Bcron

Need some starting help. 1991 sl500 won’t start when dash lights aren’t present. When I turn the key forward, if I have dash lights, it starts fine. If no dash lights, it cranks and cranks but won’t kick over. I was thinking fuel pump relay? It sometimes runs rough at idle which I’ve read is also a symptom of fuel pump relay. I’ve read conflicting info on location of the relay. It’s a 119.155 engine.

dolucasi

Sounds like you have some ignition switch issues. Rough idle is probably unrelated and not a fuel pump relay issue. Rough idle can have many culprits, fuel pump relay must be at the bottom of that list as I have never seen that come up for a rough idle.