HFS-3 not spraying ::mystery solved::

[Richard L]

Originally Posted by Arthurwiljami

OK, so ECU cable on good. I?ve checked all the wire colours and they all have the same orientation red, yellow, blue, black. I do have multimeter.

Is it a must for the thin red wire to be connected to injector (+) or does it matter if the connetion just have 12V in pre-crank position (and when the car is running of course). If I?ve understood correctly both red wires need to get 12V in pre-crank position but where ever I put the thin red the pump activates when the gauge lights up. I don?t understand what triggers the system to spray when the car is not even running. Is it possible that the injector is "short to ground" for some reason in pre-crank position.

Well either way the system will not spray when the car is running.

The thin red wire should really be connected to the fuel injector (+) so that when the ECU switches off the fuel injector (+), the HFS3 might think the fuel injector is at 100% dc because i thas lost the (+) supply.

From past experience, this does not happen on the Audi ECU but for some un-explained reason, the HFS-3's green wire (grey harness) is seeing a ground signal from the fuel injector (-) terminal.

We can check this out if you can use your voltmeter to check the voltage at test pin8, relative to ground (bottom left hand corner):

Voltage at TP8
key off .............. no voltage
key #1 (accessory position) ........ no voltage
key #2 (pre-crank/run position) ....... 12V
key #3 (starter) ............. 12V

Please re-test key position #2 after ~10 seconds (before cranking) just to confirm the ECU does not remove the injector 12V. If it does, the 12V will disappear at TP8. This will cause the system and pump to trigger in error.