How do I test a crank sensor for my Jaguar xjs 4 litre.
It will not start, spins over. It did run briefly.
Thank you

 

Well this is the XJ40 forum -but- as the XJS 4 litre has the same motor as the 4 liter XJ40's, you can get an idea if it's working buy cranking the engine and looking at the rev counter.

If it doesn't register any RPM when cranking, the crank sensor is suspect

Larry

 

Hello,
Perhap’s This tech Doc may help you ?

Bestregards

 

Hi Jerey,

Welcome to the Jaguar Forums! It's great to have you with us.

I have moved your thread from the forum for the XJ40 to the forum for the XJS. Here you will find knowledgeable owners of similar cars.

Larry and frenchfairplaydriver have already given you some excellent information. The classic test Larry describes of watching the tachometer/rev counter while cranking will give a good clue as to the health of your crankshaft position sensor (CPS). Just to expand on his instructions: if the tachometer reads approximately 200 rpm while cranking the engine, the CPS may be working properly, at least intermittently. If the tachometer reads 0 rpm while cranking, then either the CPS or its electrical circuit has a problem.

You don't mention the year of your XJS, but you can download the appropriate Electrical Guide at this link:

JagRepair.com - Jaguar Repair Information Resource

Besides the CPS, there are plenty of other possible causes of of a cranks-but-won't-start condition. The fuel pump, fuel pump relay and fuel pump electrical circuit are key suspects. Turn the key to position II (ON, but do not start the engine), and listen for a short priming burst from the fuel pump. If that is inconclusive, loosen the fuel line at the firewall/bulkhead end of the fuel rail, aim the fuel hose into a suitable container, and crank the engine briefly while observing the fuel flow from the line.

Other suspects are the ignition coil, distributor cap and rotor, and other associated ignition components. You can remove a spark plug, reconnect it to its wire, and lay it on the cylinder head cover so the threaded sleeve is grounded. Crank the engine and watch for spark.

Please let us know the year of your Jag (it's helpful if you add the year, model and engine details to your signature line so others don't have to ask).

Also, please visit the New Member Area - Intro a MUST and post a required introduction so we can learn something about you and your Jaguar.

Cheers,

Don

 

Confirm spark and fuel and move on from there. Not any particular reason to jump to the crank sensor if its not starting but you haven't gathered any info about the fuel and spark.

 

I’m all for diagnosing problems... but it’s ALWAYS the CPS. Always! Conventional troubleshooting diagrams just go out the window. Step one, replace the CPS. Step two, anything else.

My 95 4.0 XJS has eaten no less than five in the last 100k miles. I keep a spare in the glovebox.

If you buy a CPS, install it, and the car won’t start... put the old one back on and leave the new one in your glovebox. You’ll need it eventually.

 

I can't imagine the car not starting, so you go through all the trouble of replacing the CPS only to find out you have a bad fuel relay. If you try to skip diagnostic steps, you are in for a bad time.

 

I would no way disconnect the fuel hose and crank the engine, what if it did start and catch fire.
I checked my fuel pump by aiming hose into container and turn ignition on and it did squirt gas but (do not crank) the engine.

 

You'll only get a short priming burst from the fuel pump when you turn the key to position II (ON), which is not usually sufficient to judge fuel flow. Obviously, you should follow good safety practice and keep the fuel hose and catch container away from sources of potential sparks. But even if the engine starts while the fuel hose is disconnected, it is not going to run for long with no pressure to the injectors. I've cranked engines countless times with the inlet fuel hose disconnected. In roadside emergencies, I've even done it without a catch container and simply aimed the fuel onto the ground (avoiding any hot exhaust components). This is often the fastest method for confirming the fuel pump is not only running but is delivering sufficient fuel flow. An alternative to cranking the engine is to jump terminals 30 and 87 in the fuel pump relay socket.

Cheers,

Don

 

If you pull the middle wire out of the Distributor then the Engine cannot start as you Crank it over

With a 4.0L XJS it is highly likely that you have an 'in tank fuel pump' and if that should need replacing, then the Fuel Tank would need to come out

Or at least be pulled back far enough to be able to get to the Fuel Pump that's in the Top of the Tank, which is a PIA Job if ever there was one

So in the event that you're not getting Fuel, then the First thing to check is the Relay, which on a (UK) Car is accessible from inside the Boot/Trunk

Behind a Piece of Trim up by the Bulk Head, where it is tucked up out of sight, in the space under the Rear Wing/Fender (inside the car)

 

To fix any problem you need to work methodically through each system and verify it is working. An engine only needs Air, Fuel and Spark at the correct time to work.

Verify you have spark by removing the dissy input and holding it close to the engine block or other good ground. If you have spark then the CPS is most likely working as the ECU uses the CPS to time the spark.

Fuel can be verified by either a pressure gauge or cracking the fuel line at the input to the rail.

Also check the battery voltage if it is low this could prevent starting .

 

Hi!
Be sure to check the fuel tank inertia shut off switch.
My 1994 has the switch in the passenger (RH) door jam.
Also check the fuse for the ignition coil and the fuel pump.

I had a strange problem where the ignition coil fuse almost opened up.
It was acting like a spark gap/resistor. The car would have a spark and then nothing.
Use an Digital Volt Ohm(DVM) meter to check those fuses. The good old
Mark One eyeball doesn't work very well on checking fuses.

Regards,
Bob

 

Just had this turning over / not starting issue problem with my 96 XJS - replaced the fuel pump relay and all is fine. Cheap, quick and easy place to start and it is a well known fix.

 

Its difficult to test the crank sensor. I would only recommend fitting a genuine one. And crank sensors can break up so sometimes work and not. We have had them where it feels like the car has a rev limiter and it has turned out to be the CPS.

Best to just try a good used and if its not then you have a spare.

Good luck.