ECU or Power Amplifier Diagnosis When Injectors Don't Fire

Since the Service Manual treats both of them pretty much as black boxes, I'm not sure what other steps I can take. Any advice on home diagnostics of this problem would be greatly appreciated.

Thanks to those of you who responded. I'm posting this to the group for future reference. Hopefully no one else on the list will have to use this information, but just in case, here it is...

Several people suggested transposing the feed wires from the ECU to the Power Amplifier as a way of distinguishing which unit was at fault. On Series 2 XJ12's this is most easily accomplished at the four-wire connector located near the front of the right inner front fender. This has 4 black wires, labelled 9-12, going to the Power Amp, and B, KB, KG, and KU wires going to the ECU.

From my measurements (but not from any official Jaguar source) these wires appear to have the following roles:

• B: ground
• KU: 12V power supply to Amp
• KG: Signal to Injector Group A (A1,A3,A5,B2,B4,B6); voltage here measured 12V, but should have measured apprx. 2.5V
• KU: Signal to Injector Group B (A2,A4,A6,B1,B3,B5); apprx. 2.5V

To check for injector function, I turned the ignition on and turned the throttle assembly, listening for clicks. I found it impossible to determine which injectors the clicks were coming from unless the majority of the injectors were disconnected.

By connecting KU to lead 11 (normally connected to KG), I was able to make the previously nonfunctioning injector group A work, indicating that the problem lies in the ECU, not in the amplifier. If group A had continued not working, this would have indicated an amplifier problem.

Thanks again to everyone who offered advice.