How to adjust venturi switch

[Berggreen]

Hello everyone

On my 1971 280SL automatic, I have started to trace down what causes the sometimes hard down-shifts, I experience from time to time.

The first thing I checked was, if the modulation pressure solenoid on top of the automatic gearbox is activated by the venturi switch on the side of the throttle body. I did it like this:

1) Took out fuse 4 for the fuel pump to remove the annoying sound from that.
2) Put a voltage meter on the venturi switch and put on ignition. One terminal on the switch shows 12V all the time no matter how I move the throttle linkage, and the other terminal drops to zero, when I activate the throttle linkage. So it seems like it was alive!
3) Next I listened carefully, when first putting on ignition without moving the throttle linkage and in P, and I hear that the solenoid is clicking, and I assume it goes to the down position.
4) Next I listen again carefully, but now I move the throttle linkage slowly, and immediately after a few mm of movement, I hear the solenoid clicking again, and I assume it does to middle position.
5) Finally, I keep the throttle linkage at max. movement, and have a friend press on the kick-down switch under the accelerator pedal, and I hear again the solenoid clicking, and I assume it now is in the upper position.

(I know that there is a small hatch on the right side of the tunnel inside the car, but on my car sounds protection material has been heated over the hatch, and I would really like to avoid cutting this up to access the hatch.)

So, from this I assume that both the venturi switch, the modulation pressure solenoid on the automatic gearbox, as well as the kick-down switch are all working. Agree?

So far so good, but now comes the million dollar question: When I have the car in gear, the constant speed solenoid will push on the throttle linkage to get the idle up to the 7-800 RPM, where it should be. BUT that will also in my mind push the modulation pressure solenoid to the middle position, as only a few mm movement activates the venturi switch, and therefore at idle as well as in de-acceleration with the foot off the pedal, the modulation pressure solenoid will NOT be in the down position, where I assume it should be, in order to avoid the hard shifts, among other bad symptoms.

Am I right here, or have I misunderstood something?

If so, then I see only one solution and that is to adjust the venturi switch on the throttle body to first activate the modulation pressure solenoid later. But I cannot find any instructions about such an adjustment in the manual on our site.

Do anyone know how to do this?

I look forward to hear back from you.

Cheers,

Christian 

[Shvegel]

speaking as a lifelong mechanic there are a couple things that have peaked my interest,  Hearing the 3 position solenoid and seeing it are not the same thing.  I would crawl under the car (since you don't want to remove the oval cover) and make sure it is actually moving.  I am also pretty sure you car should be fitted with a Vacuum actuator on the left (from driver seat) side of the throttle body down low and the Constant Speed Solenoid is only used for stepping up idle for the AC system.  Cars without AC have the solenoid but it is just sitting there unwired.  can you post a picture of the left side of the throttlebody?  If you have a vacuum fitting on the left side it is most likely that the vacuum idle control is gone.

[Pawel66]

You are not the first one to ask this question. It is somewhere in the posts, a lot of them, if you search "throttle switch adjustment" or "switch adjustment".

I set it linkage slack + just a little bit of pedal movement. It was not great, so I finally adjusted the switch for more pedal movement that took care of the CSS "addition". I think you need to experiment a bit and you will find it.

However I would encourage to check the points Shvegel is advising to check.

The solenoid will click in its best intentions to move, but if it is bound, it will not move...

[ja17]

Christian, the switch on the intake is mounted by two screws and the holes are slotted. You can loosen them and turn the switch to de-activate a little later. I think your diagnosis is good and this would be an easy thing to try first.

[wwheeler]

Shevgel is correct. Do not go by the sound. Mine is a new 3 position solenoid and does not always make an audible click. It is easy to get under the car and see the mechanism moving. Do that with the fuel pump off, ignition on and engine NOT running. Yes, you have to take into account the throttle solenoid's effect on the throttle position. I think the factory book says something like to set the throttle switch to open @ 1200 RPM in neutral. You might want to check that, but sounds about right.

I was curious to see when the throttle switch actually opened and closed while driving. So I temporarily installed a small 12 V LED light with one lead going to the switched terminal on the switch (going to the solenoid) and then a ground for the other lead on the LED light.

My concern was that the switch was constantly opening and shutting with the throttle just barely open. Turns out, it does not open and shut as often as I thought when cruising. That is an easy way to put your mind at ease.   

[Berggreen]

speaking as a lifelong mechanic there are a couple things that have peaked my interest,  Hearing the 3 position solenoid and seeing it are not the same thing.  I would crawl under the car (since you don't want to remove the oval cover) and make sure it is actually moving.  I am also pretty sure you car should be fitted with a Vacuum actuator on the left (from driver seat) side of the throttle body down low and the Constant Speed Solenoid is only used for stepping up idle for the AC system.  Cars without AC have the solenoid but it is just sitting there unwired.  can you post a picture of the left side of the throttlebody?  If you have a vacuum fitting on the left side it is most likely that the vacuum idle control is gone.

Thanks a lot for your feedback! 

First my car has no air-condition anymore - it had one originally, but it was removed many years ago, and I have no need for it, as I live in Denmark. 

Also, my car may have had the vacuum actuator on the side of the throttle body originally, as it is a ´71 model. But the engine has been changed by the previous owner in connection with the renovation, and when I got it, the vacuum actuator was not there (see picture attached), and neither was the constant speed solenoid. So the engine and configuration today is comparable to an early 280SL, and I have even also changed the throttle body to a European model, where the vacuum port is located under the body to match the vacuum supply to the 123ignition installed in program 8 (makes it run very well and much better, than with the original US throttle body, but that is another story).

However, when I got the car, it suffered a lot from a large difference in idle speed between P/N and the driving gears, and to avoid the engine dying at idle in the driving gears, the idle in P/N was set very high - something like 12-1300 rpm. So in order to get to a point, where I could have approximately the same idle speed in P/N and the driving gears, I bought a used constant speed solenoid and installed and wired it into the car to match an early 280SL configuration. That helped a lot on the engine performance and idle speed stability, which was very un-even and rough before.

It still however suffers from large difference in %CO between P/N and the driving gears. But I have set the %CO to match around 3.5-4.0%CO at idle in gear, because I think this is most important, as this is where you drive. But I still have a much higher %CO in P/N - something like 8-9%CO. But with this setting, I at least have a smooth and very well driving car in gear, where it pulls hard and even and also has a nice and stable idle in gear. In P/N you can however feel and hear that the engine is running much too rich. So I avoid idling in P/N for too long, not to clog up the spark plugs and injectors.

But I actually had a theory that part of the reason for the large %CO difference in P/N and in gear was due to the modulation pressure solenoid (3-position solenoid), so that in P/N it is actually in the down position, as it should be in idle, but this is not the case in gear, where where the constant speed solenoid pushes the throttle and with that the modulation pressure solenoid (3-position solenoid) into the middle position. I guess there is a different resistance in the gearbox depending on the position of the modulation pressure solenoid (3-position solenoid), and this will also affect the loading and thus also %CO. But just a guess.....

But good point to first check if the modulation pressure solenoid is actually moving. Can I see this from under the car? I though it is located on the top of the gearbox?

Thanx,

Christian

[Berggreen]

You are not the first one to ask this question. It is somewhere in the posts, a lot of them, if you search "throttle switch adjustment" or "switch adjustment".

I set it linkage slack + just a little bit of pedal movement. It was not great, so I finally adjusted the switch for more pedal movement that took care of the CSS "addition". I think you need to experiment a bit and you will find it.

However I would encourage to check the points Shvegel is advising to check.

The solenoid will click in its best intentions to move, but if it is bound, it will not move...

Thanx Pawel! Good to hear that I am not the only one, who have encountered this problem.

I will look up the earlier threads and study those and first check the modulation pressure solenoid movement and then try my way forward with adjusting the venturi/throttle switch.

[Berggreen]

Christian, the switch on the intake is mounted by two screws and the holes are slotted. You can loosen them and turn the switch to de-activate a little later. I think your diagnosis is good and this would be an easy thing to try first.

Thanx Joe, that sounds easy. I will try to play around with the slots.

[Berggreen]

Shevgel is correct. Do not go by the sound. Mine is a new 3 position solenoid and does not always make an audible click. It is easy to get under the car and see the mechanism moving. Do that with the fuel pump off, ignition on and engine NOT running. Yes, you have to take into account the throttle solenoid's effect on the throttle position. I think the factory book says something like to set the throttle switch to open @ 1200 RPM in neutral. You might want to check that, but sounds about right.

I was curious to see when the throttle switch actually opened and closed while driving. So I temporarily installed a small 12 V LED light with one lead going to the switched terminal on the switch (going to the solenoid) and then a ground for the other lead on the LED light.

My concern was that the switch was constantly opening and shutting with the throttle just barely open. Turns out, it does not open and shut as often as I thought when cruising. That is an easy way to put your mind at ease.

Thanx Wallace, great input!! 

I will look into my service manual and try to find the instructions about the switched RPM and try to adjust it accordingly, after having seen the modulation pressure solenoid actually moving.

[Berggreen]

Hello,

Can I expect to have to re-tune the engine (fuel-air mixture), once I change the position of the switch and thus will have the 3-position solenoid in the down position at idle in gear?

Currently, it is tuned at idle in gear to around 4%CO, but with the 3-position solenoid in the middle position.

Thanx,

Christian

[Berggreen]

Hi All

Today I opened the port on the right side of the tunnel to check the movement of the linkage to/from the 3-position solenoid, and I made a short video to demonstrate it.

https://www.dropbox.com/s/taflu5pmmg69ekw/IMG_7883.MOV?dl=0

Just to explain what I do in the movie:

1) Turn ignition on
2) Press gas pedal until Venturi-switch activates 3-position solenoid
3) Full gas pedal to activate kick-down switch

As you can see, it moves into all three positions, so the conclusion must be that both Venturi-switch and 3-position solenoid should be ok..

I also adjusted the Venturi-switch so that it shuts off and activated the 3-position solenoid, when the the linkage moves 3-4mm away from the idle position in gear, using the tip to push the idle solenoid all the way out to simulate its position at idle in gear.

But as I still get the clonk-downshifts from time to time, then next question and attention I guess is the modulation pressure.

I have read that you can adjust the length of the linkage to/from the 3-position solenoid. But what happens at idle in gear, if I make it longer or shorter respectively? Which direction should I move the rod?

Looking forward to hear your views, and thanks a lot for you input!

Cheers,

Christian

[Paul & Dolly]

Hi Christian
Is the Venturi switch actually making and breaking correctly ?
I had rough changes and cured them by dismantling and cleaning the switch.it was full of muck and so was not giving a precise make and break.I disconnected it and tested it with an ohm meter.
Keep well
Paul

[Pawel66]

If I were you I would adjust the switch more than 3-4mm.

I do not remember if you have the constant speed solenoid - if so, the switch needs to be activated past it.

The book says the switch should activate at ca 1200rpm. I think this may be more than 3-4mm.

[Berggreen]

If I were you I would adjust the switch more than 3-4mm.

I do not remember if you have the constant speed solenoid - if so, the switch needs to be activated past it.

The book says the switch should activate at ca 1200rpm. I think this may be more than 3-4mm.

Hi Pawel

Thanx, actually the 3-4mm is measure from the end point of the constant speed solenoid. So in total the switch breaks at a linkage movement much further than the 3-4mm. I don’t know exactly at which rpm, but higher than idle in gear which on my car is around 8-900 RPM.

But I see no problems with the Venturi-switch. It is in good condition and works perfectly every time, and as you can see in the movie I posted, it successfully moves the 3-position solenoid on the gearbox.

I think attention is now on the modulation pressure in the gearbox. But I don’t know how to proceed in terms of adjustments to the linkage to/from the 3-position solenoid, and could use some advice.

Cheers,

Christian

[Berggreen]

Hi Christian
Is the Venturi switch actually making and breaking correctly ?
I had rough changes and cured them by dismantling and cleaning the switch.it was full of muck and so was not giving a precise make and break.I disconnected it and tested it with an ohm meter.
Keep well
Paul

Thanx Paul, check out the video I posted. The  switch is working well and moves the 3-position solenoid as it should. So I think the switch and solenoid is in the clear, and the modulation pressure is the next step of adjustments. But need some help with that.

Cheers and stay safe too.

Christian

[Pawel66]

I think attention is now on the modulation pressure in the gearbox. But I don’t know how to proceed in terms of adjustments to the linkage to/from the 3-position solenoid, and could use some advice.

Cheers,

Christian

If I were you I would still try to move the switch a bit more. I did it myself like that - by experimenting a bit, marking the switch position to be able to come back if need be.

As for the linkage to 3 position solenoid advise - I think you have it in the technical manual. If you are going to measure the modulator pressure and your car drives ok in essence, you would probably make some small adjustments with the adjustment screw in the modulator. I am not sure if you need to do something with the linkage. If so - it is just one rod that you can make a bit longer or shorter. Longer rod - lower pressure, shorter - higher. My understanding is that it is used for initial "more or less" adjustment, which is then fine tuned with the adjustment screw. Lots of adjustments and driving...

If you car shifts nice, more or less at speeds by the book, smoothly and depending on load and the only complaint is the clunk at downshift/stopping - I would experiment more with the switch.

But I am not an expert, just writing from the bottom of the heart.

[Berggreen]

If I were you I would still try to move the switch a bit more. I did it myself like that - by experimenting a bit, marking the switch position to be able to come back if need be.

As for the linkage to 3 position solenoid advise - I think you have it in the technical manual. If you are going to measure the modulator pressure and your car drives ok in essence, you would probably make some small adjustments with the adjustment screw in the modulator. I am not sure if you need to do something with the linkage. If so - it is just one rod that you can make a bit longer or shorter. Longer rod - lower pressure, shorter - higher. My understanding is that it is used for initial "more or less" adjustment, which is then fine tuned with the adjustment screw. Lots of adjustments and driving...

If you car shifts nice, more or less at speeds by the book, smoothly and depending on load and the only complaint is the clunk at downshift/stopping - I would experiment more with the switch.

But I am not an expert, just writing from the bottom of the heart.

Thanx Pawel, you make some very good points! 

Actually, I worried that I had actually moved the switching point too far out, so that I would run in the down position on the 3-point solenoid at too high RPMs. What is actually the symptoms, if this is the case?

You also touch upon an import point, and that is that the gearbox shifts fine up and down at speed, but only occasionally makes the clonk before stop.

Also, I was thinking that the rear differential is quite worn, and if I on a lift hold on one wheel and try to rotate the other, you can feel a bit of play. Could this also influence to what limit I totally can avoid the stop clonks?

I am dreaming of renovating the rear differential and change it to the European 3.69 exchange, but the parts are HORRENDOUSLY EXPENSIVE!! 

Thanx,

Christian

[Pawel66]

As I said, I am not an expert - far from it. I just went myself with my car through some of the similar issues.

If you set venturi switch on too high rpms - I do not remember, to be honest. The reaction of the 3 position solenoid would be late... late and hard shifts?

On differential - I have no idea what play should be there. I think there are plenty of reasons for clonks in the drivetrain. But if you isolated it to a particular situation....

[mrfatboy]

As Pawel, I'm not an expert 😁 However,  three of us (one a real pagoda mechanic🤣) recently tried to diagnose very similar issues on a fellow forum members car.

We thought the problem was straight forward but we kept chasing our tails. 

Not until we decided to start from the beginning and follow the blue book step by step diagnostics did we make progress.

We found that the throttle body switch was installed upside down and... Wait for it... The circuit board in one of the electronic relays (late 280) was incorrect.  Someone installed a 107 circuit board 😳😳😳

It was amazing. This all started with just a simple idle adjustment 😳

To quote someone one this forum "Assume everything has been messed with" 🤣

Sometimes you just have to start at beginning.