Fuel pump issue.

EDIT - sorry, should have mentioned this is a 1976 911S with a later (?SC) 3.0 with CIS injection. So, a few weeks back, the car stopped running moments after starting and about to warm up. Checked for the usual stuff and my diagnostic had me stopped at fuel, I noticed that on turning of the key to 'START' or even turn it over, there was no sound coming from the fuel pump. I have had issues with it before and it's been in the car longer than I've owned it, so I figured it had given up the ghost. I ordered a new one, installed it and wired it up. Same issue. So I had a look at the fusebox, it. was upgraded to a classic retrofit model, also prior to my purchase, so I don't know what fuse is what, but the test lights all light up green with ignition on. There are three relays and I confirmed they all work by putting them at the end and manually closing them to make the horn work. Now, with the ignition on, one relay (centre of the three) is engaged constantly, the other two open. I assume this is for the ignition. The one closest to the firewall (not the horn) does nothing if I close it manually, I am assuming this is the fuel pump relay. I hooked up a multimeter to the wiring that supplies the fuel pump, at ignition ACC and START it supplies 0.00V (with a random flicker up to 0.01V now and then). Then if I turn the car over (starter works fine), it goes to 0.12V. And yes, I had it on the right setting because I tested it on the battery at 12.4V. Oh, and same result (0.00V) if I close each relay. So now I really don't know where to go from here. I'd take it to an auto-electrician or Porsche specialist but obviously, it does not run and it is parked in an underground garage which is inaccessible by a tow truck. Any input is greatly appreciated!

Could it be the DME relay

Sorry, should have mentioned it’s a CIS car.

So the ignition switch is getting power but not passing it on in the "ignition on" position?  So why don't you think its a faulty ignition switch?  What have i missed? Suggest you download a wiring diagram just to guide your investigation.

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You raise a good point, except there is power to the CDI when the switch is set to on, so I assume that isn't the issue? Or is my thinking flawed?

Not all the contacts are necessarily shared so it is possible to have power to one circuit but not another if the switch is faulty.  I'd be surprised if the ignition and the fuel pump shared the same fuse and circuit given the current draw involved but you would need to look at a wiring diagram to confirm. I suggest you spend some time under the dash and record with your test light or multimeter what circuit have power and what doesn't at the back of the ignition switch at the various key positions.  When compared to your wiring diagram this will verify if the switch is Ok or not. However, before you do the above, I suggest you double check the relay for the fuel pump as the relay is the most unreliable component in the entire circuit. Good luck!

Remove the air filter and lift the sensor plate, can you hear the f/pump relay click? Or pump come on?  Obviously with ignition on. sounds like the rotary contact switch on the ignition switch, or something to do with the wiring when the fuse box was  installed. i would bridge the fuel pump relay out, to get the car started so you can get the problem resolved. Can assist with this, my shop is located inner nth west melb. Richard.

Thanks all for your great advice. I finally have some time off, so I will get out to the garage and have a play. If I can get it started, can I bring it to you in the next few days @Mezger Autosport When I was first diagnosing the issue, I did bridge the relay to no avail, hence why I thought the pump was the issue. But just replacing the relays is relatively cheap and easy to rule out.

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Hi Richard, I had the car up and running but it has happened twice more, spontaneously. SO, tonight I went back out and lifted the sensor plate with ignition ON, no joy. tested outward power from ign. switch with it ON - power to all wires except yellow wire (starter circuit) - i was unable to test this one as I needed a third set of hands to turn the car over. I have replaced all 3 relays just to be sure. No change. I will pop back out and check the power to terminal 30, 87, 87a. By process of elimination, I think something funky is happening to the wiring between the battery and fuel pump relay. If it miraculously starts in the next couple days, I will bring it to you.

Okay, so I went with a mulitmeter and checked the power to/from the relay. I found an online diagnostic and I will point out how my car differed in red from what was stated as normal on pelican parts. 1). Pull out the FP relay and do the test (ignition switch @ OFF position) for the individual terminal at the socket: A simple 12-volt self-powered test light will work well for this test or a regular non-powered test light, or a volt meter. 87...........no power Same on my car. 87a.........no power Same on my car. 86...........no power (87a 86 are bridged) Same on my car. 85...........no power (should be grounded at this point) No ground on my car. 30...........no power (has ground contact?) Same on my car. 2). Turn ignition switch @ ON position (not start): 87............no power No power- has ground contact 87a..........has power Same on my car. 86...........has power Same on my car. 85...........no power, presence of ground contact No ground contact on my car 30...........no power, presence of ground contact Same on my car. Now, I'm wondering if that brings up any ideas for anyone? from what I can tell, this means the ignition switch is probably not the culprit?

I went and did some further testing. No earth 85, with the air flow switch either open or closed. This is abnormal from my understanding, BUT this should mean the pump runs full time as opposed to not at all? Bridged 87a and 30 - nothing. Bridged positive battery terminal to 30 - pump ran. I am really scratching my head now, because 87a read at 12v. So it should have run the fuel pump, right? @Mezger Autosport I don't feel it's overly safe to bridge battery post directly to 30 so I can bring the car to you, but perhaps I can make a fused wire to take the journey.

Yes i wouldn’t be running a permanent power feed to your fuel pump unless you have it protected, for a shortish journey somewhere. Sounds like you have some wiring issues, probably from people trying to modify and bypass things over the cars journey. End of the day there’s no shortcuts to repairs on these vehicles as they all come back to haunt.

Did you find the root cause?  I had a similar problem on my 993, did all the diagnostics, everything checked out OK but wouldn't start.  It was the fuel pump, car had been sitting for some time.  Eventually got it to go jumping the DME relay sockets.  Pump is on the way out so now I start the car every couple of days.