Will a flip key work on a 90 420sel? I current have keys that have the red dot and ones that don't and both work. So, does this mean the 420 does not have the chips? I guess 1990 was before the chips. Any help would be appreciated.

There are aftermarket flip key systems, some with alarms, some that just remotely unlock and integrate with existing alarms, for older Mercedes. I have one for my W201 and they are available with different keyblades.

What's a flip key?

Sixto
95 S420
87 300SDL

Link to Flip Key for W201