W108 Dual Webers and Auto Trans

[n10]

I have read that if you convert to dual Webers and have an auto trans you end up getting a harsh downshift into 2nd coming to a stop.
I cant remember what the little sensor's official name is that hooks to the first Zenith carb to keep the harsh downshift from happening but that's what I am referring to...

SO...has anyone come up with a good solution to this issue with dual Webers on an auto trans?

Thanks!

[ScooterABC]

I came up with a so-so solution for my 67 250S with a 280S engine and dual webers. It involved a microswitch that was attached to the linkage. Basically anytime the gas pedal is not pushed the switch needs to be activated to send the signal to the transmission to shift down soft. I don't have pictures and I think the installation would have interfered with the AC compressor which was not installed at the time and I no longer have the car.

Bottom line - you need a microswitch that is positioned somewhere such that it is activated when the linkage is in the no load position.

[n10]

Thanks Scoot

So since the Benz "actuator" currently links to the first carb... Am I correct in thinking that it would be removed and the 3(?) wires that go to it would be reused on a new microswitch free from the new carbs. Basically mounted out of the way on the firewall?...
Is there a particular one to get or will any one do?

[ScooterABC]

Originally Posted by n10

So since the Benz "actuator" currently links to the first carb... Am I correct in thinking that it would be removed and the 3(?) wires that go to it would be reused on a new microswitch free from the new carbs. Basically mounted out of the way on the firewall?...
Is there a particular one to get or will any one do?

Not exactly. The zenith carbs have a couple of wires running to them. First of all, there is the electronic choke (this is from memory so someone correct me if I'm wrong). All it does is get +12V whenever the car is on, which heats up an element and opens the choke. When the element is cold the choke is closed. This is on both cars and the wire is daisy-chained from one to the other. Then _I think_ there is a wire to cut off fuel flow going to both carbs. That's so when you turn off the car it can run-on or anything. I might be wrong about that one. Finally, there is a wire going to the "are you pressing the accelerator" pedal switch which is part of the carb itself. I _think_ it's only on the front carb but I'm not sure.

The problem with Webers (which Redline and weber TOTALLY do not acknowledge) is that there is no way to duplicate that with the weber carb. So instead you need to use a microswitch. I don't remember if it grounds or is +12V at no acceleration.

Mine was fashioned to the accelerator linkage near the AC compressor bracket. I didn't do it and I never really liked the way it was done. I have an old thread on this topic that I will look for...

Scott

[ScooterABC]

Here, read this thread on Benzworld.
108 auto trans clunk - 3 position solonoid - weber carbs - Mercedes-Benz Forum

[n10]

Thats a great thread for sure but it doesnt outline the solution very well. I need pics!
I noticed a few threads on OZbenz about the conversion but no real end solution with images on the microswitch


I noticed the JAM carbs are on sale for the same price as the Webers now!
Any thoughts either way? I know the Webers come with dinky plastic linkage nubs...

[ScooterABC]

There is no "standard" solution - this is a hole in weber's seemless conversion process. I don't have the car anymore or I would show you how a microswitch could be mounted. It doesn't matter where you mount it as long as it is closed (I think) when you have your foot off the gas. You could have a switch under the gas pedal or anywhere along the linkage.

[ScooterABC]

My suggestion is for you to first find the wire(s) that are supposed to be going to the carb and experiment with them. If you activate the soft kickdown when you are trying to drive the car you will know it because the transmission will behave like mush. Just slipping and stuff. So if you had a couple of long wires running to the wires under your hood you could sit inside the car and manually toggle them to experience the difference when the car coasts to a stop. No clunk shifting into first. Sorry I don't have a better answer but it looks like no one else is answering either. Weber _really_ should handle this issue but they don't.

[mosu]

Based on what you described, ScooterABC, the firewall mounted switch sounds like the most robust solution. The linkage is bound to close it in the rest position. I wish I could help, but I have the manual transmission and the only wires on my Zeniths are the electric chokes.

At the risk of a minor hijack, I have questions for both you guys:

n10 - what prompted you to switch to Webers instead of rebuilding the Zeniths?

Scooter - how did you find the drivability, power and mileage of the Webers compared to the Zeniths?

I ask because I'm debating between a $6-700 professional rebuild of the Zeniths and the Weber switch at around $1000. Both are pricy and I want to understand which will yield the most benefit.

[mosu]

IIRC, the Webers had a variable throttle stop screw. Part of the tuning procedure was to ensure that it was backed off enough so the throttle plate covers the transition holes. Can a micro switch be fitted between the stop screw and the throttle lever? The screw can be backed up to accommodate the width of the switch blade.

[n10]

Hey Mosu

I am not able to take the car off the road for more than a few days as it is my DD. I have almost zero knowledge of rebuilding a carb. My mech buddy would do it but at a cost. If I was to do that I may as well go new.
I could find some spares but unfortunately Im not that interested in rebuilding them. I guess Im just being lazy. I spent a year rebuilding everything else so I just want it to go
BUT the only thing stopping me and yet slightly considering a rebuild, is this stupid throttle/transmission actuator setup

[ScooterABC]

Originally Posted by mosu

Based on what you described, ScooterABC, the firewall mounted switch sounds like the most robust solution. The linkage is bound to close it in the rest position. I wish I could help, but I have the manual transmission and the only wires on my Zeniths are the electric chokes.

My understanding is that on a fuel injected early W108 (250SE) the switch is on the firewall, but I have not ever seen this in person.

Originally Posted by mosu

Scooter - how did you find the drivability, power and mileage of the Webers compared to the Zeniths?

I ask because I'm debating between a $6-700 professional rebuild of the Zeniths and the Weber switch at around $1000. Both are pricy and I want to understand which will yield the most benefit.

I can't answer because I've never compared them back to back. My W114 250C 2.8L had zeniths that had been rebuilt and it ran great. My W108 250S 2.8L had dual webers that were rebuilt and it ran great. You might get more power out of the webers, but I can't say for sure.

[mosu]

Scooter: was the mileage of the Weber equipped car comparable to the mileage of the Zenith equipped car?

n10: That's exactly how I feel about those Zeniths now So tempted to go for new carbs if the price delta is ~20-30%.

What if you used a vacuum switch to mush up that downshift?

If I understand correctly, these are your transmission's operating modes:

Coasting, off-gas (high vacuum) - low pressure position on the transmission solenoid
Acceleration, on-gas (low vacuum) - high pressure position on the transmission solenoid
Kick-down enabled (very low vacuum) - over-rides previous settings via separate electrical switch

During cruise you'd be at moderately high vacuum and the transmission will be in low pressure mode. You don't care as once you step on it, it'll firm up pressure.

Do you guys think that would work?

[ScooterABC]

My mileage in both cars was about 15 in the city and about 18 on the highway.
I don't think the vacuum switch would work. Under light acceleration you would still have mush which wouldn't work.
You might want to read about the transmission here, at least it shows the terminals.
Pagoda SL Group Technical Manual :: Automatic / Start

[mosu]

Run it off a ported nipple. Throttle closed, no vacuum, throttle open, manifold vacuum. It can even tee off the vacuum advance line.

Just a thought if you can find a vacuum operated switch...