injector voltage
Discussion
It's difficult to get a true measurement.
With the ignition on but engine off, the ECU is not firing the injectors. So they're not grounded and the only current passing through the resistor pack is that being taken by the multimeter. Because it has such a high input impedance it presents hardly any load at all to the resistors so the meter sees the full 12v.
With the ECU firing the injectors, the voltage across the resistors is rapidly being dragged down and then let rise by the switching action of the ECU. Add in some back-emf fom the inductive load presented by the injector windings, plus the response time of the meter and you probably wold be looking at anywhere between 5 and 10 volts.
What you need is an oscilloscope...
With the ignition on but engine off, the ECU is not firing the injectors. So they're not grounded and the only current passing through the resistor pack is that being taken by the multimeter. Because it has such a high input impedance it presents hardly any load at all to the resistors so the meter sees the full 12v.
With the ECU firing the injectors, the voltage across the resistors is rapidly being dragged down and then let rise by the switching action of the ECU. Add in some back-emf fom the inductive load presented by the injector windings, plus the response time of the meter and you probably wold be looking at anywhere between 5 and 10 volts.
What you need is an oscilloscope...
dickymint said:
Should be reduced to 3 volts at the injectors courtesy of the resistor pack.
Sorry should have said-
It depends where you are measuring and what you are measuring with!
If you look at the diagrams in the linked pdf you can see that the ecu grounds one end of the injectors with the other connected to a resistor. The resistors are about 12 ohms and the injectors are about 4 ohms so when the injectors trigger you get about 1/4 of the battery voltage across the injectors.
The voltage at the injectors is not constant but pulsed as the injectors are triggered this means that the voltage you see will depend to some extent on the meter you are using to measure it. If you are seeing 8.7 on cranking you are probably/hopefully looking at the battery side of the resistor pack, what voltage do you see on the battery positive as you crank?
Edit Took too long replying ...
Edited by toltec on Friday 2nd February 12:37
wedg1e said:
It's difficult to get a true measurement.
With the ignition on but engine off, the ECU is not firing the injectors. So they're not grounded and the only current passing through the resistor pack is that being taken by the multimeter. Because it has such a high input impedance it presents hardly any load at all to the resistors so the meter sees the full 12v.
With the ECU firing the injectors, the voltage across the resistors is rapidly being dragged down and then let rise by the switching action of the ECU. Add in some back-emf fom the inductive load presented by the injector windings, plus the response time of the meter and you probably wold be looking at anywhere between 5 and 10 volts.
What you need is an oscilloscope...
With the ignition on but engine off, the ECU is not firing the injectors. So they're not grounded and the only current passing through the resistor pack is that being taken by the multimeter. Because it has such a high input impedance it presents hardly any load at all to the resistors so the meter sees the full 12v.
With the ECU firing the injectors, the voltage across the resistors is rapidly being dragged down and then let rise by the switching action of the ECU. Add in some back-emf fom the inductive load presented by the injector windings, plus the response time of the meter and you probably wold be looking at anywhere between 5 and 10 volts.
What you need is an oscilloscope...
Shirley though - if the measured resistance is 3 ohm then all is well with the resistor pack?
chedder said:
3 ohms on the injectors, 10.7ohms on the resistor pack, just cant get my head around this problem though. The resistor pack does not appear to have an form of inversion coil, so therefore should not produce the required 9v drop.
Is the voltage drop produced by the ecu?
Is the voltage drop produced by the ecu?
The resister pack and the injector act as a voltage divider, in a steady state the Voltage is shared in proportion to the resistance. So that 3/13.7 of the battery Voltage across the injector, and the remaining 10.7/13.7 across the resister. If you do the arithmetic it works out to roughly 3V across the injector and the remaining 9V ish across the resister.
toltec said:
dickymint said:
Should be reduced to 3 volts at the injectors courtesy of the resistor pack.
Sorry should have said-
It depends where you are measuring and what you are measuring with!
If you look at the diagrams in the linked pdf you can see that the ecu grounds one end of the injectors with the other connected to a resistor. The resistors are about 12 ohms and the injectors are about 4 ohms so when the injectors trigger you get about 1/4 of the battery voltage across the injectors.
The voltage at the injectors is not constant but pulsed as the injectors are triggered this means that the voltage you see will depend to some extent on the meter you are using to measure it. If you are seeing 8.7 on cranking you are probably/hopefully looking at the battery side of the resistor pack, what voltage do you see on the battery positive as you crank?
Edit Took too long replying ...
Edited by toltec on Friday 2nd February 12:37
It could be down to the meter you are using, to really see what is happening you need an oscilloscope as mentioned above.
The ecu is only triggering the injectors for part of the cycle of the engine so the 3v only exists for a proportion of the cycle the rest is at 12v - ish.
12-12-12-3-3-12-12-12-3-3-12-12-12-3-3
12V Injector closed
3V Injector open
It is possible that the meter you are using is showing you the average voltage which is dependent on the proportion of the time that the injectors are open.
Edit - ascii does not work - bggr
The ecu is only triggering the injectors for part of the cycle of the engine so the 3v only exists for a proportion of the cycle the rest is at 12v - ish.
12-12-12-3-3-12-12-12-3-3-12-12-12-3-3
12V Injector closed
3V Injector open
It is possible that the meter you are using is showing you the average voltage which is dependent on the proportion of the time that the injectors are open.
Edit - ascii does not work - bggr
Edited by toltec on Friday 2nd February 15:37
Edited by toltec on Friday 2nd February 15:39
I think you're looking for a fault that doesn't exist. Injectors and resistor packs EXCEEDINGLY rarely fail, at least on a car in regular use. Even then the injectors usually don't fail electrically, they just sieze up from lack of use... but can be freed.
See Uglymug's experences of a couple of years back...
See Uglymug's experences of a couple of years back...
What about the fuel pressure - is it adequate? Presumably you think the injectors aren't opening because there is no smell of fuel in the exhaust after a bit of cranking?
Try clamping the return line from the fuel regulator and - briefly - cranking. If it fires up maybe the fuel regulator is shagged and letting the pressure drop far too low.
Try clamping the return line from the fuel regulator and - briefly - cranking. If it fires up maybe the fuel regulator is shagged and letting the pressure drop far too low.
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