Intermittent starting problem on 73 450SL. Relays?

[300TD4x4]

My recently acquired 1973 450SL has a starting problem. Some times it starts up and runs great. Other times it cranks normally but will not fire. I have played with the relays behind the passenger kick panel. I believe that I have determined that the main relay and fuel pump relays are the two upper most relays toward the fron of the car (or, when looking directly at the bank of relays they are the two that are up and to the left).

When I can hear the fuel pump pressurize the system like it is supposed to after the key turns to run then the car will start every time. When it does not pressurize it will never start. I tried swapping the main and fuel relays with other ones and it still starts sometimes but not others. When it is in the will not start mode if I jumper the fuel pump with an external wire from the pump itself or by removing the fuel pump relay and jumping there it still will not start.

Any ideas?

Brian Roth

[Richard Wooldridge]

Hi there,
It could be a fuel pump with very worn brushes - try tapping the pump with a hammer when it won't start, and see if the fuel pump starts running. If it does, then it's time to replace the pump. They seem to have a life of around 100K or so miles.

[300TD4x4]

I am pretty sure it is not the pump because I can make it run whenever I want by running a wire from the pump to positive.

[Earlybird]

Try the fuse box. I have a 73 and a 85 sl and they both have fuse contact problems. Get out your circuit tester light and check both the in and the out path above and below each fuse. This may save you a lot of time. Wish I had done it right off the bat.