Jaguar Repair: XJS V12 HE wont start, resistor pack, haynes manual

Question
QUESTION: Hi Howard
I have a problematic XJS V12 HE 1985
Originally the car would not start. Fuel and spark ok but voltage to injectors were intermittent.
I replaced the ignition amplifier and got the 6CU ECU repaired.
When the ECu went back in it fired immediately and worked fine.
Today the garage tells me the car wont start again with similar issues as before the repairs to the ECU.
I am at a bit of a loss at to what it could be?
Perhaps the replay on the raditator cold start is it called?
Or distributor somewhere or crank sensors?
If you can help I would be really appreciative

Regards

Mark

ANSWER: Hi Mark,
I am not proficient on the V-12 injection system but do know a few things to check. The main relay next to the ECU powers the injectors and the ECU grounds the injectors in groups of 3 at a time. The ECU grounds them through the resistor pack mounted in the right front of the car. I believe your model uses the same ignition amp as the XJ-6 uses which has a GM amp inside. If you have ignition that is not the problem. However, you need to know if the ignition signal is being received by the ECU and that is best done with a Logic Probe (12v type)(it does not interrupt any signal) As I remember all other info sent to the ECU are just modifiers of the pulse duration. And only the ignition pulse is the trigger to start doing something. But you need to know that the ECU has power and ground.

I have three factory diagrams of the ECU wiring to the car but they are by Vin number.
up to #133460
up to #133460 with purge mod kit JML 832
from #133460 on to #139052

Let me know if you would like a diagram and which one and I will post it on my web site for you to copy.
Howard  

---------- FOLLOW-UP ----------

QUESTION: Many thanks for this Howard.
The vin is 122292
Out of interest is it possible that the wiring for ignition signal to the ECU is short or shorting? Sadly the Haynes manual I have doesn't show the route for this cable but I am guessing it moves through the bulkhead into an area which is either really hot IE burnt cable or boot area which is likely to have been damp at some point.
Ill check these and hope something comes to light.

Regards

Mark

Answer
Hi Mark,
I have always learned to look at "End Result" in testing electrical or mechanical. "What don't work? Then what tells it to do something?" Then I work my way back toward what tells it to do something Electrical or Mechanical. This method has served me well for 50 years of working on British cars.

The wiring diagrams are posted on my web site in two sections.

http://mg-tri-jag.net/XJ-SVin133-11x1.jpg  
and
http://mg-tri-jag.net/XJ-SVin133-21x1.jpg

The amp signal must be checked at the ECU plug (pin 18) to know that the ignition signal is reaching the ECU. (The signal that tells the ECU to Start doing something)
Also check ECU ground (pin 1) and power (pin 22)
The "Start" signal is probably to enrich the fuel mixture for starting (pin 26) so it would be good to see power on it in the "Start" position of the key.

It is a waste of time to look for burnt wires or any other fault until you for sure 100% that a signal is not reaching it's destination. Then trace backwards from it's destination. This is true on any electrical component on the car.

Over the years I have only found a few failed ECUs and have at times had to confirm that EVERY wire in the ECU plug had it's appropriative power, ground or signal. This can be done very fast with the use of a Logic Probe (12v) I bought mine at Radio Shack but was just in there the other day with a friend who needed one and they don't handle them any more so you will need to check with a electronics store to locate one. They are not expensive and you will probably not use your test light any more once you start using a logic probe.

Let me know how you do.
Howard