I have a 420sel. It starts fine but when the key is released, the car dies. If I hold the key on, it revs, so fuel should be fine, just the spark dies.
It's an '87 with less than 80k and in almost perfect condition otherwise.
Thanks

 

I got this fixed. I just happen to have a wrecked 560 out back. I took relays, one by one from the 560 and replaced in the 420. The very last one, one of the two large black ones (between the firewalls) did the trick.
I've learned a lot from these forums. And fixed a lot from what I learned. I hope this helps someone else.

Thanks!!!