1991 XJ6 Vanden Plas NO START


I am new to Jag and am very lost on this car. car is a no start. it cranks over but has no fuel pressure, no injector pulse, and no spark. the car is a XJ6 Vanden Plas that was built 7/1990 LHD. From what i can tell this is really more of a xj40 then an xj6? i really appriace any input anyone may have. What i have checked.

  1. fuel pump relay has power and when jumping 87-30 terminals i do get fuel pressure. on this im pretty sure that the ecm is not controlling the replay to turn on the fuel pump. this is the relay located on the left front near the fire wall. im not sure what the ecm is looking for to control this.

  2. injector pulse checked using a noid light. has power to injectors. with this being said i believe it to not be receiving the ground signal from the ecm to fire the injectors. not sure where the ecm receives the information to fire these cam sensor input?

  3. ohmed out the ckp senor i don’t remember what the value was but from what i can find it was in spec.

  4. Checked power to the ecm from the main relay and the power is present there.

  5. Checked power to the ignition amplifier and i do have power there.

Maybe someone has something i have not checked, or has an ideal on these cars that would take down all 3 systems.

Thank you!

These point to a dysfunctional crankshaft position sensor (CPS). Resistance being ‘in spec’ does not indicate a working CPS. I would try a known-working CPS first.

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Check if the rev counter registers <>200rpm when cranking, If not then this is a good sign the CPS is not working.

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The rev counter does indeed not move. My question to this though is if it has a failed cps would it also not prime the fuel pump for its .5 burst?

M though on this is yes it possibly could if it is taking the whole sensor side of the ecm to ground (earth). But with that being said if i unplug the cps i should regain the fuel pump prime correct?

I may maybe over thinking this i’m just trying to make sure I have a clear understanding how this system works.

Thank you

If you pull one end of the coil wire out of the distributor cap and hold it close to a grounded bolt on the engine while your assistant turns the car over, do you get spark?

No, if the ECM is not seeing the correct signal (waveform) from the crankshaft position sensor (CPS) it will not cause the fuel pump to run.

No, think of the CPS not as an inhibitor, but as an enabler. It is not that an invalid signal from the CPS will inhibit fuel and spark, it is more so that the CPS must enable the ECM to provide for fuel and spark by outputting a valid signal. (The CPS signal is how the ECM knows the engine is rotating, and at what speed.) So disconnecting the CPS will not remove an inhibitor, it - well, it really would have no effect in this case. As far as the ECM is concerned no signal is the same as a bad signal whilst attempting to fire the engine…

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Missed this sorry,

I have made progress, I replaced the cps and now it is illuminating my noid light. however it is illuminated all the time with the key on and cranking . I believe I read somewhere on here that the cps on these have a correct air gap so I am going to check and see if I can find those readings again.

Ok so are you saying that your tach is now registering movement but still no start?
Have you tried starting the car with your foot to the floor on the accelerator? Just in case the engine is now in a flooded state, this cuts off the injectors and can clear the cylinders of fuel. Just a thought.

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I fell like I’m going circles here lol. doesn’t look to me as if the tach is moving even with the brand new cps. so then i decided i would go back and check my injector pulse with the noid light with the cps unplugged. This time the noid light is illuminated with no pulse during prior testing I had no noid light illumination at all.

So things I’m sure of,

  1. I have fuel pressure
  2. I do not have injector pulse

Do you happen to know what 2 wires at the cps to scope to look for signal? Maybe a bad new sensor?

You might be able to test the CPS by removing it and connecting the two connectors to a DVM set to AC and strobe a piece of steel across the face of the CPS to see if the DVM registers any signal?
An oscilloscope would be a better piece on equipment to use but not every one has one.
I might be wrong in my assumption, maybe someone with more electrical skills will confirm or correct my thoughts.

StevenD57 Sorry for the slow reply, no spark out of the coil. and noid light lights up but no pulsation.

There is NO WAY to set the air gap in the crank position sensor. It installs into the bracket on the engine ONE WAY and ONLY one way. The only way you could have a problem with the air gap is if your CPS bracket was somehow bent or displaced or the teeth on the crank damper were extremely F’ed up.

Are you certain your coil is good? As Robin said you can test the CPS with a volt meter set on AC volts (lowest possible setting) and turning the engine over should generate a small AV voltage signal from the CPS.

I Will check the cps signal in the morning. I am not sure yest if the coil is good. i will report back.

I believe the XJ40 CPS is a three wire unit so only two of the wires are the ones that will carry the AC volt signal. I think the third wire is the shield ground for the wire. You should be able to see this on the engine management wiring diagram.

Try changing the rotor too

Good thought on the rotor and also check that it isn’t shorting out though the dissy shaft.

If it was the rotor, then the test I suggested of checking the spark through the center coil wire would have still given a spark to ground. I know this for a fact because of recently experiencing a rotor failure myself and did this exact test.

Did you get a CPS with the same plug as the one you replaced or did you splice the new component’s wiring into your harness?

I got a cps with the same connector

Ok, just a WA guess here, (maybe someone with electrical experience can confirm) …I believe there is a relay (ignition on?) that is involved in these no-start conditions?