i have an 89 420: While driving recently car quit running will not start checked timing chains. Replaced fuel system relay, and fuel pumps and strainer. Getting electic to pumps. Still will not start motor turns any suggestions on wat else may be. In my 1970 280se had to change a diod at one point not sure if could be the same with 89 please help

 

Did you check for spark to the plugs??

 

On my '86 it was the crank position sensor; the car would die when it got hot (ignition stops firing when it loses the signal from the position sensor), but restart when it cooled off.
It could be the ignition module: check for spark (pour a little fuel in the intake when cranking, and if it tries to run for a bit it has ignition, but no fuel so check the pumps near the right rear tire, and the pump relay. you should hear the pmups run when you turn the key on).
The ignition module on my car is bolted to the inside of the steel inner fender on the left-front, and there is white heat transfer paste under it, as the inner fender is a heat sink to keep the module's temp down. I removed my module, cleaned out the old, dried-up past and put in some new past (can get it at Radio Shack) on the advice of a technician who said the lack of cooling helps the module fail.
Hang in there; I hope thishelps you some.