Been a visitor to the sight and can't quite solve a no start with a 1989 xjs v-12.
I have a nice 1990 xjs v-12 runs beautifully, good for some comparisons.
The 89 really want's to start but won't give me the satisfaction, lol.
The 89 is showing 12.4 +/- volts on 4 of the 8 pins at the injector loom connection, the voltage goes away when I pull the fuel pump relay. The
voltage remains when I pull the connector to the ecm.
The 90 shows no voltage on all 8 pins when the key is in the run position.

seems like I should have a relay of some sort stuck with this symptom, just can't seem to find it.

any ideas?

Many thanks, Steve.

 

Seems to me you should only have juice going through the injection harness (if engine is NOT running) when you first turn the key, and only for a few seconds as it primes for starting. Constant voltage tells me either the ECU is faulty or there is a short. Are both cars Marelli ignitions? If they are the same, you can disconnect the batteries (VERY IMPORTANT) and switch the ECU's to see if that is the problem.

 

Appreciate the response.
I'm thinking like you that I must have a short that is feeding the loom.

I've pulled the ecu connector off and still have the voltage reading on 4 of the 8 pins, with the key in " run" position well after the fuel pump as primed the system.

The only way to kill the voltage was by removing the fuel pump relay. I have a new relay in place, stll have the voltage.

I thought rotation of the engine might move the voltage to other pins, it does not, in all rotations is is the same pins showing voltage.

Steve.

 

that definitely sounds like a short in the harness. I would suggest unwrapping it and follow the wires back to where it enters the fender.

 

By "+/-" do you mean "approximate" or "positive/negative"?



Which four pins?

Is this when the connector is plugged in, or unplugged?

I'm not sure if I understand the problem

To clarify, you're taking about the block-style connector on the actual fuel injector harness, right?

If so, the four pink/black wires are 12v supply to the injectors...and should show 12v "+" wth the key on.

The remaining wires are injector grounds from the ECU.





Odd, as the same relay that provides 12v "+" to the injectors also powers up the pump relay.




Supply "+" voltage on the pink/black wires? I dont think unlugging the ECU would change that


To me this sounds very odd but obviously I'm misunderstanding something!


Cheers
DD

 

Doug, shouldn't the 4 powered wires cease voltage a few seconds after the key is turned on? Or do the injectors use an intermittent ground to cycle them?

 

Not to the best of my knowledge, no.

At minimum, if the fuel injector harness is unplugged the pink/black voltage supply wires should show voltage whenever the key is "on", for as long as it is on.

Some more study of the diagrams might be in order but I'm not aware of any ECU control over *supply voltage* to the injectors.



Yes, intermittant ground to cycle them.

My understanding is that the injectors always have 12v "+" supply voltage (from the main relay).

The ECU provides the ground to actually "fire" them.

But let's wait. Either my understanding of the description of the problem at hand, or my understanding of how the circuit operates, might need clarification or correction.

Cheers
DD

 

Really appreciate you fellows looking at this thing.

1. Approximate, me and the meter, 12.4/12.5 volts
2. Yes, the Block Style connector that feeds the injector loom.
3. Connector unplugged
4. Yes, they both ar Marelli ignitions.

with your last explanation it sounds like that connection is o.k.

I'll re-check the one on the 90 xjs, had no voltage on any of the 8 pins
with the key in the run position.

earlier before I put the injector loom back in place, I did have 12.5 volts on all connectors with the key in the run position.

I may be barking up the wrong tree, I've replaced the CPS, have fire, have fuel.

Wanted to head toward the AAV, will this cause a no start?

Many thanks, Steve.

 

Hmmm

I'll have to do some mulling

Hopefully others will chime in sooner


Cheers
DD

 

Still at a dead end,anyone know the position of the fuel pump relay on an 89 xjs. Can only find the main relay by the ecu,any ideas anyone.

 

I think you're right, Doug, that the injectors get constant 12v and the ECU grounds them to make them fire.

The fuel pump relay is right next to the main ECU relay on my '85. Its the relay closest to the front of the car (right side of the trunk.... Err, "boot"). They moved it for later cars?

 

If it isn't up there by the ECU I don't know what to tell ya. All my 1989 diagrams say that's where it should be, and that's where it was on my '88 V12.

Hopefully someone will chime in with a good answer

Cheers
DD