Over the last two weeks I have been stranded twice due to a faulty fuel pump relay. "Ava", my 2000 E-320, runs great and is a true dream car...except for the recent spate of fuel pump relay problems! Does anyone know if there are TSB's out on a problem that causes premature relay failure, or is this a common problem with this model MB? I cannot imagine that this is something I am "just going to have to deal with" over the proposed life of Ava! In both of the failures, everything was normal until the point where I decided to go somewhere. Today I drove a few block, stopped for a brief time, returned to find the relay had decided to go **** up. Last time pretty much the same issue - drove to work, parked, came back at the end of the day and no fuel pump. At $250 a pop for towing and repair, this does not need to be a bi-weekly routine.

Help!!

cdavid46

 

Had a similar issue with my 99 E300, dealership told me it was a faulty fuel gauge and i ran out diesel.

How didu isolate that this was an issue w the fuel pump relay ?

 

The first time this happened and I had it towed to the local dealer, they "diagnosed" the problem to be the faulty relay. But, since this has happened twice since they said it was a relay problem, unless MB has a boatload of bad relays or there is something else causing this to happen. It is once again down after only about 4 days on the second replacement relay was put in. I don't believe that it is only the relay causing this. I think something else is causing the relay to get fried or something. All I know is if I replace the relay, it works like a charm until about 4-5 days until it quits again!

I'm running out of patience with this since I don't like getting stranded at odd times.

I cannot tie a faulty fuel gauge to this problem, but I guess stranger things have happened!

 

Therelay has two basic cicuits within the casing.
1. An input circuit made up of a solenoid (electromagnet) connected to the switch so that it is activated when the ign is on.
2. AN output circuit that passes +12 volts (battery) on to the fuel pump by means of contact points.
The solenoid is quite robust and seldom gives trouble unless it has been subjected to overvoltages. Was the replacement relay OEM?
If the points are getting burnt prematurely, the pump could be drawing excessive current from the relay due to electrical problems within it. Yet again, if your replacement relay was not OEM, it may not be able to cope with the normal current demands of fuel pump.
You need not have to tow your car to the service station if the relay fails. I have read a thread (I can't recall where right now) where it was shown how to bypass the relay& directly
connect the pump to +12 V when ign on.
Hope this helps.
Shan.

 

Shan, thanks for the info and for curing one question I have had since the second stranding - Why does a fuel pump need this relay anyway? Why not bypass it and be done with faulty relay issues?

Having said that, my very limited mechanical knowledge would lead me to ask whether or not bypassing the relay is a good idea, especially if the problem is somewhere else. I am sure a fuel pump is a much more costly repair and I certainly would not want to have to do that every 10 days! And, somewhere else is where I think the problem must be. After not being able to start the car after this last incident, as I was about to pull the relay and go to the local parts house for a replacement, I decided to give it one more try. Without missing a beat, it started right up with no indication the relay was bad. Will a relay act in this manner if it is bad? I would have expected it to die and not come back to life as this one did. You mention the ignition and this is something that has been nagging me since the second failure. Could this be an ignition issue, like maybe something in the switch? Everything else (dashlights, locks, etc) seem to function normally, but I still can't help but wonder if the fuel pump thing might be something misfiring in the ignition switch.

I'd appreciate your response when you get a chance.

Thanks!

 

Hi David
In our cars the relay also has an additional function. It cuts off the fuel supply (by cutting off power to the fuel pump) when you overrev (go beyond the red zone in the RPM meter) the engine. This is for the protection of the engine.
Bypassing the relay willcut off this importantfeature. As I see it, this is the only danger you will face. So it should safely take you home if you don't use the kickdown to accelerate.
From what you have mentioned, it could be just bad contacts. Unplug the wires to the relay and give the clips in the plugs a liberal spray of contact cleaner or WD40 and see if the situation improves.
Shan.