1991 Classic Collection- won't start

XJ-S
1

Car suddenly won’t start, I checked the distributor cap center plug and there was no spark.
Any suggestion where to start trouble shooting?
Thank you,

2

Marelli cars suddenly won’t start if the CPS is dead. The sensor itself is probably good (needs cleaning), check the connector next to the t-stat housing on the B-bank.

3

Mai,
I suspect the Crankshaft Position Sensor (CPS), it’s wiring, and it’s connector are good places to start looking for problems. You should also check the Jag-Lovers archives for “crankshaft position sensor” for posts from others who had CPS engine no start problems and what they did to fix it. The CPS part number is DBC12507, they are relatively inexpensive and easy to source. This is a common failure.

Paul

4

Has the car been sitting outdoors for a long time? Condensation can build up under the distributor cap…

5

Thank you,

Where do I locate the CPS?

6

Mai,
The CPS is located at the front of the engine. The sensor head is mounted on a bracket that is next to and beneath the pulley at the front of the crankshaft. The long (2 foot?) CPS wire goes up the left front of the engine and along the left side of A/C compressor where it get connected to another cable.

You will have to put the car on a lift, jack stands, or ramps in order to access the CPS from beneath the car. Removal of the spoiler undertray is required.

Paul

7

Check the connector first, as I suggested. The location of the sensor itself was described. There is another ongoing thread where it was mentioned just a few days ago:

8

Thanks again, I found the CSP, checked the resistance, it stayed ~700 ohms and did not change when I cranked the engine.

Looks like it is the problem.

I found this part on Ebay : Formula Auto Parts CAS376 Crankshaft Position Sensor

Is this the correct part?

Thank you,

9

700 ohms doesn’t sound bad; I’m sure others can chime in. Not sure if you should see a variation when cranking with a VOM; it might require an O-scope to see what it’s doing.

There is another sensor on the flywheel. That one will also keep the engine from starting.

10

Thank you,

I will replace the CPS first to see if it works.

11

Mai,
As described in the Jaguar XJ-Series Repair Operations Manual (ROM) the test of the CPS requires an oscilloscope. You can not check it with a multimeter. I tested mine a few years ago with an oscilloscope when I was trying to sort out an “engine cranks but doesn’t start” problem in my wife’s 1990 XJ-S convertible (5.3L V12 with Marelli ignition). A neighbor, who is an Electrical Engineer, came over with his personal oscilloscope and we tested the CPS and the flywheel sensors (they are the same part number). They tested fine and I eventually found the problem elsewhere.

As I previously mentioned the correct Jaguar part number for the CPS is DBC12507. I have no idea if the aftermarket part number that you provided will work. After 19 years of Jaguar ownership, and purchasing hundreds of parts for my five Jaguars, I have learned to suspicious of aftermarket parts from unfamiliar sources.

Paul

12

Paul is correct, and I agree with him completely, as my previous rants will attest. I think if you check CPS or Speed sensor (same part #) with an ohm meter and get infinite resistance (open) or very high resistance then you have a problem. But I have never seen any compelling evidence that one can diagnose beyond that… if it reads 700 ohms it still might be bad. Or not.

13

Where do I locate the flywheel position sensor?

Thank you,

14

It reads off of the flexplate teeth that the starter uses. Technically it is the Engine Speed Sensor. You can see the loom that goes to it in this picture, by the oil filter. The other end is a plug, up behind the left side cam cover.

image

This is sort of spelled out above, but to be clear:
There are two sensors. The CPS or Crank Position Sensor and the “flywheel sensor” or Engine Speed Sensor. They are both the same part number and are interchangeable.
Either one will cause a “crank but no start” situation on a Marelli ignition car.
The best test is to use an oscilloscope AT THE IGNITION computer in the passenger side footwell. This confirms (or not) sensor function and harness integrity. If no signal at the computer, work backwards from there.
The second best test is to determine (listen with screwdriver or stethoscope) whether or not the injectors are firing. If they are NOT, then likely one of the sensors is not working.

If this was my car and I didn’t have a scope, I would:

  1. Listen for injectors. If not clicking, assume CPS or ESS is bad.
  2. Unplug CPS and ESS from harness and use contact cleaner on the plugs, make sure there are definitely good connections. These are very low-voltage and low current signals, they do not tolerate poor connections. As previously mentioned these plugs are next to B-bank t-stat housing and behind B-bank cam cover. They look like fuel injector plugs.
  3. Ensure that the sensor end of each is tight and clean. Clean with brake cleaner or similar. If they are sloppy in the mount or all gunked up they may not be reliable. Now try to start.
  4. If still no start, check thin white coaxial wire that is buried in the harness on the left side of the injector rail. There is a spade connector in there that fails. Make sure it is clean and tight. This is part of the ESS harness and is a common failure point. Now try to start.
  5. If still no start, Buy two new sensors. Replace CPS. If start, save second for spare. If no start, replace ESS.

Regards
Bob

15

Thank you very much for the help.

16

Thanks everyone for helping. I replaced the CPS and the car started right away.
Thanks again.

2 Likes
17

Mai,
Congratulations on fixing your problem and for posting back with the good news results. Hopefully this will help others encountering the same no-start symptoms in a 5.3L V12 with a Marelli ignition.

Paul