No Start XJS

Does the engine turn over but not fire? Or is the thing just dead? If the former, check the ignition amplifier, these were known to go bad, especially if they get too hot.

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Steve:
Does the engine turn over but not fire? Or is the thing just dead? If the former, check the ignition amplifier, these were known to go bad, especially if they get too hot.<hr></blockquote>

The engine is turning over nicely with no start heppening. What is the ignition amplifier? Where would it be?

The ignition amplifier looks very much like one from a stereo, an unpainted aluminium box shape with cooling fins mounted on the top. These were always prone to breaking down if they got too hot, the only recourse being to replace the unit, no repair is possible. On early V-12s the amplifier was mounted right in the middle of the 'V' created by the two banks of cylinders, and on the later cars, of which the '89 should be, it should be placed on the top of the panel going across the very front of the engine bay, which would be the bonnet shut panel if the Jag bonnets opened normally! This applies to the saloons, although I don't see why the KJ-S should be any different.

A very good source of information is fellow forum member Diggs, who is a master Jaguar tech. He should be able to give you an idea if the amplifier should check out okay.

I think I know what those are. We saw wires coming off of the ignition coil to two packs located on top of the radiator. We scratched our head. They don't have cooling fins that I remember, but they were two silver boxes. How do we test them out?

I forget how a guy once tested one out for me ( twenty years ago), but I'm sure that your instructor should know, failing that a friendly Jaguar dealer might. After all he wants to sell you a new one. Keep in mind there was only one amplifier, about three inches by four, unpainted. It just unplugs from a loom. I was merely an enthusiatic owner operating a dream car on a budget, so my technical "nous" was low. An MGB I can handle!

Your idea is appreciated. I am sure a closer look at the wiring diagram may have some clues. YEs an MGB is a much better car for shadetree mechanics. However, this is a biggie for me because I want to work mainly on European cars from the 70s and 80s. Jaguar and SAAB being two that make most mechanics turn tail and run. After a close look under the hood of the XJS I can see why!

The 12 cylinder is simple to diagnose a no start condition. First you have to isolate the problem. Put a spark detector (like a timing light) on one of the spark plug leads, put a fuel pressure gage on a junction of the fuel rail like the regulator and also put a detector on one of the fuel injector electrical input plugs. The injector signal detectors are commonly available for Bosch fuel injectors at NAPA stores. You also have to have a fuel pressure regulator that registers at least 70 PSI.
Try to start the car with these items installed. You must see about 40 psi on the fue lpressure rail.
You must see the timing light registering spark (the spark should be correctly synchronized with the firing order).
The fuel injector signal should be blinking at a rate of two blinks per revolution of the engine speed.
This will tell you what area to look for the problem. If all of the above conditions are met, you have an interesting problem. If one of the conditions is not met, tell me which one and I will give you further advice.
Best regards, Jack Farrington

Ok I am in Engine Performance class, and an instructor has left his 1989 XJS V-12 for us to look at. It has a no start condition. We looked at the crankshaft position sensor and the TDC sensor and both are supllying voltage to the computer. The TDC sensor is creating a sine wave which the teacher thinks may be wrongs, and it should look more sawtooth. We also checked that both of the coils have power to them. This one has the later design ignition with the tach running off the lower coil. We have only begun, but the instructor has little experience with Jaguars so we all feel a little over out heads. Where to look? Also, we noticed some animal droppings and acorns in the crevices under the fuel rail. Some wires may be chewed but it is so hard to see, everything surface level looks good. Any clues would be appreciated.

We have ignition at the coils. Only when the instructor grounds the computer at its casing. (?) We did not hook up a fuel pressure indicator, but there is a signaling problem from the computer to the power modules. We think the computer needs replacing. Its a $1200 call.
This one has the two unit later design power coils, not the single one you had replaced on your XJS.

One thing I have learned in getting a dead car back to life is to check all possibilities and to make no major assumptions without demonstrating that they are correct. Make the effort to step through all the tests that I suggested, Let me know what the results are. We can go from there. Best regards, Jack Farrington

If you have two coils, you have the later Marelli (spelling) ignition. What do you mean when you say power module?

The "power modules" are the power boosters that are located on the radiator ahead of the engine. This one has the two module set up with 6 volts going to each. 1989 models had two different arrangements, and this is the later one, the earlier one having a single power module on the radiator. We have a signal to the modules from the computer, and spark at the ignition wires to the distributer. Last term They were able to get the engine to sputter and turn over by grounding the casing of the computer. This is how we saw spark at the noid light.

With the Marelli ignition the amplifiers are indeed the located on top of the radiator. I had a very similar intermitent problem. In the end, mine was corosion from moisture that got into the conector for the front crankshaft sensor located near or under the air conditioning compressor. Don't rule that out as a cause.

Also check the connection and condition of the magnetic pick up that reads the teeth on the flywheel. Both sensors have to work correctly.

Good Luck!!

The power modules that you describe are the ignition amplifiers for the solid state ignition. Marelli uses two of them, one for each cylinder bank. If these amplifiers are not well grounded on their case, they will not work and the ignition will be dead. Check how these amps are grounded. Hope this is the problem,
Jack Farrington

Greetings from the UK. Every Pre Ford Jag has problems with earth leads! make sure all ignition components have a good earth especialy the amplifier units. Ensure that you have a good injection pulse at the injectors. Snap-On and Blue point do a small test light that plugs into the harness at the injector. When doing this make sure it lights up quite bright if the system is not working it still may produce a small pulse! The crank shaft sensor, once a gain even if this is not working properly it will still give an output. when turning the engine over does the rev counter move? if so is the movement iratic if this is the case the crank shaft sensor is fualty. next is the second coil located infront of the radiator definatly working and connected the correct way round? due to its location it is prone to filling with water and or the connections corroding. also it is quite easy to connect it up the wrong way round if it has ever been removed as the wires are not clearly labeled. Finally this may be a silly question but is the roter arm and dizzy cap in good order and are the HT in the correct order it is an easy mistake to make with so many cylinders and so little room! Hope this has been helpfull!!!!!

Chris, You are getting good feedback. One more thing to keep in mind. The marrelli ignition system uses two posistion sensors that must feed the ECU for the ignition spark function to be correct. There is one on the bottom front of the engine That measures engine RPM. There is a second sensor called a crankshaft posistion sensor that also inputs the ECU. It is located on the bottom rear of the engine at the flywheel Both of these sensors hae their harnesses connectors over their position on the engine but on top. By ;the way both of these sensors are identical and interchangeable but both must provide signals to the ECU to have a functioning ignition and fuel injection system. Check them out. Best regards, Jack Farrington

So you guys don't think it is the PCM at all. We found a bad diode when we took it apart and began to check the circuits. Like I said we have no signal going from the PCM to the ignition unless we manually ground the PCM and try and act like the computer signalling the system. THey were able to get the car to start this way.

There is no movement at the Tachometer when the car is attempted to start. I appreciate all of your help. My classmates have been less willing to follow your specific diagnostic procedures even though I think we should go back and check everything. What we need now is a breakout box or PCM from a working XJS to see if we are indeed correct.

Does anyone have a break out box or service manual so we can check to make sure the output signals from the PCM are correct. Up the creek.....

Awright! What's going on with this XJS? Did it ever start? What was the fix?

This is like missing the last half hour of a mystery and not knowing whodunit.

Thanks for asking.

The news on the car is that we have had to push it in and out of the shop every day because it still hasn't started. You ever have to push 4000 lbs of XJS in the freezing cold up a small hill with ice on the ground? Thats one heavy kitten to steer with no power assist. Most of the students have given up on it completely, leaving me to take the lead.
We spoke to two area Jag specialists who were as confused as us. Finally, the owner, a retired ATC instructor, has found someone in DE who can test the two computer units to see if that is indeed our probalem. There are only eight days left of class, so my hope of seeing it start while I am there is slipping away. I am sure the instructor can invite me back when they reinstall the units. I planned on writing next when she fired up, but kitty kat still hibernates.