Guys
On Thursday I started up the XJS again, second time in 4 days, to do a second check on my airbleed/purge system. All totally fine. Car ticked over for 20 mins and I switched her (yes HER, but no name) off. Continued my other playtime fun on the tractor as the Management was out patriotically boosting the French GDP.

15 minutes later, went to start the XJS to return her to the winter lair. Engine spun over fine, NOT a sausage of firing. Tried a couple more times, lots of throttle etc etc. Nothing. I mean, this is not supposed to happen!

First off, I pulled a plug, which, had there been fuel, would be soaking wet. Dry as a bone, and VERY white deposit. Second, cracked a fuel join, and found lots of pressure;

Conclusion, injectors not firing. Next step, carefully detach injector loom plug from the bottom of the Power resistor. This aluminium box with Lucas on it, from early pre-restoration photo:


I understand that the Power Resistor's function is to hold open the injectors for the required milliseconds once they have been initially opened by the ECU transistors. There are no moving parts inside, not even any electronic components, just a circuit board with four resistor tracks on it. If the contact with the injector loom is lost, the injectors do not open.

Pulled out the can of contact cleaner and the recycled Firm Bristle toothbrush and did my best to clean the 10 contact pins and holes in the respective sides of the plug at the base of the PR. Plugged it in again and tried.

Engine started right up. So, guys, this is definitely a job to add to the yearly maintenance schedule. Pulled the plug again today after 10 mins. tickover, plug colour back to mid brown. So even before the thing actually failed, it must have been failing, as clearly the white plug indicated far too lean a mixture.

Greg

 

Good one Greg.

I reckon a lot of the "no start", "hard to start", V12's may be traced to that plug and socket.

 

nice bit of fault finding


BB

 

Nice one Greg, thanks for sharing.

As stated above by Grant, it might be a good idea to do a FAQ / HOW TO write up on this and add it to the XJS info database

 

Is there a similar module for cars with Marelli Ignition?

My XJS is so bedded down, that I can't get the bonnet up to look.

 

Yes, OB, identical.
Greg

 

Thanks Greg

That was a brilliant Tip!

 

Great work, not only in terms of troubleshooting but, also, as a reminder to always remove/check/clean/lubricate/adjust as needed *before buying new parts*. Many others would've just replaced the part without trying to save it. So many supposedly dead and inoperative components can be brought back to life with a little TLC

Cheers
DD

 

Thanks for shareing Greg, good advice. since I am going thru ALL the wireing under the bonnet, I have been doing the same with ALL the contacts, \cleaning the male and female ends. When I start putting back together I plan on putting on a bit of contact grease to help prevent corrosion, read somewhere, don't remember where, that putting on an anti corrosion grease helps with the older Lucas connectors. so instead of replacing add the grease

 

Hi Greg,

Just a little info on what the Power Resistors actually do. Power Resistor is a bad term as these are really current limiting resistors. They do not hold the injectors open, they limit the current drawn by the injectors.

The injectors in the XJS are low Z approx 3ohms with 6 injectors in parallel (pre Marelli) the ohms seen by the ECU would be 0.5ohms with this low resistance the injectors would draw 24amps from the ECU. Adding the Resistors limits this current to a max of around 7amps.