Injector Voltage

[Pete]

Anyone about with a petrol engine lupo, preferably a GTI with the capability to whip off a fuel injector plug and see what voltage is at the pins with the ignition on/engine off?

 

Cant get my injectors to fire and checked all the usual good stuff but I don’t have another GTI to compare some of the readings I’m getting that seem a bit odd.

 

Thanks!

[mk2]

No idea TBH, but usually they're on a 12v bus controlled by the ECU via a relay and separate fuse. Might be the relay if they're all out? So when the engine is not running, the fuel pump and injector power is isolated for safety. If sparks are ok, then it's not the crank sensor. Unlikely to be the cam sensor.

Although ignition switch has been known to cause weird issues like this across all Lupos and Rossas. Lights and radio all good?

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[Pete]

Hello sir, thanks for the response. I’ll tell you the full story, the fuel pump in the tank had burnt out, so I’ve replaced that initially as there was no fuel being sent to the rail, but that is now fine, I’ve had the plugs and leads out and checked for spark already, that’s all good, so I pulled out my injectors on the rail and cranked over and no fuel was being sprayed, so I checked the injector fuse and fuel management fuse, all ok.

 

The wierd thing is I’m getting almost 4v constant to all injectors with the ignition on, which I didn’t think was right. I figured it may be a shorted relay but wasn’t sure which one would be responsible for it, so I removed all of them and I still had the 3-4v supply, so I’m starting to wonder if the ecu is faulty and sending out the voltage. I don’t often work on vw but I do know some manufacturers have strange ways of controlling their injectors and didn’t know if this was normal or not. I didn’t want to start ripping looms out to check for shorts or buying spare ecus if it’s normal to get that 3-4v to the injectors and it’s actually the injectors that are at fault!

[mk2]

I'd be very surprised if it's the ECU. The power outputs are all current limited (as in they have a self preservation circuit in there). 

Have you checked to see if the injector feed bus momentarily 'blips' to 12v when you power up? 3-4V is something you often can see in the form of a computer logic system voltage, that leaks through circuitry that is off. 

Are you seeing something around 4.8V on the sensors, which is typically their supply power volts? That tells you that the internal ECU voltage regulator is working fine.

But thinking about it, there is an external feedback power circuit. So when you first switch on the ECU (ignition), the ECU does it's self test, then activates the low power feed control signal- turning on a relay, which then provides 12Vpower back into the ECU, which then is regulated down to 5V, to power the various sensors and aux circuits. Then another line is powered up to power the fuel pump and injectors- only after the crash (airbag) sensor has passed its checks (designed to cut off power to stop fuel being sprayed everywhere in case of a collision).

You could connect up small 12v lamps to the injector feeds to see if they flash. 5 watt size. Anything bigger you wouldn't notice flashing (they take too long to get glowing). Although a 12v LED circuit might also work.

Is the fuel pump running when you crank? Could be a stuck fuel pressure regulator, filter purge valve? 

Sorry for the essay...

[Pete]

I’ll be taking home some more kit from work tonight, only had a home made test light with no probes and an old multimeter over the weekend so will take some back probes and croc clips home and have a play. I did attempt to slip in some core to the injector terminal and using something heavy to hold the other end of the test light to the engine lifting eye but it didn’t prove anything, hard doing it on your own without anything to properly keep a guaranteed contact to ground!
 

I’ve not tested any of the other sensors at this stage, but I do seem to remember at the fuel pump getting around 3.7v across main live and earth, also with the ignition on but not being ‘actuated’. It’s always there, just like to the injector terminal.

 

And yes the fuel pump runs during cranking. I have looked online at relay designations but it seems that my car isn’t fitted with the fuel cut off/airbag/crash relay. Not sure if that is correct, but either way with all of the relays removed from under the os dash that constant voltage stays present.