As a point of clarification, the USA (and perhaps all of North America) C280 does *not* have a reference resistor, however, European C280's do have the "dial-type".

It is very difficult to retrofit since the leads coming out of the ECU aren't there to "splice" into (I've checked with my 95 C36 ECU & harness vs. the European/Netherlands 95 C36 harness which has the "dial-type" reference resistor).

Not only would you need the Euro wiring harness, but even the ECU may not have this "feature" activated.

:-) neil