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Discussion Starter · #1 ·
Last night I was driving my vert and it died on the road close to a friend's house. It has spark but was not delivering fuel or engaging the injectors. Pump works if you throw 12v at it on the fuse.

My first question is how is the fuel injection relay powered? I want to check if the fuel pump relay is get power to pin 86, but that requires me to check if the injection relay is closing and powering the pump relay through the boost pressure switch (? It really does not make sense to me why this guy is stuck between the two relays electrically speaking). I just realized I was testing it without the ECU but the diagram seems to imply the ECU is the one that grounds the the injection relay. Which then makes me wonder how the ECU gets powered and knows it is time to turn the injectors on. The later is probably due to the Hall sensor info. But, how does the pump relay powers the relay? The diagram in the Bentley was not that helpful.
 

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I don't have a wiring diagram, but I think you're confusing "powering" with completing a circuit.
All the ECUs in automotive use prefer to manipulate grounds rather than power. The LH ECU turns on the fuel pump relay by grounding the pull-down circuit (the pull-down circuit is the coil in the relay that becomes magnetized and "pulls down" the contacts that carry the current). Pull-down circuits require very little power, that's why the ECU can handle it.
The ECU also opens the injectors by completing a ground circuit; the injectors always have a 12v supply.
The ECU will refuse to supply the grounds unless it sees an ignition pulse. The pulse is provided by the amplifier (triggered by the Hall Effect sensor or magnetic pickup).
I think you're chasing power when you should be looking at the ground side of the circuit to understand how it's switched.
 

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am i correct in thinking that the fuel pump relay will not activate without the ECU receiving a signal from the hall effect sensor in the distributor?

that is relatively quick and easy to check, far moreso than anything found under the dash
 

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Discussion Starter · #4 ·
One thing I did before yanking the ECU and fuel pump relay out was to connect my spare distributor to the hall sensor wire and then spin it by hand. This way I can hear the injectors and see the coil being fed without having to spin the engine. This time the coil was beinf fed and was giving out a spark, but the injectors were dead.

The reason I was checking the power supply on the injectors was that, based on my Volvo experiences, I would expect them to have 12V as soon as the ignition is on. Kinda like the coil. Then the ECU can ground the other wire and close the circuit at its leisure. But, if the injectors are not getting 12V that would make me thing the problem is not on the ECU side but on its supply side. I could, however, check if the ECU is grounding the injector ground wires when the distributor spins though.

I hope now my madness makes sense. :cheesy:
 

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Discussion Starter · #5 · (Edited)
Update: after marking sure I got the tach signal on pin 1 of the ECU connector,

I decided to bypass the ECU alltogether and simplify my testing. That was done by taking the ECU out of the circuit and then grounding the pin 85 in the fuel injection relay just like the ECU would when it turns itself on. That meant that pin 86 in the fuel pump relay now has 12V (pin 85 too as it is on the other side of the coil). So far so good.

So, I then emulate the ECU, which would ground the pin 85 when it got the Hall sensor signal through pin 1, by grounding that pin against the body. Well, the relay did not close. Now, if I remove the fuel pump relay and run a jumper to emulate the closed relay, pump works and injectors get power and I have no problem running the car.

Is it me or this sound like there is some kind of current loss here?
 
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