w124 4sp to 5sp Auto conversion

[aka$h]

After much thought I decided to go ahead with the transmission swap. I had a donor car, whcih Ive taken the engine out of for the turbo conversion.

Having stripped the car apart, here is what I now know about the system.

The 5sp box uses electronics only for 4->5->4.

But the wiring required to do this uses 11 feeds into a HGS transmission control unit. 4 of which go to the ECU (the ecu is also different), one to the OVP, one to the Kickdown relay, and the remaining go to a plug in the fusebox. If your car is a 4sp the other end of that plug is not present either. This loom then goes from the fuse box, to the kickdown switch, 2 plugs on the gear selector (one for the bulb), the last part goes under the floor into the gearbox.

Without a donor car this would be a Very expensive conversion. It did take hours to srtip the main harness to seperate the gearbox wires.

Now just need to rebuild the box, and swap the props and diff's.

 

[fnkhan68]

Good luck with the gearbox, that 5th gear really makes a difference to the fuel economy, its strange but my 320e is actually more economical than my 260e even though the car is heavier and has a bigger cc engine.

 

[bolide]

Someone, maybe on this forum, suggested there was a sensor on the speedo that drove the 4/5 switching. Any signs of this?

Nick Froome
www.w124.co.uk

 

[aka$h]

Someone, maybe on this forum, suggested there was a sensor on the speedo that drove the 4/5 switching. Any signs of this?

Nick Froome
www.w124.co.uk

it takes speed signals from the ecu by the looks of it.

 

[aka$h]

Good luck with the gearbox, that 5th gear really makes a difference to the fuel economy, its strange but my 320e is actually more economical than my 260e even though the car is heavier and has a bigger cc engine.

Ive done it to keep the rev's lower when the turbo's are fitted, at 100mph the car revs at 4000rpm, with 2 turbo's running full wack. Hopefully this should help keep the revs lower.

 

[Dieselman]

It would've been simpler to just upgrade the diff, especially as the engine should now have enough torque to allow for the wider gear ratios.

Does the 5 speed actually turn slower than the 4 speed given that the diff is lower ratio iirc?

 

[aka$h]

It would've been simpler to just upgrade the diff, especially as the engine should now have enough torque to allow for the wider gear ratios.

Does the 5 speed actually turn slower than the 4 speed given that the diff is lower ratio iirc?

The top gear on a 4sp is 1:1, on a 5sp its 1:0.75.

The diff ratio is 3.06 on a 4sp and 3.69 on a 5sp.