Fuel Pump Relay??
 

Hello,

I have an ‘86 300E with about 153k who will not start. I have a very quiet fuel pump (no juice back there on the contacts). Fuel Pump relay clicks to ears and touch when key turned on. Car does not start, not at all, but cranks away nicely. Just did tune up, plugs, cap, rotor, wires, oil and filter, air filter. I do still need to do fuel filter. Car was running the day before, albeit rough idle and slightly difficult warm starts (usually needs a little pedal), which was way better than before the tune.

So, is this for sure a fuel pump relay, or are there other possibilities which would make my fuel pump so quiet??

Thanks

I am pretty sure I have answered my question. I pulled the relay and bridged 7&8. The pump was whirring away like a champ.

Does it start and run with 7 & 8 bridged?
Gilly

Yes, but not with the relay installed

Sounds like you found the problem to me. I have had very good luck in the past by opening the relay up and resoldering the broken solder joints inside. I fix 9 out of 10 this way. The relay is fairly cheap at about 150 peanuts. Look very close to the connection that controls the relay too since they can cause the relay not to close also. Very common problem on this vintage Mercedes, all models.

fasthair

Thanks!!! I will check the relay and rebuild to keep as a spare. I already bought a new one. I will check the connections to relay as well.

Question 1: if the relay is fried and toasty inside, is that an indication of an additional problem, like a failing fuel pump? (The pump sounds pretty much fine.) The relay looks great on the outside. BUT, it is date stamped 2/23/00, so it is not very old, which gives me concern (at least enough to carry a spare).

Question 2: And oh yeah, if the 7&8 jumper works, the connections are good, right?

FWIW I killed a new relay once checking the fuel pumps for current. I accidentially grounded the hot wire. Feeling I had nothing to lose I opened the cover and found the open circuit. A little solder and it's back in bussiness. :)

You can connect a ammeter to pins 7 & 8 and monitor the current draw. I don't have this memorized but I want to say around 12 to 15 amps for a dual pump set up, or 8 amps for a single pump.
On question 2 you are correct.
Gilly