89 XJS, marelli car. Just purchased, engine partially taken apart by previous owner:
car cranks, won't start, have spark. Can start with starting fluid into intake.
-new cap, rotor (potted, dizzy shaft shaved down), wires and plugs, new front crank sensor, rear tested at ~670 ohms, have spark
-checked water temp and air temp sensor, both within spec on ohm meter
-new fuel pump, filter, gas tank and surge cleaned, fuel injectors cleaned and tested, new wiring on fuel injectors, have gas at rail, haven't pressure tested but spurts quick with a slight disconnect of hose rail fitting
-new battery

-plugs are dry, have checked grounds I see, swapped Lucas trunk computer from a car that ran,

problem seems to be fuel injectors not firing so can I get the experts to lay a diagnostic path for me? (considering previous owner [or present owner] did something stupid)

Thanks!!

P.S. Wait til i tell you my plan for this car...

 

There are 2 crank position sensors on your vehicle, you changed the front one I assume? Your car won't start if the one that's closer to the firewall (drivers side) isn't working. I forgot the exact location.

 

the flywheel sensor is under the engine on the left hand side bolted to the bell housing of the gearbox, there should be a small heat shield over it to to protect from the exhaust

if this is not plugged in then the car will not start

but

there is also the water temp sensor on the engine water rail, again , if not plugged in the ecu will not fire the injectors

injectors get their power from the trunk , next to the fuel pump relay , the ' main relay ' must have a double outlet pin for the power ( pin 87 ? ) the injectors have a permanent power supply , and the ground is switched through the power resistor behind the headlights

i would check the ' main relay ' and that there is power to the injectors

hope this helps

BB

 

The injectors are controlled by the Lucas ECU. If the car has fire, the Marelli isn't suspect in my book. I am very comfortable with diagnosing the Marelli if you need to discuss. The power amplifier for the injectors may be suspect. There is a thread that goes into great detail about refreshing one very (maybe free) inexpensively. Feel free to send me a message for my cell number if you need help (encouragement), lol.

 

Thanks for the replies!

So, I've done a little more research and I'm leaning towards the main relay and fuel pump relay.

I've ordered a couple of spare relays and am waiting on them to arrive. In the meantime, I have these relays which I believe are the main relay on left and fuel pump relay on right-correct?




1. The fuel pump is pumping fuel to rail and pressure seems high, (haven't measured but it spurts strongly if a fuel line is cracked) so does that mean that the fuel pump relay is working?
2. The plugs are dry, injectors do not appear to be injecting (been tested as good) so is a possible cause the main relay is bad?
3. What else could cause injectors to not fire?

(disclaimer, sorry for any redundant questions, between the complexity of this car and the fact that it was partially disassembled when I bought it the troubleshooting has me dizzy)

 

yup

the main really is the one with the red stripe on the top ( whats left of it ), this will have two output pins, and these relays are known to fail , any ' matching ' relay will work ,but,

when replaced the owner does not necessarily recognise that the two output pins are there and put in something similar, and then this won't send power to the injectors , and then the injector relay won't run either

hope its as simple as this

BB

 

btw,

if memory serves, there are three temp sensors

one for the instrument dial, one for the warning light, and one for the ECU , only the one for the ECU NEEDS to be functioning for the engine to fire , it won't fire if its fubar

BB

 

New info!

So, read through suggestions:
I have 12 volts at injectors with ignition on, so if I understand correctly then power resistor and fuel pump relay and main relay are working-correct?

This seems to leave the marelli ECU, temp sensor and crank sensors as options left for injectors not firing-correct?

 

I wouldn't suspect the Marelli ecu if injectors aren't firing. The Marelli ecu only gives the Lucas ecu engine speed so it can fire injectors. If the car is firing, both coils!!, the the Marelli most likely isn't suspect. I installed a inline T on my return fuel rail so I could check fuel pressure. Needs 30psi

 

almost

the good news is you have power at the injectors, this then goes to the power resistor behind the headlight, the connections on these are known to corrode and cause issues , a good clean will get them working again , then from there it goes to the ECU

tricky , but one step at a time will find the fault

BB

 

I have voltage at the power resister but haven't checked back at fuel injection ECU.

But I did read in another thread that the flywheel sensor gives a "tach signal" to the ECU that allows the injectors to fire (can't recall if the tach signal went to Marelli ECU or fuel injection ECU).

So I've tested the plug in the engine bay of the flywheel sensor (tested at about 680 ohms).

Seems like next step is get under the car to see if its connected to bell housing and measure the distance to flywheel and then check wiring in engine bay harness back towards ECU.

I need to quit my job so that i have time to troubleshoot my XJS.

 

New info! Same problem....

(for refresher, front crank sensor is new and ohms 655)

I replaced rear flywheel sensor with different used sensor, new/used sensor ohms at 680, filed mounting boss so that sensor is .020 away from flywheel and still no start.

Injectors aren't firing.

What am I missing?

 

The tach signal for a Marelli car is tied to the A side coil. Nothing to do with the Lucas ECU. The rear sensor gives the Marelli ECU engine speed. The front one does timing. Again, if your injectors aren't firing, I really doubt it has anything to do with the Marelli ECU. The power resistor reduces voltage to the injectors. The injectors only need about 1.5 volts to open. For whatever reason, it still sounds to me the power resistor is at fault here.

On the car I have, i pulled both coil wires, placed them near one of the metal tubes connecting the manifolds, and spun the engine over. It would start firing from coil wire to the metal line for two to three seconds, then B bank would drop out. I chased it for days on end.

If your coil wires will fire to ground consistantly, look at the power resistor. I still bet the problem is there, not the Marelli, or the Lucas ECUs.

 

GOT IT!!!!!

The shielded tach wire in the wiring harness in the "V" was "repaired" by PO by cutting and splicing and soldering...the shielding of the wire.

So, no tach signal to ECU.

Jesus what a hassle, you can't make assumptions when it comes to cars like this and previous people working on them.

 

What a great feeling! These cars are really overly complicated looking, yet, persistence usually wins out in the end.

 

well done for sticking with it and getting the right result

as you say , you can't allow for the PO syndrome

and at least you know everything else in the system is perfect , lol

on to the next issue

BB