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Discussion Starter #1
Went to start the car after a fresh engine rebuild and rolled snake eyes with spectacularly engineered electrical system. No fuel pump to be heard from. But it gets stranger, outside the car the fuel pump works perfect, 12 volts is getting to the plug in the trunk, and there is no high resistance to ground on the side of that plug. With the pump plugged it and cycling the key voltage drops to .8 or lower at the pump and the pump tries to turn but isn’t getting moving. My suspicion is the supply Amperage is high enough but I haven’t a clue where to start looking. Advice would be greatly appreciated.
Thanks, Brennan


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Discussion Starter #2
Update: there is 12v being supplied to the relay and 12v coming out on to the red wire on pin 87 while fuel pump is still only seeing .8 to 1 volt


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Update: there is 12v being supplied to the relay and 12v coming out on to the red wire on pin 87 while fuel pump is still only seeing .8 to 1 volt


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Note: the Bosch 0332014113 NON red stripe fuel pump relay DOES NOT have a red wire at pin socket 87. It is pink-white wire that goes to fuel pump.

Pin 30 PINK wire is 12v wire from ignition switch via Engine Services F22 fuse. Pin 86 of fuel pump relay has a smaller 12v red wire coming from pin 87 of the Bosch 0332014112 red stripe relay for the Motronic fuel injection system.

If you remove fuel pump relay from relay socket turn on key and check for 12v at pin 30 pink wire and then jumper pin 30 pink wire socket to pin 87 pink-white wire socket with key on fuel pump should run. You should have 12v at pink-white wire at fuel pump with jumper in relay sockets 30 and 87. If fuel pump won't run with 12v to connector in trunk then black ground wire in tank cover or to chassis ground is bad since you say pump runs when tested outside of tank.
 

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Discussion Starter #4
Yeah I came to the conclusion I was poking at the wrong relay after looking at the wiring diagrams. At some point the relays swapped spots so my position diagram was misleading me. Jumpered the fuel pump runs and I’ve traced the problem to a lack of ground on the grey and black wire


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Discussion Starter #5
It appears ground 131 is at fault but my diagram is vague in its positioning saying “located on the upper cover”. Where exactly is this


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Discussion Starter #6
Scratch that, grey and black runs to the motronic computer on pin 20. Is there any reason not to supply that with a constant ground?


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Computer supplies the ground to gray-black wire to ground fuel pump relay pin 85 once engine is cranked over and stays grounded once engine running. That is the way system designed so fuel pump does not run if engine isn't cranking or running. Externally grounding that wire will start fuel pump as soon as key turned on same as removing relay and jumpering pin 30 to 87.

As for G131 grounding wires they are on the end of the intake plenum by upper dogbane top mount.
 

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Discussion Starter #8
From what I can tell it’s not getting a stable ground while cranking, relay is jittering back and forth and my multimeter doesn’t pick up fast enough to see it iether. Other than the fuel pump running constantly is there any harm that could be caused to the computer by providing a constant ground to the relay


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Discussion Starter #9
Alright, disconnected the wire that goes to the ecu and put in a constant ground, fuel pump now runs but no spark and I’m assuming no injector pulse since the ecu is no longer spitting out it’s blink codes. What gives and why is this so complex?


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Discussion Starter #10
Oh boy oh boy do I have a story for y’all. Decided to check voltage with key on and while cranking, multimeter on jump post and intake for ground, key off 12v key on 1v cranking nothing, hmm that’s odd. Checks all ground points, clean. Decides to check small ground strap from intake to block, ****ing open circuit. Tools flew across the workshop and much head pounding ensued. I’d like to take a moment to honor the moron who decided to put the most important grounds of the car on something that is completely isolated by rubber mounts and tubes and then ground it with a ****ty little wire. The car now runs and drives perfect and I need to seek professional help. Moral of story, check your god **** grounds first, especially between isolated engine components.



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"Dwight's rule, check the ground [wires]." Here was my memo on that issue on later 164s with rubber mounted intake: Stumbling & no power when warm
 

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Discussion Starter #12
Thanks for all your help Steve, you really are a great asset to this whole community. Keep up the good work and stay healthy.


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