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Vic · Joined: 05 Jan 2011 Posts: 42 Location: australia

Hi all , having trouble with my 924 turbo , i have no power at fuel pump fuse , car will try to run but wont rev, i thought one side of fuse would be live with ignition on ? Even tried with test light on terminals and cranking the motor , still nothing , fuse is ok, can't work out why no voltage either side of fuse terminal , any ideas greatly appreciated

chuck21401 · Joined: 20 Feb 2005 Posts: 607 Location: Annapolis, MD

I'd start by checking the fuel pump relay which is before the fuel pump fuse, yes you should be getting 12V at the fuse.

Here's a pic of the current flow diagram.

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1981 Porsche 931 w/S1 engine & euro g31 transaxle. Water-cooled intercooler

Vic · Joined: 05 Jan 2011 Posts: 42 Location: australia

cheers mate , appreciate your help

924RACR · Posted: Sat Oct 24, 2020 8:57 pm Post subject:

Actually - the fuel pump relay won't be activated without the tach signal... so if you're trying to check voltages without the car running, they won't be right. I would not expect power at the fuse just by having the ignition on. You'd have to jump the relay - terminal 30 to 87.

That's a worthwhile test to run, try running the car like that.

Very common to have issues with the tach lead wire; should connect to the tach and the coil. Circuit/terminal #1 in the diagrams, IIRC, the green and white wire.

Gotta check your grounds too, of course, both for the pump (between the taillights backside of the license plate) and behind the fusebox on the firewall for the relay and dash...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Vic · Joined: 05 Jan 2011 Posts: 42 Location: australia

many thanks , will check !

chuck21401 · Joined: 20 Feb 2005 Posts: 607 Location: Annapolis, MD

924RACR · Posted: Sun Oct 25, 2020 5:16 am Post subject:

Correct; the fuel pump usually will be run briefly when first switched to key on, to prime the system, maybe for 2-3s.

Yes, if the tach jumps when cranking, you can test the relay - but you also MUST ensure that the relay has a good ground, and that the tach signal can actually make it to the relay as well - check continuity between tach or coil and relay socket.

Otherwise you're just pissing in the wind.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Vic · Joined: 05 Jan 2011 Posts: 42 Location: australia

great suggestions , really appreciate it guys !
great description is I truly am pissing in the wind ! lol
I can't hear relay kicking in at all or in fact pump running , unless I bridge relay , never seen a car with so many shitty earths ! Tacho does pulse whilst cranking motor over , now I have oil smelling like petrol , suspect leaking injector(s) ,

924RACR · Posted: Sun Oct 25, 2020 8:57 pm Post subject:

usually the fuel in the oil will be from cranking without running. Obviously you'll have to change that out, but maybe get it to fire first.

If you can get the pump to run while bridging the relay - good. Wiring or relay are suspect, debug the wiring first, using the wiring diagram posted. Power, ground, and tach lead.

Note that the fuel pump relay is grounded THROUGH the overboost switch on the intake charge tube, just before the throttle body. So make sure that is giving you a ground - can jump that lead to ground if needed. That's terminal #31 on the diagram, you can see the switch.

I've gone on enough about the tach lead for you to understand the need there.

I trust you have a multimeter or ohmmeter? Can't sort the wiring without.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Vic · Joined: 05 Jan 2011 Posts: 42 Location: australia

many thanks for all the tips !!! Yes have been giving the ohms meter a hiding , car has sat for too long !