AC and cruise control have NO direct effect on the ECU and having removed these, have no influence on your symptoms.
Injectors have no intelligence of their own. The ECU controls them and there are only 6 inputs to the ECU that determine what happens to the injectors:
- By far the most important is the vacuum signal from the centre of the cross-pipe that goes to the ECU;
- The remaining 5 inputs are: Coolant Temp sensor, Intake Air temp sensor, full load enrichment switch, battery voltage and the throttle position sensor (mounted below the throttle pedestal). TPS only comes into play when the throttle is actually moved.
Think of the injectors like a relay. When you switch the ignition on, the injectors will have 12V on one of the two pins - like a relay. When the ECU wants them to squirt fuel, the ECU will ground the other pin - like a relay. So, the injectors have no ground of their own. Its the ECU ground that comes into play here, but a bad ECU ground won’t make the engine run rich, it will run lean - or not at all.
There is one major difference between OEM JAG V12 injectors and a relay (from an electronic perspective): As you have measured your injectors, they have a resistance of only 2.8Ohm. These are called “LOW IMPEDANCE injectors”. A relay’s resistance is MUCH more an can remain active indefinitely. But a 2.8Ohm injector switched to directly to ground (via the ECU) is at risk of being fried. THIS IS WHY there is a resistor pack. A low impedance injector switched to ground can end up drawing I (Amps) = V (Volt) / R (Ohm), I = 14 / 2.8 = 5A per injector. If we now calculate the Watts for this injector we get W (Watts) = I x V = 5 x 2.8 = 70 Watts. Think of how hot your 55W headlight lamps get to put that 70W into perspective…
So what happens now is that the ECU has TWO ways of switching an injector to ground:
- one route is directly to ground;
- the other route is also to ground BUT, via a resistor (inside the resistor pack). The resistor reduces the amount of current flowing (thus preventing it from over heating) but its enough to keep the injector open.
When the ECU wants an injector to open, it will switch on BOTH routes simultaneously but after about 1mS (0.001 S), it switches the route directly to ground off, thus leaving only the route via the resistor pack.
The route directly to ground is to open the injector as quickly as possible and the route via the resistor pack is simply to keep it open.
On the jag the injectors are grouped in sets of 3, so 3 injectors are supported by 1 resistor.
So to answer your questions:
"why would the injectors not adjust their pulse width correctly? "
They only do what the ECU tells them to do based on those input parameters.
"can a bad resistor pack do this? "
No, a bad resistor pack can only reduce or break the current to a group(s) of 3 injectors causing them to run lean or not at all.
“Can a bad ground to the injectors?”
A bad ground can affect the injectors INDIRECTLY through one of the 6 input parameters or directly by not allowing the intended current to flow through the injectors via the ECU to the ECU ground.