Flywheel and Crank position sensor test

I’ve replaced both DBC12507 sensors last night. I took the old ones home and did some measuring.
Room temp of around 24C, resistance 700ohm
Apply hairdryer to sensor and it sweeps from 700 to 850 within about 2 minutes. Both. Then from 850 to 900 takes another 2-3 minutes.
Even after heating there is some output of around 4mV when I wave a drill bit infront of the sensor as close as I could get to it without the magnetic force grabbing. If I let it grab the drill bit it hits around 40mV.
I know the output should be in the region of 400mV from the specs of SEN8 (D-K) but I don’t have an oscilloscope so sorry that’ll have to do (and I suspect the specs for our SEN8 S are the same).

Forgot to do the same test with new sensors and don’t feel like crawling back down again.
Can someone who is about to replace their dbc12507 and who has access to a multimeter AND a hairdryer do the same test on a new sensor.

This is the OPPOSITE effect of what John J. Lynch reports in Marelli Intermittent Failures

Warning, I am getting on my soapbox :slight_smile:

For years we’ve been back and forth on this forum about the CPS and speed sensors on Marelli cars. Resistance, temperature, wiggle the leads, how to test, etc, etc.

My .02: If you are going to drive a Marelli car (I do) then you need an oscilloscope. There are several sub-$100 handhelds on Amazon that will do the trick. The ROM is very clear on how to test using a scope.
If you really want to make you life easy, crack open the passenger side foot well and carefully tap into the Marelli injection computer harness. I wired in a simple harness that allow me to see the 3? wave forms that are expected here. (I think it’s CPS, engine speed and a pulse to? the ECU in the trunk…) The ROM is clear on this as well, I am going from memory at the moment. The end of my harness is up under the glove box. I keep the scope in the boot. I can do an initial sensor troubleshoot in about 1 minute. This allows me to confirm the sensor function and wiring all the way to the Marelli computer. If one is bad, I can start working backwards towards the sensor, checking things like the goofy white wire on the left side of the fuel rail, sensor plugs, etc. It also tells me that the Marelli computer is outputting the pulse that it is supposed to the ECU.

For about $100 and a couple of hours of careful interior panel disassembly and soldering, my sensor worries are now minimal.

Off soapbox!

Well your soap boxing just cost me $70. An oscilloscope is on its way from Amazon. I had read that two wire CPSs aren’t tested with an oscilloscope but it the ROM says they are, then they are.


When I was trying to sort out a no-start problem with my wife’s 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) this spring a neighbor came by with his Tektronix 2465 oscilloscope to read the 4 Crankshaft Position Sensors (CPS) that I had (two from the car and two new ones that I purchased) to make sure they were not the cause of the problem (and they weren’t). It was good to be able to eliminate them as the reason for the no-start, and that left me with the possibility of stuck fuel injectors as being the cause. I had originally eliminated the fuel injectors as the cause of the problems because they were recently serviced (within the past 3 months), but little by little I began to suspect them. The CPSs all tested good, and so I removed the fuel rail that I had just recently installed and confirmed that 11 out of 12 of the fuel injectors were stuck and not firing. The attached picture shows what the waveform looked for one of the CPSs, which is reasonable similar to the one in the XJ-S ROM. My friend tested the sensor both at the CPS connector at the front of the engine, and again at the Marelli ECU cable located in the right hand footwell (it’s a LHD car) and he seemed to get the best result at the connector at the front of the engine.


Paul M. Novak

1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA

Since I already spent money on new sensors I don’t feel like spending more money to be able to confirm that they are brand new, namely I know that, I ordered them from jcp myself and they were well packed.
What I do need, however, is for someone who is about to replace theirs to do a simple test and confirm whether resistance climbs with temperature so quickly on a new item as it does on a 25-year old one.
But thanks for your sugestion, I might buy me an oscilloscope once I feel the need to test things regularly (other than just 2 sensors on one of the cars I drive)

Interesting!..whilst all my Jags are pre-80s, I am on another forum about '90s Jeeps, and those guys are always having “which sensor is crook” questions, and although there is nothing in the Factory Service Manual about scopes, I have thought a Hantech auto scope for ~US$200 would be a good piece of kit and pay for itself first use in diagnosing the problem…most vehicles built in the '90s, while having OBD11 wont necessarily generate a code that tells what sensor is crook, (unless that sensor affects pollution), and it can be difficult to find technicians with experience to work on them.

And for all of you itching to buy an oscilloscope just so you can test your cps when enjoying a sunday out driving around countryside then may I suggest you order yourself one of these rather than spend $100

When you tapped into the harness, did you solder on to a wire going into that big 25 pin connector, or double up some wires by soldering onto the pins of that connector or…

The reason I ask is because some of those coaxial are delicate/subject to interference and I don’t want to ruin a signal in an attempt to monitor it!

It’s been several years but as I recall I picked up a couple at the pin and one farther back on the wire… I do recall that it was a delicate situation. Obviously disconnect battery, the plug from the computer, etc. I believe I also picked up the ground wire from the harness; the thinking being that that is the same ground the sensor is expecting to see, so why not.

Hi Posix-
My rant was not directed at you, so my apologies if it came across that way. The same topic comes up on the forum on a regular basis, and there is always lots of back and forth about resistance, visual inspection, check for injectors firing, distance from sensor to flex plate, etc, etc., none of which will definitively identify a bad sensor. Rather than running a process of elimination, this is one time where the xjs owner can perform a definitive test and have a quick answer on a common failure point, which is a bit of a rare treat on these cars, especially if the fault occurs while enjoying a country drive.
Regarding the resistance rising with heat, it is an interesting data point to know. Resistance in a wire will always rise with heat, so the changes you are seeing seem normal to me. As you suggest, perhaps someone with a new sensor can confirm.

How tough to assemble the kit? Does it self calibrate, ???etc. ???
Thanks, JW

I believe there is a fully assembled version that can be ordered. And even on the kit there is very little to assemble, instructions are provided. If you’re a tinkerer type that is. And since you drive an xjs you better learn to be…

I looked up the kit on the internet.(your net address as supplied)…Chinese kit is a disaster according to the commentators who spent their money and tried to make the thing work. CAVEAT EMPTOR!, will purchase a newer digital unit at regular prices. Just another diagnostic tool for my shop, as I am contemplating purchasing Marelli system machine(have owned and operated 3 XJ-S v12 coupes since 2005). The Lucas system is very stable and reliable after the problems are sorted.
My friend Merle Elles has a 90 XJ-S convertible and has sorted his nicely using an O-scope.
Merle is a graduate math and electronics engineer (many years a deep water sailor, now aground in Kentucky) He has prepared several documents on the electronics in his 1990 convertible and they will be submitted to Peter Crespin at JCNA for publication along with a portfolio of documents from our friend from OZ…the Maestro himself…Grant Francis. Grant continues to work and repair/solve problems with even the newer Jaguar models…looking forward to a continuing stream of intel. on these later machines. Best, JW

If you were looking at cheap oscilloscopes on Amazon, the E-one was useless with chinglish instructions. I sent it back. The hantek for 60 bucks that pulled into your USB on the computer was able to read my Flywheel sensor pretty well. The crank sensor is supposed to have a different pattern, and either it’s a brand new sensor and no good or I have no idea how to use the Oscilloscope properly for that particular range.

Sensors have a lot of turns in coil formation.
The turns are enamelled copper wire.
Copper has a significant temperature coefficient.
Heat it and its resistance increases quite a lot.
Common faults in these sensors are open circuit where the solder termination has parted company with the connector.
If the resistance is not close to the standard specification you probably have shorted turns in the coil, and that will degrade the output signal.
Shorted turns are caused primarily by heat and age breaking down the enamel insulation on the copper wire.

Not sure if there is a signature line attached here, am using an IPad ( instrument of the devil) in China where Google Mail is no longer blocked.