The largest and best supported Jaguar cars enthusiast site on the Internet!

Serving Enthusiasts since 1993
The Jag-lovers Web

Currently with 3,166 members

ECU or Power Amplifier Diagnosis When Injectors Don't Fire

ECU or Power Amplifier Diagnosis When Injectors Don't Fire

Jon Schrock
Jon Schrock wrote:

Hello everyone,

I posted a request for help about a week ago, when my Series 2 XJ12 refused to start after I'd left the ignition on long enough to completely drain the battery.. . I found that one entire major group of injectors (1A,3A,5A,2B,4B,6B) wasn't firing. From the way I read the manual, this means either my ECU or Power Amplifier has expired..

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.

Looking for or selling Jaguar Automobilia? Then use our absolutely free Jaguar Collectibles Exchange! Got some pictures you would like to share with other Jag-lovers?
Then try our automated Photo Album and post some images!


Please help support the move to the new site, and DONATE what you can.
A big Thank You to those who have donated already!



Go to our Homepage
Improve your Jag-lovers experience with the Mozilla FireFox Browser!

  View the latest posts from our Forums via an RSS Feed!

Jag-loversTM Ltd / JagWEBTM 1993 - 2024
All rights reserved. Jag-lovers is supported by JagWEBTM
For Terms of Use and General Rules see our Disclaimer
Use of the Jag-lovers logo or trademark name on sites other than Jag-lovers itself in a manner implying endorsement of commercial activities whatsoever is prohibited. Sections of this Web Site may publish members and visitors comments, opinion and photographs/images - Jag-lovers Ltd does not assume or have any responsibility or any liability for members comments or opinions, nor does it claim ownership or copyright of any material that belongs to the original poster including images. The word 'Jaguar' and the leaping cat device, whether used separately or in combination, are registered trademarks and are the property of Jaguar Cars, England. Some images may also be Jaguar Cars. Mirroring or downloading of this site or the publication of material or any extracts therefrom in original or altered form from these pages onto other sites (including reproduction by any other Jaguar enthusiast sites) without express permission violates Jag-lovers Ltd copyright and is prohibited
Go to our Homepage
Your Browser is: CCBot/2.0 (, IP Address logged as on 18th Jun 2024 06:04:05