Fuel pump permanently on...

[TonyJerseyUK]

Hi,

On my RHD '66 230SL, the fuel pump is now permanently stuck on, until I disconnect the battery.

The problem started a while back when I had to tweek the ignition key a little to get it to come on before starting, but now it's led to this, where the car remained with the engine on, even after I took the key out!  Is this a problem with the ignition or a stuck relay?  I've checked fuse 4 but that's okay.

Would very much appreciate any advice.

Cheers,

Tony

[ja17]

Try it again and close the driver's door this time. (This is not a joke). Otherwise the electrical part of the ignition switch may be defective. It can be removed and replaced.

[TonyJerseyUK]

Try it again and close the driver's door this time. (This is not a joke). Otherwise the electrical part of the ignition switch may be defective. It can be removed and replaced.

Hi Joe, 

Many thanks for the suggestions.  Defo not the door (if only...).  I've just tried again (with the door shut!) and she starts fine but then won't turn off, even with the key out of the ignition.  And, as I mentioned, even when I manage to turn the engine off (by turning the key to start it?!?!) the fuel pump is permanently on.  Having spoken with MB Classic here in the UK, they suggest checking the wiring first and then, if not that, replacing the switch on the ignition. Would that be your diagnosis too, and what is most likely??

Bests,

Tony

[Cees Klumper]

I've come to appreciate that electrical issues like this are often due to Bad Ground. Not an expert by any means, but Bad Ground is apparently responsible for many electrical gremlins. So that is what I would try and diagnose in your case (after checking Joe's suggestion of course!).

[WRe]

Hi Tony,
have you read this: http://www.sl113.org/forums/index.php?topic=20425.0 ?
After that you should follow wiring diagram to the fuel pump. There must be a wrong connection (door switch, brake fluid warning light, any other connection that fires the pump).
...WRe

[TonyJerseyUK]

Hi Tony,
have you read this: http://www.sl113.org/forums/index.php?topic=20425.0 ?
After that you should follow wiring diagram to the fuel pump. There must be a wrong connection (door switch, brake fluid warning light, any other connection that fires the pump).
...WRe

Hi, thanks all for the ideas. Upon further inspection, there is every chance that it's not anything to do with the fuel pump, but simply the fact that the ignition is on (red light in dash) all the time, and a byproduct of that is obviously that the fuel pump is on all the time!

I've read how to replace the ignition switch, so that now takes me out of the equation altogether!  Mechanic on his way tomoz.  Pity, as I'd like to have diagnosed it myself, but there are so many old elec cables running all over the place, that it's just beyond me! :-(

Thanks again,

Tony

[TonyJerseyUK]

Problem solved!  After reading that article referred to by WRe (where door courtesy lights are connected to brake fluid Lights are connected to fuel pump!) it made me wonder why there was an extra switch coming out of the lower dashboard, very close to the courtesy light. Errrrr, that'll be a manual override switch for the fuel pump, which for some reason I'd flicked to the left instead of it's normal right-sided position.

I can only guess that someone may have had this problem with the car before I bought a year ago.  That switch surely can't be standard??? I'd take a photo and post, but not sure if that facility is available to me.

Still, a very much cheaper fix than I was bracing myself for!! :-)

Best wishes and thanks again to those far brighter than I!

Tony

[Garry]

Definately non standard. Someone has fitted it to bypass the MB electrics.

[ja17]

Ah yes, there is always the possibilities of problems, caused by previous  re-engineering attempts.  Sometimes creative, likely inferior, sometimes useless, often just not working, they are most often the most difficult and non-typical problems to find.

[Garry]

Totally agree Joe and normally electrical

[hands_aus]

Just wondering, did the switch also turn the ignition light off too?

[GGR]

I think we have wonderful cars:

If the car stays on, shut the door. If still on, check the brake fluid. If still on, switch the car on to switch it off. Eventually check the courtesy light, or the switch next to it!

Someone at the NSA must surely be wondering what we've been smoking?

[TonyJerseyUK]

Just wondering, did the switch also turn the ignition light off too?

Hi,

Good question.  Yes, that switch turns on the ignition light as well, so it appears to be a second ignition bar the actual starter motor.  Another kind member has pointed me towards a thread that gives exact info and photos on how to get the ignition out to replace the switch (which I've now bought), so hopefully I can solve the prob with the new ign switch and then remove this extra toggle switch and restore to the orig spec.

Would that be your fault diagnosis as well - duff ign switch?

All the best,

Tony

[hands_aus]

There is another possible reason the ignition light stays on.

If the light goes off while the engine is running but stays on when the engine is turned off this could indicate a blown diode in the alternator regulator.