1974 911 with a 3.2 Died while driving

Hcarraro · 05-25-2009, 04:43 PM

While out for a drive on Friday in my 1974 911 coupe (with a 1984 Eurospec 3.2 liter Motoronic F/I engine) it abruptly died and would not restart. I had driven the car earlier in the day and did experience some bucking and missing that seem to disappear with acceleration. I decided to drive the car to my local Porsche shop for a diagnosis when it died.

I was able to coast to a parking lot but like I said, it would not restart. As I was coasting into the parking lot I did see a small amount of smoke on the driver side in the front of the driver seat but not enough to identify by smell. I do believe it to be electrical but I an't say for sure. I disconnected the battery and called a wrecker to have the car towed the last few blocks to the Porsche garage but my technician friend and I didn't get a chance to look at the car. As it turned out, he badly needed my help in delivering a customer's car and closing the shop for the holiday weekend and I gladly agreed to help.

The way it stopped running abruptly is almost like someone jerked the coil wire off the coil. And because I did see the smoke I am leaning towards electrical. I just recently replaced the turn signal and winshield wiper stalks and the light switch. Could I have created a short or perhaps inadvertently loosened a critical wire on the ignition? I don't have a wiring diagram but may be able to download one from somewhere.

I know my friend is really busy right now and he is short handed not to mention that he facing a reduced work week. As a result, I would prefer to fix the car myself if at all possible without bothering him but I am not at all sure where to start. Any suggestions would be greatfully appreciated.

Thanks in advance for your help.

Regards,

Henry

Roc Doc · 05-25-2009, 04:52 PM

Start by checking all the fuses. And make sure you don't have any loose connections at the fuse block or at the ECU. If you let the smoke out of the computer you're going to need a new one, [captain obvious].

old man neri · 05-25-2009, 04:54 PM

The infamous DME relay perhaps?

Hcarraro · 05-26-2009, 04:30 AM

Thanks guys. I will let you know what I find.

Henry

scarceller · scarceller's Garage

The DME and the DME relay are located under the driver seat, my bet is the smoke was from one of those. Hopefully the relay and not the DME.

Hcarraro · 05-27-2009, 04:44 AM

Hello guys,

Well I dodged a bullet. The DME and the DME relay are functional. Thank goodness not all of the smoke got out of the DME. Grin

We have isolated the issue to an electrical problem feeding the fuel pump. We bypassed the OEM wiring to the fuel pump and the engine started. So my problem appears to be a shorted wire somewhere between the fuse panel and the fuel pump.

Anybody have a PDF of a clean wiring diagrapm for a 1974 911? What I am working with is a a copy of a copy of a copy and it is wurfless

Thanks for your inputs.

Henry

scarceller · scarceller's Garage

Not sure how the 74 fuel pump is wired but in the 3.2L Motronic the pump SHOULD be feed from the DME relay (pin 87b). The relay feeds the pump 2 ways: first turns the pump on while cranking then once engine starts keeps the pump on. This is done via pin 20 of the DME the DME grounds this pin to activate the pump at the appropriate time, the pump only runs while cranking and once the engine starts.

See this:

I also have a full wiring diagram for the Motronic please PM me with you email-id I'll send.

Hcarraro · 05-28-2009, 04:31 AM

Hi Sal,

Thanks for the DME wiring diagram showing fuel pump. Yes please email me the full Motronic wiring diagram, I would appreciate having it. My email address is hcarraro@hughes.net.

I didn't get a chance to work on the car yesterday but I will today.

Thanks for all the input.

Henry

Hcarraro · 05-31-2009, 06:01 AM

Hey guys,

First of all, thanks Sal for your DME email attachements. Those will come in handy I am sure in the future although my problems currently have to do with the 74 911 wiring.

Here' is a follow up to what was found and what we did to get my car running again. As I mentioned in a previous thread the DME itself and DME relay were fine and it appeared at first glance that I had I lost a fuel pump. However, by hooking an external electrical source to the fuel pump showed the fuel pump to be working. This isolated the problem to the cars fuel pump electrical circuit. After repeated attempts to further isolate the problem, and with no useful wiring diagram for a 1974 911 to work from, it was decided to create a new circuit as a temporary measure.

It is impossible to tell from the picture, but essentially what we did was to create a circuit from an un-used (air conditioner) fuse position to run the rear mounted Bosch fuel pump. The heavy yellow wire is the first of two wires I added to create the new circuit.

I took the car on a 300 mile fun run yesterday with about 20 other Porsches and the car performed flawlessly. Okay, well uh the fine oil mist from my leaking rear main seal pissed off several folks.

Thanks again for your inputs.

Henry

Roc Doc · 05-31-2009, 09:43 AM

When I did my 3.2 swap into my 76, I used a 80's VW Jetta fuel pump in the factory location under the tank. At first I used a "free" one from a buddy at a wrecking yard, and later when it died I bought a new one at the import parts store. They're Bosch, and the fuel pressure is right for our application. Fuel pumps for EFI always work better pushing, rather than pulling fuel from the tank.