W116 350se Foot brake kills fuel pump

[Mad Mick]

Good Morning America.
My car broke down recently coming to a stop at a junction and I could not get it started and had to tow it home. The fuel pump was only coming on sometimes when I turned on the ignition and she would then die. I replaced the relays and no improvement. I ordered a new fuel pump which is on the way to me in Ireland from USA. Checking the car this morning I had my feet out of the car when I tuned on the ignition and heard the pump and she started right up, only I noticed my fuel gauge did not register at all. When I sat into the car properly and put it into gear she died and would not start. I then put her into neutral and turned the key, the fuel gauge registered her full tank but no sound from the pump. I then took my foot off the brake pedal and turned the key and I could hear the pump working so I started her again and after a few moments pressed the brake pedal as normal to put her into gear and she died again.
Weird as this seems, pressing the brake pedal kills the fuel pump.
I can hear my car ticking over perfectly outside as I type this. The fuel gauge is showng halve full now when running now even though the tank is full and when the engine is off the guage then shows full.
On searching the only thing I can find in terms of a wiring fault is the connector at the handbrake is disconnected and there are bits on the floor from this connector. Is there any way this could be related.
This is not a vacuum issue as with the engine off and my foot on the brake the pump does not work and with my foot off the brake the pump does work.
Hope one of you guys can help.
Forgot to mention she is a 1975 model.

[Mike D]

As weird as this problem sounds, I'm thinking wiring problems under the dash? Maybe a bad ground leading to the relays?

What was the last thing, no matter how simple or seemingly uninvolved, did you do to the car? I changed a battery once and my electrical system went nuts. Turns out I moved the old wiring harness when I replaced the cables and it rubbed the insulation bare and created an intermittent short. Drove me bonkers for about a week.

[Mad Mick]

Thanks Mike,
I recently relaced the rear window seal which has two wires for the heated window, can't see how that might affect this as she ran for weeks after with no problem.
Can't figure why the footbrake kills the pump and affects the fuel guage.
I'll keep looking.

[Mike D]

Does the rear defroster work? Sounds like it's "disconnect the dots" time. Unplug the dash display. Same problem? Disconnect the brake switch. try it again. If disconnecting the brake switch doesn't do it then look for a wire being moved by the pedal assembly. Go on to the actual rear lights fixtures. Lather, rinse and repeat. Cruise control? Rotate ALL of the fuses in the fuse box. Make sure they are clean. Pull and inspect the flasher unit for the emergency flasher. All kinds of crap is run through it's grounding system.

It sounds like a bad ground somewhere. A corroded or partial ground will allow voltage but not amperage. I'm guessing when your brake switch engages it creates a "draw" which the ground cannot sustain and trips the relay for the fuel pump.

Uggghhhh! Electrical problems! I hate 'em! Somebody ought to invent a vehicle which is completely organic.

[Mark DiSilvestro]

Look for a faulty ground, most likely one shared by the fuel pump and the brakelights. Could be under the dash or in the rear or trunk.

Many years ago, I worked on a mid'80s fuel-injected Buick Regal that would speed up or slow down depending on the setting of the dash-light brightness control. Turns out the in-tank fuel pump was grounded to the trunk floor with the same terminal as the tailights. The shared ground terminal had broken from the grounding screw but was still grounding the pump through the light circuit to the dashlight rheostat.
This resulted in an adjustable ground to the fuel pump and variable fuel pressure to the fuel injection system, via the dash-light control!

Happy Motoring, Mark

[Mad Mick]

Thanks Guys,
I removed the instrument cluster and the same problem. I removed the connector to the foot brake light switch and problem's gone. I then got my wife to apply the brakes with the foot brake switch connected and the right rear brake light has a dim light indicitive of an earth problem. I now think there must be an issue under my rear seats which were removed when I replaced the rear window seal.
The good news is that Mayo beat Dublin by one point in a fabulous all Ireland Gaelic football semi-final this afternoon which was a credit to both teams and the celebrations here will go on well into the early hours.
In view of knowing ones priorities I shall join in with the celebrations.
I hope to track down the fault tomorrow and thank you for your help with this.

[Mike D]

You could always just move to Rome. No need to apply the brakes there. Use of horn and hand gestures are all which is required.

[GaryF]

Originally Posted by Mad Mick

The good news is that Mayo beat Dublin by one point in a fabulous all Ireland Gaelic football semi-final this afternoon which was a credit to both teams and the celebrations here will go on well into the early hours.

Only Kerry to beat now....could be a short lived celebration

[cth350]

There's a heavy cable with about 15 leads in it that runs from the driver's knee back to the rear of the car along the channel under the doors.

You've described a case where either there's a short or something mis-wired.

Unplug the cable from under the dash and get a multi-meter and start testing leads. The 116 chassis manual has the wiring layout.

-CTH

[Mad Mick]

Quote:
Originally Posted by Mad Mick
The good news is that Mayo beat Dublin by one point in a fabulous all Ireland Gaelic football semi-final this afternoon which was a credit to both teams and the celebrations here will go on well into the early hours.


Only Kerry to beat now....could be a short lived celebration

That was true for both of us!;

The problem is now fixed and thanks to you all, including the true blue:

The earth connection behind the rear seats was ever so slightly loose, this connection is high up behind the rear seats and there is a very small bolt with a tiny seven millimetre nut that carries four brown wires and one black.

The black is for the earth of the rear window heater element and the browns' must be for the earth to the rear right tail lights, as well as the fuel pump.

This now has the Mother of all earth connections!!!!!!!

It just doesn't get any better than being able to drive a beautiful machine like this.


Well maybe beating the Dubs comes close!


Beating Kerry?


I'll always have the pleasure of the old Merc.

(And who knows?)

[cth350]

my old boss grew up in Dublin. He got to see U2 when they were still a pub band. As he put it, they hadn't yet learned to play their instruments, but clearly, they had something. -CTH