W126 4 speed auto transmission query

[Rico]

Having recently acquired a very nice 1991 300sel W126 I've got a query about the S and E options on the autobox.

I'm assuming this stands for sports and economy modes. If so what should be the specific difference between these modes.

I'm asking because mine doesn't seem to do anything different regardless of switch postion. Also I feel the car revs higher than i would expect at motorway speeds so thought maybe the economy mode had diffrent gearing on 4th gear ?? :?:

Even if someone could tell me what rpm should equate to 70mph :?

 

[Malcolm Brown]

S changes up later than E. Also, I've never noticed 1st engaging in E, but it does in S if you floor it from standstill or force 1st by moving from D to 2 & back before moving off.

No difference in gearing between settings.

Generally I drive in S so engine is not labouring. E is better in slippery conditions, combined with a gentle right foot.

Can't remember what revs are at 70, but it is certainly more than someone used to a more modern motor with 5 speed box would expect.

 

[Rog_E]

Hi.

In the 'S' mode the gears change at higher revs and first will engage if you really toe it pulling off from a stand still.

There is no effect on the overall ratios, revs will be the same can't recall of hand what revs equate to 70 (approx 3000?).

My 124 will change from 3rd to 4th at 90 however, in 'S' mode and 'pedal to the metal'!

Should feel two gear changes under normal throttle.

Moving selector from 'D' to 2 and back to 'D' will force a first gear start.

HTH

 

[Rog_E]

70 = 3250 RPM (I checked by driving a little more sedately to work this morning.

 

[talbir]

I've re-wired my transmissions on my W126's, do that they always start in first gear - regardless of which mode is selected, S or E. And you don't have to floor the throttle, since the modification I have made means the transmission actually IDLES in first gear.

The only reason MB had these cars start in 2nd gear was to met CAFE emissions regulations - a car idling in first gear will idle very slightly higher, thus giving higher emmisions. From 1995, MB reverted the trannys to first gear start.

The beauty of first gear start is that the car will actually idle in first gear, and take-off is amazing. On a standard 126, you have to select 'S' mode and floor the throttle - the car will idle in second, then when you floor the gas, the gearbox will think about the message from the kickdown switch and half-a-second later will complete the hydraulic ritual of moving into first and then take-off. Whereas on my 126's, the car is already idling in first and hence the 0-60 time is significantly reduced.

Not to mention it's far better for the gearbox and engine to be tkaing-off in first rather than second.

 

[Rico]

Thanks for info guys.

Car has 157,000 miles on it but with FMBSH and everything seems good but switching on box was only thing worrying me but 3200 rpm ties in with what mine does. Just felt revvy for 3litre straight six but i have been driving about in an old volvo with 5 speed manual which does about 2500 at 70mph and i've got a XJS which i haven't driven for quite a while but seemed lazier at 70mph think it only did 2500ish rpm.

cheers

 

[Leylandp76]

Talbir, I'd like to know how you did the rewiring. It sounds like a sensible mod, can you explain?

thanks

Alistair

 

[talbir]

Alistair, (apologies for the following long post)

Yes, it's quite simple, however you need to have the OEM module, part no. 002 545 4132.

This module was standard fitment on all pre-1986 W126's, which were V8's, i.e. the 380, 500, SE/SEL/SEC. However the 12V feed to the module was only activated when kickdown was applied or 'B' was selected on the SEC tranny's. So the cars always pulled away in 2nd unless kickdown or 'B' was activated. 'S' and 'E' mode were introduced in 86, when the module was removed. The module only gives true first gear start on the V8 engines, which usually have different tranny's to the 6-cyl models(but not always the case). If the module is fitted in a 6-cyl car, the car will idle in 2nd and when pulling away normally, tranny will kickdown into 1st and then move into 2nd gear at 8mph. On the V8's, with the module fitted, the car will actuall IDLE and pull away in first, always.

The module effectively 'shorts' the kickdown switch, BUT also takes a feed from the speedo pulse, so that once the car attains a speed of approx 8mph, the module stops the 12V feed to the kickdown wire and the tranny shifts into 2nd gear.

If you just simply connected a switch between the kickdown wires, you would also pull away in first, but then you would have to manually turn the switch off - otherwise you would be in first gear all the way until the revs red-lined, and this would FORCE the tranny into second gear. Not an ideal way....the module takes the speedo pulse to stop the signal and let the tranny shift into 2nd at the correct speed.

The module requires 4 inputs :

Pin 1 on the module - this is the kickdown wire, which is 12V when the kickdown switch is depressed.

Pin 2 on the module - 12V, hot with start

Pin 3 on the module - empty

Pin 4 on the module - ground

Pin 5 on the module - pulse wire from speedo

Pin 6 on the module - empty

The speedo pulse is best taken fom behind the speedo - there is a green/yellow wire coming from the back of the speedo (two wires if the car has cruise control). Feed into this and run to pin 5.

The kickdown wire can be taken from the kickdown switch - you need to feed into the wire that turns 12V when the kickdown switch is depressed.

The 12V wire is straightforward, but it would be reccomended to run this through a switch, so that you can revert back to 2nd gear start when conditions are wet/snowy. 1st gear start with a V8 on a slippery surface is a nightmare. I have wired the 1t gear start module through the original MB ASR switch, which is mounted along with the other switches in the centre console wood. It is the original MB switch, with the red LED. It is a rocker switch and thus requires latching relays to be wired up with it - the red LED can be made to illuminate whenever the car is in first gear.

I have also fitted the MB factory Lock/unlock switch, which was fitted on later Mercedes models. This switch enables me to lock /unlock the doors at the press of a button, which is so convenient. Again, this is a rocker switch and requires relays and since the 126 series has vacuum locking which requires a 3 second pulse, it requires timer relays.

I have linked up my locking module to my 1st gear start module, so that the doors automatically lock at a speed of 8mph - just as the system is on present day Mercs.

The 1st gear start module is about £120 plus VAT from MB and about £55 from MB breakers. But it will only function correctly with the V8 126's, i.e. the 560, 420, 500, 380. For 6 cylinder Mercs, the tranny valve body needs to be modified, which is a bigger job.

 

[Leylandp76]

Talbir,

Thanks for taking the time to post such a detailed reply. As you noted, I have a W124 6 cyl with 722 box so it may be different to your W126, however I do have the same basic 'features', ie it starts off in 2nd normally, but can be forced to start off in 1st by pre-selecting 2 then returning to D.

Until I saw this thread, being a recent MB owner, I didn't know this. Having tried the 'select 2 then D' approach to forcing a 1st gear takeoff I do feel that the car moves off much better from a standstill. I figure there is enough information in what you have told me to try to achieve the same result on my W124. I have cruise control and ASR, but no ASR on/off switch. I'll look to see what else matches up.

thanks again,

Alistair

 

[kiwi_bloke]

comment

One of you said the mercs start in 2nd for emmision reasons? Im just curious to what the correct reason is as ive heard its for economy reasons

josh