Change diff ratio - automatic gearbox

[jeremy156]

I'm curious whether changing the final drive ratio would require any alternation to an automatic gearbox.

Just a mental exercise at the moment, swapping the existing 3.46 for an easily obtainable 3.07 would drop motorway rpm a little - so I'm wondering whether the 722.6 would be happy with that, experiencing what feels like a heavier load at any given revs... or whether it would require changes in the logic that controls shift points etc. In other words, if I were to just physically swap the diff, would the car drive OK?

In a manual car the driver would adapt his gearchanges... but I think that's more about engine load and rpm than it is wheel speed, correct?

 

[LostKiwi]

You'd definitely upset the gearbox electronics I think.
The car has speed sensors in the gearbox and I'm pretty sure that if they don't agree with values from wheel sensors it'll get it's knickers in a knot.

 

[jeremy156]

That would make sense I suppose. Pity, as I've got to replace seals in my diff and it seemed like a "while I'm in there" opportunity to quieten the car down a little.

 

[jeremy156]

You'd definitely upset the gearbox electronics I think.
The car has speed sensors in the gearbox and I'm pretty sure that if they don't agree with values from wheel sensors it'll get it's knickers in a knot.

A bit of digging has shown up this document which appears to show the sensor in the diff is (roughly) inversely proportional to the ratio, therefore my current 5-speed C200 3.46 has 28 teeth on the ABS gear while a 3.07 from a 4-speed C200 would have 31 teeth, compensating for the higher resulting wheel speed per given rotation. Likewise a 2.87 from a C280 has 33 teeth on the speed sensor.

With that I'm inclined to try it, experiementally... install a used diff, see whether it pukes. Worst case I re-seal the current one and put the other back on eBay.

"No risk, no fun"?

 

[V6Matty]

I think you’ll have problems with the lockup clutch and shift points, but as long as you can analyse the live data when doing your testing then go for it, it’s only time and money at the end of the day, once fitted you’ll know soon enough if your about to damage the gearbox