. I can actually hear my trans solenoid click on and off while in the car with the ignition on and the engine not running. So sometimes I push the throttle and release to listen for the click of the solenoid. That is how I knew it wasn't working. Now maybe with the engine running and higher voltage, it would still activate the solenoid?

Either way, I decided to add a 4 post relay for the throttle switch operation. Why? For one thing, that trans solenoid is a big one and surely draws more amperage than the starting aid solenoids which all have relays. Harder on the delicate throttle switch points. Second, during driving, that switch open and shuts many, many times. More times than just about any switch on the car. I am amazed that Mercedes never installed a relay for it. True the kickdown switch is the same deal, but how many times do you use that? So after installing the relay, I have never had any issues with the points going on four years. Maybe the W113s are wired differently than the W111 cars, so not sure it is a problem there.     

[Pawel66]

Wallace, thank you again!

Two points, if I may:
1. I have read your post briefly (on the procedure) and it seemed clear, but now I read it again and need to ask: the switch should open
a) after the far throttle position (to where solenoid pushes it) or
b) just after normal or "zero" idle position, when solenoid starts pushing it?

sorry for this, but you wrote: "You don't want the switch to open at the precise position the solenoid pushes the throttle to, rather a bit after that", which to me is a), and then
"The switch should open just when the helper slightly raises the throttle (foot firmly on the brake!) above full idle." which to me sounds like b). Maybe I misunderstood "full idle".

Let me just proudly underline, that the method helper+meter at disconnected cables etc. I figured out myself! 

2) it is interesting with the relay. May do it with modern relay hidden somewhere, or - I have the FIP solenoid relay there, next to the CSV relay, but it is not hooked as I do not have solenoid on FIP. Might use this one if need be...

[wwheeler]

Yes, using a newer relay will sure be less expensive and take up less space. I just happened to have one extra spot on my W111 where the relays go and so I used the old style. I believe the relay saves lots of stress on the points. Even though the switches are now available again, they are at high prices. 

Sorry for the confusion and I will say in a different way. When the engine is at idle and in park, the throttle is closed and lets say at position "A". When put in a drive gear, the solenoid pushes on the throttle linkage to open it slightly to say position "B". This compensates for the transmission load and in theory, the idle RPM should remain constant.

At position "B", you want the throttle switch to still be closed. If now you were to open the throttle a bit more past "B", that is where the throttle switch should open. That bit more is a tricky balance because if it is too close to position "B" and as you cruise, the switch will open and shut a million times while you put on or take off the throttle. Also if it is too close to "B" it is then possible, given slop in the linkage, the switch may not close and the trans stays in medium pressure which is too rough for downshifting. But too far past "B" and the transmission stays in low pressure when it should be in medium pressure. I think it is best however to err on further away from "B" rather than too close to position "B" given the consequences. Hopefully I said that better. 

[Pawel66]

Yes, thank you very much! Now we will have this topic here from A to Z for the generations to come .

This is how I set the switch now - a bit after point B. But it may be too far as, when I drive, I can hear gear engage/disengage as I press or let go the throttle pedal. Not always, but often. Upshifts and downshifts are smooth. I will experiment a bit as this adjustment is easy.

On relay - yes, I have the same on 280SL. I have wiper relay, CSV relay and FIP solenoid relay, which is not connected - probably a candidate for switch relay. I will look into it.

Thank you again!

 

[Pawel66]

I did some driving with various adjustments of the throttle switch. In essence every time whe it is adjusted past point "B" (closed while gas pedal is at rest when gear engaged, open when you touch the pedal, which is, I believe how it should be) - I hear the clunk of the gear engageing. Is it supposed to be this way? Or do I have something more out of adjustment?

Please kindly advise.

[wwheeler]

When you say "clunk of the gear engaging", do you mean a harsh downshift into 2nd from third as you are coming to a stop? The transmission pressure needs to be in the low pressure position during this downshifting to eliminate the harsh feel. Have you confirmed that in fact when the Venturi switch is closed that the trans solenoid is at the low pressure position?

[Pawel66]

Thank you for your kind attention!

No, it is not that - upshifts and down shifts are very smooth. Downshift you recognize just by momentary rev increase, no noise at all.
What I have most often is: you slow down before the corner, apply brake to slow down, then press accelerator again as you going out of the corner - you get a clunk as you press accelerator. It is not a loud clunk, but there is a clunk.

Three position solenioid operates as it should - moves up on throttle opening and on kick down, goes back if no signal from these two is present. 

[wwheeler]

Not sure about that clunk. Could be related to the switch, but honestly that is getting out of my knowledge range. The inner workings of transmissions and possibly the rear axle are not my strong suit. Maybe someone else can lend a hand?

FYI, the transmission solenoid has three positions. Two are activated by switches, the other is simply neutral. The high pressure (kick down) and low pressure (idle) are powered positions meaning activated by switches. The medium pressure (solenoid position in the middle) is the neutral. So when no power is applied to the solenoid, it is in the neutral or medium pressure position.   

[Pawel66]

Wallace, thank you!

I had a go last year with the transmission, so I read a great deal about it here, in the Tech Manual etc. - the solenoid is where it is supposed to be. I think the clunk comes from transmission rather than from the back of the car. I also have the impressoin that I had no clunk when the switch was adjusted slightly more before the point described.

ok, I will keep looking into it - thank you!

My guess is that I have the switch set a bit too far now and revs may get a bit too high before the switch cuts off the voltage to solenoid to make it move back to middle position... I will experiment a bit with it.

[rgafitanu@gmail.com]

Here's what I did to adjust it. Set the car in Drive or Reverse, pull the emergency brake (assuming it holds), you can get some wheel chokes too. Go to the engine and adjust the switch so it is still on but it will be off if you open the throttle a couple of mm. May not get rid of clunks completely but at least you will have that set-up done correctly. The clunk that you describe is like a lag between the engine and the car: when slowing down you car inertia is pushing against the engine then when you press the accelerator it switches to the engine pushing the car?
Get under the car and with the transmission in neutral check how much play you have at the drive shaft when rotating back an forth. Check the flex disk and the coupling at the diff. I hope it's not excessive play inside the diff or the transmission.

Radu

[Pawel66]

Radu, thank you for the helpful voice! I will get to the adjustment and lag check ups!

[Pawel66]

Wallace, I think you were right. Clunk is not about gearbox, it is about the rear axle.

I have not yet done a proper examination of it, but with a friend we just looked under the car as it was standing while engageing gears. Indeed the clunk is accompanying movement of the right hand side of the differential housing and the noise is coming from there. Quite funny, when you sit in the car it seems to you those noises are coming form completely diffeent places vs. reality. When my friend just stood next to the car, he immediately pointed at the back of the car.

I am not sure if the movement (0.5-1cm) of the differential housing while engageing gear is normal and it clunks becuase something is lose or it is not normal and the movement happens despite it should not be, causing clunk as it moves. Not sure if I make myself clear, but I think there is a difference.

I now need to take a good look at what is moving/making noise and why.

[wwheeler]

Pawel,

I know that if the transmission has a hard downshift or if engaging in gear with too much internal pressure, it will cause a clunk in the rear axle. Now the older the axle, the more wear it has and probably the more noticeable. Still the cause could be too much slop in the rear axle, I am no expert on that.

Not surprised about the noise though. I had a broken baffle in the front muffler and it vibrated at a certain engine speed. I looked high and low not really sure where it was coming from. I stood in one spot and thought it was from the front. Stood in another spot, thought it was the back. Finally, I had a friend like you stand and listen for it. Sure enough, isolated it to the front muffler.

Friends (or spouses) come in handy with these old cars trying to figure out a whole host of problems and fixes.

[Pawel66]

Wallace, yes, thank you - we will look for visible causes (something lose) but pressure related causes also came to my mind. Thanks to your suggestion I dropped the silly preconception that clunk comes from gearbox or its vicinity - that's a progress  .

[ja17]

Precise adjustment should be carried out with a pressure gauge to set the modulator pressure correctly. The different pressures for different solenoid positions and throttle positions can then be checked and adjusted. The process is a little complicated, but once everything is checked-out you can move onto other possibilities.

[Pawel66]

Joe, yes, thank you. I think I understand now what might be happening. The transmission is upshifting and downshifting great, therefore I thought I had the pressure set ok. But I see it might be too high apparently. We will see.

I am scheduling this for Friday. I think I know what and how, I just need to arrange for a bit more convenient lift for the car and nice weather for driving.

The only thing I could not clearly identify in the Tech Manual was the port for measurement of the modulator pressure, but I think I found it in the BBB.

Another point I found and wrote about it was the units. In some procedure descriptions we have ca 41PSI pressure, in some other 4bar (idle, vacuum disconnected). Well, 41PSI is around 2.8 bar, not 4 bar. I thought we might have had some discrepancy there or I misunderstood something. I will ldouble check in the BBB.

[ja17]

I will try to expand on this topic a bit later when I have more time.

[Pawel66]

Joe, that would be very helpful indeed and appreciated!

[Pawel66]

Ok, for an update. We were searching for the reasons of a clunk while selecting R or 4 or depressing accelerator pedal while car was running with engine in idle.

We took a close look from underneath at everything that might have gone lose (a nearby Mercedes dealer are making a lift available for me when I ask nicely - great guys!). I have:
- some slack in rear Y joint on the drive shaft (which is a sad surprise as the joint is new)
- we saw something like threads of fibre appearing from under one of the bolts on the flex disc on the drive shaft (have you something like seen this???)

And I will have to take care of those, unfortunately.

Then we checked the modulator pressure in the transmission. And it was too low. I had 0.45bar, 2.5 bar and something slightly above 4 bar at kickdown. We adjusted the pressure to the BBB specs. We have also adjusted reverse gear pressure. As far as I can tell on the short drive from the dealer to my place and, while standing still, selecting quickly R and 4 intermittently - the clunk is gone!

I think it is quite important to note, given the amount of posts I saw on clunks in the back - the clunk may appear not only if modulator pressure is too high (what many of you indicated and what is intuitive), but also when it is too low. So clunks may happen if the pressure is just wrong.