New fuel pump; failure in 5 miles

[MunichTaxi]

I replaced the mechanical fuel pump on my 1977 Euro 280S.

The new one failed quickly, and pumped fuel into the crankcase.
I used the spacer (gasket) that came with it.
I haven't inspected the diaphram inside, but I assume it is perforated.

Could the plunger shaft been too long, created excessive 'throw', and damaged the diaphram?

BTW, Carter P4070 (electric pump) will be the replacement.
It draws 4.5 amps.

Any ideas on a power source, that shuts down with the key?
Maybe the wire to the ballast?

[Mike D]

Don't use the ballast resistor wire feed. You can use that wire terminal as the trigger wire to a voltage relay which is how you should power the fuel pump.

A fused "hot" lead to pin#30 on the relay. Ground to pin #85, supply to the fuel pump on pin#87 and the "trigger" wire (from the ballast resistor) to pin #86.

This eliminates any chance of a voltage drain conflict on the ignition wire leading to the ballast resistor.

[todds]

The Carter is a great pump, I have one myself and its very well made. Where did you end up mounting yours?

[MunichTaxi]

A fused "hot" lead to pin#30 on the relay. Ground to pin #85, supply to the fuel pump on pin#87 and the "trigger" wire (from the ballast resistor) to pin #86.

Mike, is the Relay you are referring to a standard Mercedes relay, or a specifc one?

P.S. I've got spares from my parts cars.

[Mike D]

A relay is a relay. They are pinned the same. I use the newer plastic bodied relays because they have the standard "spade" type terminals instead of the round "bayonet" style on the originals. It just makes it easier.

You should solder the connectors on rather than just crimp them.

[MunichTaxi]

I found a post by Todds in the Search process; he asked the same question, a year or 2 ago, about the power source, but he never got an answer.

Interesting, the instructions I received today indicated using the power source from the starter solenoid.
They also recommended (required) a Pressure Safety switch # A-68301. This switch would prevent the pump from operating,
with an engine stop, ignition 'On' condition.

Hmmm...

BTW, when these mechanical pumps fail, the cranckase volume increases by 1-2 quarts.
Anyone need 8 quarts of fresh oil+gasoline, mixed 8:1?

[MunichTaxi]

Where did you end up mounting yours?

Although the concenous is to mount close to the fuel tank, on a W116 the fuel tank is behind the rear seat. I going to try a low location on the inner fender, to simplify routing the wire (+), and finding a good ground (-).
Gravity works in my favor.

BTW, the W108 has an aluminum fuel tank, while the W116 has a steel fuel tank.

[Mike D]

The "pressure safety switch" is a standard disclaimer from all electric fuel pump manufacturers. It's a good idea but not always a possibilty.

Instead of using the starter feed wire why not use a feed from the positive terminal of the battery? Same thing and it makes it more convenient. Whichever you use, make SURE you have an in-line fuse in a convenient spot. I wouldn't use a circuit breaker because any "short" will reoccur when the breaker resets itself. For the electric pump you only need 18 gauge wire (10 amp capability) so it's pretty easy to route.

Even though gravity is working in your favor remember electric fuel pumps are designed to "push" fuel more than to "pull". Mount your pump as close to the fuel tank outlet as possible.

The Carter is a good pump and you shouldn't, repeat, shouldn't, have any problems with the fenderwell location.

I just went through the push-pull routine on my '73 280 and my wife's '72 250C. They ran fine in town but on the highway they would cough and stutter when you went into a full-on accelleration mode. The electric pumps I had mounted on the fenderwells, a small electronic solenoid style and another "spinning vane" style, could not "pull" the fuel hard enough to match the demand. I solved the 250's problem by mounting the pump at the rear by the tank and I put the Carter on the fenderwell on the 280. So far, so good.

I mounted the 250C's pump in the bracket space where the vent diaphragms were originally mounted. I discarded the diaphragms and connected the vent line with a coupler. I might have to put a diaphragm in the vent line if I run into any problems but with the continous flow of the electric pump I don't forsee any. Famous last words I know, but we'll see.

[450slcguy]

Are you sure its not the fuel pressure reguator? When that diaphram breaks, the overflow gets routed into crankcase.

[Mike D]

No regulator on the "S". Just a "diverter valve" on the fuel supply line which sends excess fuel directly to the tank. Vacuum operated diaphragm on the valve working off engine vacuum.

[MunichTaxi]

Mike, your solution of connecting to the battery:

Okay, I continue to use a Relay, addittionally, with the 12v from the battery. No problem soldering; lots of experience on R/C truck motors.

Do Relays typically trip at 1 amp or less?

[Mike D]

Put the inline fuse in the hot lead between the battery and the relay. 10 amps MAXIMUM! Use a 7.5 amper if you can. Run a ground wire with the fuel pump feed to eliminate the need for finding a good ground. Small speaker wire (22 ga.) will work. Make sure you label the wire, "fuel pump" to prevent confusion down the road.

[MunichTaxi]

Fuse #6 (25 amp) controls the Cigar lighter and heated Rear window.

I don't have a cigar/cigarette lighter, and this circuit is closed only when the Ignition key is 'On'.

This appears to be a good source; I can place the Relay and fuse inside the Fuse box, and run the +wire to the Electric Fuel pump.
An alternate is to connect/solder to the fuse bar that controls Fuses #4,6,8 & 10. This is also closed when the Ignition key is 'On'.

The whole Chassis is Ground source; it is interesting the Transistorized Ignition is grounded to the Left side of the radiator cavity, and the Coil is grounded to the inner fender.

[Arthur Dalton]

>

That is b/c the points do not trigger the coil as a standard point system does. The points on Trans. Ign. systems triggers a transistor, which then switches the coils Postive signal side. The advantage is the transistor takes the load of the coil, so the points do not tend to burn up so fast. The problem still lies with both systems still have mechanical switching points..so many dump the points and transistor switchgear for Electronic Ignition modules that use Hall Effect switching.. [ Like Pertronix]
Much better and short $$$$.
Standard points systems switch the coil on the Negative/ground side , thru the points to ground [ switched ground system]

[MunichTaxi]

This 1977 Euro 280S has a Transistor box, and points.

It has roll-up windows, and no Smog equipment. It equipped it with a Weber 38/38.

Funny about the failed Meyle mechaical pump. It lasted 3 miles.
When I inspected the inside, the only question was the un-sealed diaphragm. It looked like a bad design.