Just going to post this in-case anyone is thinking of doing a similar conversion.
All w210 e300 turbodiesel's use the eis key setup, the ecu also talk's to the 5 speed auto and will go into limp mode if the 5 speed is removed. This makes the fitting of one into a w124 a bit of a challenge.
However, you can do the following: Find yourself an early w202 c250 om605 turbodiesel. These cars don't have the eis key, and if it was a manual car the ecu doesn't need to talk to the gearbox. The ecu and wiring from this car will run the om606 with it's original electrically controlled fuel pump, so you can now fit the whole lot into your w124 to control the om606.
Now early cars without the eis key still used the old 4 speed auto bolted to the om605, Mercedes never fitted the 722.6 without using the eis key.
Sprinter engines work well in a w124, but the sprinter gearbox cant be used because of it's gear lever. Using the sprinter box would put the gear lever inside the w124's heater matrix! You would also need to change the output flange on the gearbox to take the w124's propshaft coupling. The sprinter doesn't use an eis key, so you can use the wiring and ecu, ect from a 311 van to get a 220cdi engine running in a w124.
There is no realistic way of getting a 320cdi to work in a w124. But you can get the 270cdi 5cyl to work, either by gutting an early ml270 or one of the ssangyong 4x4's which used this engine. Both these options dont use the eis key.
When using a 722.6 in a w124 you will need to work out a solution to the speedo problem. The w124 has a cable drive for the speedo, but the 722.6 has no provision for this. So you need either a speedo from a v8 w124, or a w126 item modified to fit the w124 cluster. Another option is to use something like a cable x box to drive the original speedo head.
W124 e300 diesel's use the 188mm medium case diff, for fitting a tunned om606 I recommend swapping this to the larger 210mm diff and half shaft's.
