Jaguar V12 HE engine fuel injection power resistor connector maintenance.

Background
The V12 HE motor installed in the Jaguar XJS, in 5.3 litre or 6.0 litre form, uses fuel injection and transistorised ignition to provide the fuel and spark to the cylinders. There are a number of common faults that can occur, mainly though age and lack of maintenance, that can cause engine failures and failure to start.

Common faults with these systems are, in no particular order:

• On all versions of the engine, injector harness cracking and shorting to earth because of heat in the engine bay, leading to cylinder flooding caused by constantly open injectors;
• On Lucas ignition variants, poor connection between the Lucas amplifier and the ECU, owing to the cable exiting the amplifier deteriorating in the heat of the engine bay, or even the amplifier itself failing though age and heat stress;
• On Marelli ignition versions, amplifier to distributor wiring cracking and shorting, again due to engine bay heat, engine speed sensors (one the flywheel and on the crankshaft front pulley) failing. Also, the double height Marelli distributor shorting out and disabling one bank of the ignition, leading to exhaust pipe fuel fires.
• Incorrectly gapped spark plugs and old high tension leads can also stress the ignition systems so that they fail.

Regular maintenance and preventative renewing of these items will ensure that failures are extremely rare. The downloadable book by Kirbert Palm entitled “Experience in a book – help for the XJS owner” deals comprehensively with these matters and is recommended for all serious owners and maintainers of the V12 XJS.

An unusual failure mode
Although mentioned in the Kirbert Palm book, the writer has recently experienced a less well known and far less common failure mode of the fuel injection system. The writer’s car is a 1985 V12 HE coupe with the Lucas ignition system and the 5.3 litre V12 engine. On this car, as on all previous HE versions of the engine and on all subsequent versions of the V12 as installed in the XJS including Marelli ignition versions, the fuel injection system incorporates a Power Resistor located on the right hand side front of the engine bay, just behind the headlight nacelle. This unit is plugged into the main harness and if it is unplugged, the engine will not start because the injectors will not open and deliver fuel to the cylinders as intended. (see attached photo of the unit). Beware that several other models of Jaguar and other UK manufactured cars, use a power resistor that looks exactly the same and has the same name, but their electrical characteristics are different. Only Lucas PR2 power resistor, Lucas number 73196B, Jaguar part number DAC 2044, is correct for the HE V12.





The writer’s car started perfectly normally and ticked over for 20 minutes while some other maintenance work was tested for correct operation. The engine was then switched off for about 20 minutes. Whereupon when the writer tried to start the car, it turned over but would not fire. Brief investigation indicated the spark plugs were completely dry, in spite of the engine having been cranked for several seconds four or five times. The bone dry plugs implying no fuel at all was entering the cylinders, this being the cause of the failure to start. The spark plugs also had bright white-coloured deposits on them, indicating ultra-lean running, implying that even when the car had been ticking over, the injectors had not been opening for the correct length of time.

As the injector loom was fairly new it was unlikely to be the cause, however the power resistor is such an essential item for injector operation, it seemed the best thing to examine next. The power resistor was unplugged from the main loom and both halves of the 10 pin multiplug connector (one on the loom and the other on the power resistor) were cleaned with a toothbrush and electrical contact cleaner from an aerosol. The power resistor is bolted to the bodywork by two small bolts that screw into captive nuts. It is easiest to clean the connector on the unit when it is unbolted, as the connector faces downwards and is hard to get at otherwise.

Once both the loom plug and the power resistor plug halves had been cleaned, the power resistor was reconnected to the loom, the ignition key turned and the engine started immediately. The power resistor was then bolted back to the bodywork.

Maintenance advice
The design of the power resistor multiplug is not as good as modern connectors. However it is by no means bad. It is just that, over the space of nearly 30 years, the plug pins and the sockets they go into can gradually develop a build up of light corrosion sufficient to prevent the very low amperage electrical current being properly conducted.

Therefore, when owners are doing their annual maintenance, such as spark plug changes, high tension lead examination for deterioration, oil changes, and so on, it is strongly recommended to unplug the loom from the power resistor and clean both halves of the connector with contact cleaner.

After having done this work, the writer’s V12 started noticeably more quickly, indeed practically instantaneously, and ticked over more evenly.

 

Well done Greg. You shouild be an Author, and that is not meant sarcasticly, that is a briliant scribe of words.

Have a drink or 3 for that effort.

 

Thank you Grant. I bask in the praise of the Great. This was put up again at the request of the Moderators so it can be made into a "How to" link.

Greg

 

Sweet.

The time zones will align eventually and it will happen.

Just keep drinking.

 

This is excellent information. Thanks Greg!

I will be doing a tune-up on my '90 this weekend and will include this maintenance of the resistor.

 

Superb Greg, thanks for taking the time to post this.

I'll make sure it's added to the "HOW TO" database and quick linked

Best

Jim

 

I removed my Power Resistor today to perform the recommended contact cleaning. I noticed that one of the pins is pushed back and may not be making good contact with the connector.




I have decided to replace this unit to ensure proper electrical connection and operation.

 

Great info Greg. Thank you.

Mate I have an electronic background (not automotive though) and was curious if you knew or anybody else knew, exactly what is the "power resistor" pack.

In electronics, we sometimes use "power resistors" in current sensing circuits. They are physcially larger in nature than typical 1/4W resistors you might see on a circuit board as these "power resistors" are so designed to disapate a fair bit of heat.

Anybody know whats going on inside that box just out of interest sake?

I dont wanna smash mine open to find out

 

It is held together by tiny nuts and socket headed screws, and can be undone easily. Inside there are no moving parts or transistors etc, just two small circuit boards with resistor tracks on them. I am told that the resistor is needed to prevent the ECU circuits and components from being overloaded with amps. It takes a surge of power to open the injectors, but it is death to the ECU circuit if this keeps flowing through them while the injectors are open. As holding the injectors open takes way less amps that opening them to start with, the resistor pack reduces the power flow through the ECU circuit/components while the injectors are open. Maybe this helps? It needs a proper electronics person to really explain it. Modern ECU components are much tougher and do not need this protection, apparently. Greg

 

Greg you are a wealth of knowledge mate. Cheers.

It actually kinda makes sense.

My workshop manual has some electrical schematics in the back. Might have a gander at them when I get home.

So I think I have asked this before quite sometime ago when I was less familiar, but where is the ECU situated in the 85 model?

I have a picture of an engine bay of an 81 XJS at home . It looks like they used to mount the power resistor pack on top of the engine. After looking at the picture I had and looked at my own engine and found what looks like mounting holes on top of the air intakes.

Was this a common mod back then?

 

Paul,

Your ECU is in the boot, up inside the RH buttross. Slide that carpet trim vertical panel out of the way, and it is attached to the REAR of the same BLACK bracket that has the 2 relays, and 1 Interface, mounted on it.

11mm nuts (2) secure the lower edge, and the top edge is a slide fit into a hooked like clamp waaaay up inside the buttross.

That resistor pack Greg mentions is bolted to the side panel IN the engine bay behind the RH headlamp.

 

The mighty Grant! G'day mate! Good to hear from you.

Yes I have spotted the resistor pack previously. Always curious as to what it was for exactly.

Yes I think I recall now you mentioned the ECU was in the back. I'll have to have a sticky beak.

Nothing wrong but just trying to learn my way around the XJS. I like the electrics as it doesn't scare me as much as the mechanical stuff lol!

 

I have been tracing down my wet plug flooding issue for a couple of months now. I replaced all sort s of electronics and fuel regulators without help. a couple of the gentlemen here in this thread have helped, (Adelaide ) and I think the crumbling insulation on the injector wires may be a culprit, in addition to the broken toggle switch on the tower. I also cracked open the power resistor in front wall, and both sides had orange crust corrosion that I brushed off, and they might have been jumping solder points. I hope taping up the injector wires with electrical wrap and seperating them a bit will do the trick... I am sick and tired of dropping the gas infused oil (all $65 worth)out of the car after wetting the plugs and cylinders. The warmer weather is getting me back into the garage and trying to get this 5.3 to run is my priority now! I wish I saw this thread earlier, and didn't invest in a CPU and other costly parts, I thought it was a vacuum problem at first but I am sure the injectors are to blame now. I don't know if it makes a difference that I haven't replaced the wide open throttle switch on the tower that snapped, or if it will keep feeding too much fuel until that is fixed as well.... any help would be appreciated

 

Wet plugs are almost certain to be an injector loom problem. The loom earths on the engine, or shorts somehow, and this holds the injectors permanently open. My strong advice is NOT to bodge the injector loom; but to unplug it from the plug by the power resistor, remove it from the V and injectors and either rebuild it or buy a new one from Jaguar. On reinstallation, tie it to the fuel rail and keep it out of the "valley of death"!

Brake Buster has a really great illustrated thread on how to rebuild it. If you have a Marelli car, also replace the loom from the amplifiers on the rad cross member to the dizzy, etc.

Greg

 

Just another post to show how useful this information is.....

On my green '89 I replaced the fuel pump and filter, built a new fuel injector wiring harness, new plugs, distributor cap and rotor and new fuel pressure regulators.

Despite all of these new parts the engine still had a slight surging at highway speeds. A cleaning of the contacts at the Power Resistor pack eliminated the surging.

Thanks again Greg!!

Mark

 

I am very pleased it helped. Also, I take a more benign view than most about the "renew everything if you have a problem" solution to maintenance. None of those things you have renewed represents wasted time or money. What you now have is a car with its systems in top class condition, and you have prevented all sorts of later problems as well as solving the most pressing one!
Greg

 

All of my Jags were purchased in neglected conditions so replacing those parts were necessary just to get the car back to a proper operating condition.

I have my eye on another XJS that has been sitting in a garage for 8 years. Not started or driven. Many parts will be replaced to get the car back in service.

Mark

 

Thanks Greg for your fine in depth info on that power box. On my 91, 5.3 I want to install "cold air induction". That power box is right in the way where I want to drill a 3" hole. My question is does anyone know where & how to move it in regard to making the harness wire's longer & are any of those wires resistor type ? I noticed the recent thread on this subject (cold air intake) but no one mentions what they did with the Power Transistor Box.
Lawrence

 

Hi...

I was just provided the link to this thread by a member as a response to a question (a BUNCH of them) that I had in another thread possibly to do with the same, a similar or a combination of issues - this being a part. (see here)

Pls, If you don't mind and are able - where can I find the "Power Resistor" on a 1990 xjs v12? Under the bonnet or in the boot - somewhere else?

Thanks in advance. Your wisdom and experience is greatly appreciated...

 

Under the bonnet, in the engine bay RHS front, behind the RHS headlight nacelle.
Greg