my xjs 1991 worked fine for about a week after i replaced all ignition wires, distributor cap, rotor and both crank shaft sensors. Then I went for a short drive and it wouldn't start back up, quickly dying when starting. Now it just turns over and doesn't start. 1) Checked spark on spark plugs - nothing 2) checked - spark from both ignition coils - nothing 3) checked if there is 12v on the coil when the ignition is on - yes 4) checked 12v remains on the coil when i crank the engine. Recently replaced upper coil to the new one.

Any ideas what could be the problem?

 

Okay, sounds like it doesn't trigger a spart (in the Marelli ignition the ECU will trigger sparks according to the mapping and that will advance/retard like that). So check both sensors. They are readily available and don't really cost that much. The front and rear sensors are the same.

Iirc the rear one is used for the ignition timing when running and the front for TDC triggering.

As they are magnetic impulse sensors, they do lose their magnetic charge (is it classed as charge? When they get less mag etic).

They are only screwed in their brackets. The rest of the way is clipped.

 

Hi Daim

Are you talking about the Crank Sensors, at the Front and Rear of the Engine?

If so any chance of a Pic while the engine is out of your Car, so I know where to find them.

 

Yep. I can do a pic or two. Give me a couple of days though please

 

Front sensor is ignition timing and rear sensor is engine speed. Check the connectors

Front sensor is mounted to the timing cover and the rear is in the aluminium cover over the flywheel.

 

Thanks for the correction Warren, though the rear is also, according to the workshop manual, used to determine a signal for the ignition for starting. So it was the rear for initial TDC and the front after that just for impulse...

 

There is no index on the rear sensor as it uses the flywheel ringgear so this can not tell Marreli ECU where the engine is in its cycle, the front TDC sensor does this for ignition timing only.

Rear sensor tells the Marelli ECU that the engine is turning this is relayed to the Lucas ECU and below a certain RPM usually 300rpm injection is in startup mode (additional fuel is injected), when rpm is over this the ECU switches to running mode and reduces the fuel injected.

 

Hi Daim

Cheers!

A Pic would be very handy to locate the one at the rear

 

Hi Daim,

I checked both crankshaft sensors (front and rear) although they are brand new and were replaced couple of weeks ago, it seems that the impulse from them is very low, much less then 1 volt (see video attached). Multimeter was set to minimum 200m. Assume this could be the problem. But another question what could happen to the new sensors?

 

Out put from a VR sensor is usually a dirty sine wave and a DVM may not measure this very accurately, also looks like you have the DVM on DC.

Try switching the DVM to AC volts. I doubt the problem will be the VR sensor these are pretty reliable and the only ones I have seen fail usually have physical damaged to the tip.

I would be looking at the connectors, wiring and connector on the Marelli ECU.

 

Hi Warrjon!

Thank you! you were right. ECU connectors were loose but now everything works fine