The fuel injectors in my wife’s 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) are not firing and the car will not start. I searched the archives and came up with a great write-up from Trevor Beckwith (some of it is copied below) that covered much of what I needed to know, however it was for an earlier car with the Lucas ignition and not the Marelli ignition. I am trying to fix this injector no fire problem in a car with a digital ignition, a crankshaft position sensor, a flywheel position sensor and two power modules (and no amplifier) and not the configuration of car he was working on. I hope someone with knowledge of the Marelli equipped V12 XJ-S can provide me with some feedback so that I can fix this. I have some idea of what the problem is (failed coax wire) but I want to be sure about this before I tear into the engine bay wiring harnesses. I have the benefit of the S57 Electrical Guide for the 90MY and the XJ-S Repair Operations Manual, but there are some things that I need help to understand about the wiring in the engine bay for the speed sensor and how that signal gets to the EFI ECU to get the fuel injectors to fire.
First the good news. When I turn on the ignition switch the fuel pump runs for a few seconds and I get a good 39 PSI on the fuel pressure gauge that I have located on the return line on the left side of the fuel rail in the engine bay. Fuel pressure holds pretty steady with power off for an hour or two then starts to drop slowly. The fuel filter is new and the 91 Octane fuel is fresh. The starter cranks the engine and fuel pressure is renewed each time the starter is cranked. The Inertial Switch is verified in the down (closed) position. I have 12VDC to the fuel injectors with the ignition switch on. When I have an assistant crank the engine when I have my inductive timing light on the spark plug or distributor to coil wires I see the timing light flashing while the engine cranks so I am pretty sure that the Crankshaft Position Sensor is working properly as well as the wires from the distributor to the upper and lower coils and all of the spark plug wires that I have tested (6 or 8 of them). I have a spare crankshaft position sensor that I have moved from the crankshaft position to the flywheel position sensor location (they are the same part number) and so I believe they are both working properly because I always get the timing light flashes when the engine is cranked with the key in the ignition. I don’t have access to an oscilloscope so I have not verified the Crankshaft Position Sensor waveforms but it appears that both the Crankshaft Position Sensor and the Flywheel Sensor are both working properly from the timing light flashes.
Now the bad news. When an assistant cranks the engine I do not hear any clicking at the fuel injectors with my mechanics stethoscope like I do when my assistant cranks the engine on my 1990 V12 VdP (5.3L V12 w/ Lucas ignition) with the fuel pump disabled. However, with the ignition switch ON I have 12VDC to the injectors. So the signal to ground the injectors appears to be missing. I traced the wires one by one from the Marelli ECU (DAC5871) connector pins to the engine bay sensors and also from the Lucas 16 CU Control Unit (DAC6337) and came up with a two surprises. First I must mention that there is a large black plastic connector in the engine bay (see attached picture with connector circled in read) that I believe carries both the White Speed Signal wire in addition to the White Slate wire for the Tachometer. This black plastic connector isn’t mentioned in the write-ups for injectors not firing in Lucas ignition equipped cars, so it may be unique to the Marelli cars (as are the Crankshaft Position Sensor, Flywheel Position Sensor and 2 Power Modules). While testing the female pins on this black plastic connector I found that there was no continuity between the white wire and pin 24 of the Marelli ECU when I thought there should have been. However I got continuity between that White female pin and Marelli ECU pins 4 (Ground Black), 10 (Power Module B Green Pink), 12 (Ground Black), and 15 (Power Module A Slate Purple). That was when I had an “Ahaaaaaaaaaaa” moment. Why would there be continuity between White Speed Signal wire and any of the Ground wires?
I would like some feedback that these readings likely show that I have a broken White Speed Signal wire and a short between the Speed Signal wire and the coax shield that is resulting in the loss of Speed Signal and a short of that signal to Ground. Because of the lack of continuity between the white wire in that connector and the Marelli ECU I am a little confused over the purpose of that large black plastic connector which carries the tachometer signal from the coil to the tachometer, but I would like to know for certain that it is supposed to carry the Speed Signal to the EFI ECU. I checked the other wire on that connector and it goes to the tachometer. Still, it seems like the White Speed Signal wire should not have continuity to Ground. I hope that I am on the right track and that someone will give me feedback that it is time to open up that wiring harness (it’s at the firewall and will likely be a challenge to find what I believe is a damaged wire).
I plan to get some coax wire to attempt to locate and repair/replace the suspected broken coax wire, but it would be good to know if my approach is on the right track before I do that or if I missed anything
.
I would also appreciate any suggestions for additional tests that I should try to sort this problem out and get this car back on the road.
This is my first attempt at a serious digital ignition/EFI problem and I hope I am on the right track. Your comments and suggestions are welcome.
Regards,
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA
-----Original Message-----
From: owner-xj-s@jag-lovers.org [mailto:owner-xj-s@jag-lovers.org] On Behalf Of Trevor Beckwith
Sent: Friday, September 02, 2016 2:09 AM
To: xj-s@jag-lovers.org
Subject: RE: [xj-s] Not Firing
You turn the key and the fuel pump engages (should be able to hear it) to pressure the fuel rail but it can only happen if the cut out inertia switch (Right hand pillar on LHD and RHD) is closed.
Then the motor cranks and the pick-up coil in the bottom of the dizzy turns and sends a pulse signal to the amplifier (black box on the left intake manifold) via two shielded wires on my 82. The amplifier then sends this pulse to two places…
a). the coil which then sends it to the dizzy and from there to the spark plugs.
b). it also sends this same pulse via the legendary coaxial cable to the ECU in the boot to tell it to pulse the injectors.
If you have spark but no fuel then the amplifier is getting the signal so the wires from the dizzy and the pick-up coil are good.
If the fuel rail is getting adequate fuel at an adequate pressure then the problem will lie between the amplifier and the ECU or the ECU and the injectors.
My check list would be.
- Fuel filter, has it been replaced in recent times.
- Fuel sump filter, needs to be cleaned every couple of years.
- Fuel pump, could be that it’s not producing enough pressure. Also check fuel pump relay in boot.
- Injectors firing (you will need a noid light to test). Not expensive and if you are working on fuel injected cars it pays to have a set. You can hear the injectors fire but this does not check the four banks of injectors.
- Coaxial cable. They are frail and brittle and the sheath (due to heat) is often is found to be shorted with the main cable following work under the bonnet. Replace the section under the bonnet (up to the main loom) as a matter of course for it will break before long any way.
Don’t get drawn into paying the big bucks for the cable. I used a data coax (not a big fat TV coax) that contained two sheathed cables and costs peanuts. If you need more details let me know. - Power resistor to the injectors. On the inside of the right front fender to the front. Oil and gunk gets into the connector. Worth a clean anyway.
- Fuel regulator. Only the outgoing one is essential for fuel rail pressure. (the incoming one is really only a check valve)
- ECU. You want to exclude all else before going for this one as they are pretty expensive. Some wreckers will lend you one to test (here down under) if they have one.
The list can go on but those are the main culprits.
To save yourself time jump right to # 5. The coaxial cable to begin with. Unplug the cable from the amplifier and do a continuity test across the sheath and the central cable. This will show you if they are shorted. If so replace the cable. If not then place a jumper wire between the two then go to the boot, unplug the ECU and test for continuity between Pin #18 (for ECUs 6CU and 16CU) on the connector on the cable and earth. If you get continuity then the cables are ok but if not, one or both are broken. Replace the coax under the bonnet.
Cheers Trev
