As most of us like to carry a spare crankshaft sensor, I found it curious as to why XJ40 CPS costs were so high compared to the practically identical X300 units (aftermarket, in Canada costs were $650.00 vs $22.00!)

As nothing will stop you dead in your tracks like a rotor that goes high resistance, I was in the ignition category on the Rockauto site ordering a couple of spare rotors when I came across the CPS options. There were the numbers. The astronomically priced three wire sensor for our cars and the quite reasonable two wire sensor that suited the X300.

I thought, what the heck, I'll get an X300 2-wire unit and see if I can modify it to fit the 40. I had a old dead CPS in the workshop so took the plug and wiring from it. In the 3-wire setup, there's the red, blue and black wires. The black wire is connected to the CPS shielding and only the red and blue are used for the signal.
When the 2-wire X300 sensor arrived, it red and blue wires too, but the shield wasn't connected to anything, it just terminated at the plug. The pigtail was also quite short, maybe 6" compared to the the XJ40's roughly 12" wiring.

I cut the 2 pin plug from the X300 sensor and using the plug and wire from the old dead unit, soldered the signal wires together then took the braided shields from both ends and soldered them too.

With the X300 unit now sporting a 3-pin connector I installed it in the car. Guess what. It worked just fine right through the rev range without radio or CD interference.

So if you're faced with spending an inordinate amount of cash on one of these overpriced 3-wire units, if you're a little bit handy you now have options!

 

That's a really great discovery, Larry!

Looking at the schematics, on the '93-'94 XJ40 the shields for both the O2S and CPS are grounded at the CPS body. The metal mounting flange of the CPS must ground to the CPS mounting bracket.

In the X300, the shield for the CKPS (the X300 equivalent of CPS) and several other sensors connect to ground at the left bulkhead stud.

So your solution should work great!

Cheers,

Don

 

Actually Don, the X300 sensor shield isn't grounded at the stud at all!
I assumed, as you did, that it must be the case but after running tests, it seems that the (very short) 2-wire shield isn't grounded at all, it just terminates at the plug and has no continuity to any part of the sensor assembly.

After installing the test on my car I was tempted to cut the black wire at the sensor side of the plug to see if that made any difference but thought I'd just let sleeping dogs lie. Could be that the harness doesn't need to be shielded from the plug to the sensor but I was too chicken to find out!

I'll leave that to the next guy!

all the best

Larry

 

According to the 1995 X300 wiring schematic, the shield surrounds both of the CKPS wires and the shield is supposed to connect to ground at the left bulkhead stud along with the shields from several other sensors. I don't recall if or how this is actually accomplished on the X300, but it is clearly shown on the schematic, which I am attaching. Now that I look at the schematic, it looks like the shield could end at the connector for the CKPS pigtail harness. If that is correct, the shield on the sensor pigtail harness does nothing except provide some physical protection for the two wires.

My understanding is that the shield is not there to protect the radio from EMI, but to protect the CPS signal from corruption by EMI from the alternator and ignition components.

Cheers,

Don

 

On the AJ6, I believe the designers shielded the sensor pigtail through the connector due to the length and proximity of the pigtail to the distributor -which could possibly generate enough interference to affect the sensor signal.

The AJ16 design may have eliminated the need to shield that section as there's no actual distributor nearby and the pigtail is considerably shorter.

Larry