How-to: 126 automatic (switchable) first gear start

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john rh

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Feb 7, 2010
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This technique works on my 1986 v126 500SEL. It may work on other models/years, but I have no information on that.



One of the few things I dislike about my 500SEL is the original design of the gearbox to move off in second gear. The car feels 'heavy' and less eager away from rest than I expect from a large engined car.

First-gear can be obtained using a full throttle (kick-down) launch, but that's not really my slyle! Also, first gear can be selected manually by moving the gearlever to the B-position, but having to select gears manually defeats the object of an automatic gearbox.

I decided to wire my car to move-off in first gear automatically, but with an ability to switch back to second-gear move off for driving in snow and ice. I bought a module to achieve this, but in the end it wasn't necessary to use it.


New driving characteristics

I made a small wiring modification, adding one simple relay, which gives my car the following behaviour:

Gearbox mode switch in E position:
- Behaves exactly as per original specification
- Second move-off, unless kick-down or B-switch are used to engage first gear

Gearbox mode switch in S position:
- Automatic first gear move-off
- Changes to second gear at 8mph on light throttle
- Changes to second at progressively higher speed with larger throttle opening
- Original kick-down and B-switch fist gear hold work as normal
- Changes down to first gear at 5mph when braking

In S mode, the car feels far more keen, spirited and torquey when pulling away, in normal driving.


Method

This isn't a blow-by-blow guide. I focus here on the wiring changes needed, and I'll assume that if you’re reading this, you can do basic electrics and know how to access the components (or you know how to use google search to find out!)

My car has the factory-fitted first-gear move off relay, and I have used this to deliver the behaviour I wanted. I'm not an expert on Mercedes, but I understand that not all 126s have this relay fitted. If your car does not have this feature, I am 99% sure my method for first gear start won't work for you.

The principle of my modification is to use the brake light switch to engage the first gear move-off relay. Therefore, whenever you stop using the footbrake, the gearbox will behave as if the B switch has been operated. I use an additional relay to disable this feature when the gearbox mode switch is in the E position, and diodes to avoid back-feeds.

1.
Test whether your 126 has a working first-gear move-off relay:
- Select gearbox mode S
- Remaining stationary with the engine running, move the gear selector to B and then back to D
- Now pull away gently. If the car sets off in first gear (listen for an upchange at approx 8mph), then you’re in luck, your car has the first-gear move off relay

2.
You will need:
- a simple 5 terminal relay, one which has 87 and 87a outputs
- three diodes
- wire, connectors and cable ties

3.
- Remove the wood panel around the gear selector to access the B and S/E switch wires
- Remove the panel above the brake pedal to access the brakelight switch
- Decide on a location for your extra relay. I mounted mine under the coin tray, which works well

4.
As per wiring my diagram:
- Splice a wire into the brakelight switch, output side
- Splice a wire into the B-switch, output side
- Slice a wire into the S/E switch, output side
- Connect diodes in line
- Connect up the relay

DSC_0994.jpg



5.
Test drive, then refit console components. Job done!

Have fun,

John
 
Does it not take off in first gear while in 'E' if you select first and then select Drive? Or am talking rubbish and just confusing things?
 
Does it not take off in first gear while in 'E' if you select first and then select Drive? Or am talking rubbish and just confusing things?

I don't know. I remember reading on a forum somewhere that the feature operates in S mode, but I didn't try it in E.

John
 
Anyway, lets move on. ;) What's the story on the 164 'RS'? Sounds very, very interesting to me.
 
Oh, that's just lovely, brings back memories of when I was knee high to a grass hopper! Will have a read later. Thanks for sharing.
 

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