Magnetoresistive CMP Case Study and Service Caution
I have seen a few posts on here about the CMP trigger/reluctors on the 3.6 Pentastar engine. The vehicle in this case study is the first gen 3.6 without VVL and liquid cooled EGR, but the information here applies to the second gen as well. Many manufacturers are using magnetoresistive sensors (MRS) for cam/crank position and wheel speed, they are more sensitive and accurate. They do not require a metal trigger wheel, instead they can use magnetic coded triggers. These triggers should never come into contact with anything magnetic. In the case of a camshaft, the two parts should not touch each other during service. Here is a picture of the trigger.
These engines have quite a few upper end issues including rockers, bad camshafts, reluctor shifting (pressed on cam), and valves. If a cylinder head is purchased from the dealer the cams and followers are transferred over, so if the camshaft touched anything magnetic (or each other) in that time you are out of luck. Also, on a side note, I would not recommend transferring over the followers, they may look okay visually but have the tendency to stretch out and the valve does not open as far, causing a misfire code from the cylinder imbalance monitor.
Today I looked at this vehicle that had a intermittent sensor code. Just as an FYI, the wiring diagram can be confusing. There are two physical cam sensors, but 4 signals total. The sensors sit between the two cams and have 4 wires; power, ground, two cam signals. The wiring diagram simply label the sensors as 1 - 4, and the code does not specify intake or exhaust.
Make sure you look at the code chart to determine what camshaft sensor is coding, and look at the component location for each sensor to determine what bank it covers. The cam sensors are actually numbered in order on the engine:
CMP Sensor 1: B1 Exhaust
CMP Sensor 2: B1 Intake
CMP Sensor 3: B2 Intake
CMP Sensor 4: B2 Exhaust
I scope the cam and crank sensors with a sync. I have scoped these engines before and know that both intake signals should match, and both exhaust signals should match. That is not the case for the exhaust cams on the subject vehicle (missing one pulse), but the intake cams match.
Using the falling edge count between rulers you can see 3 of the cams have 8 events, the cam signal in question has only 7. It is possible for these sensors to intermittently not pull to ground, but the missing pulse was consistent in the waveform.
This engine needs a camshaft, one spot on the trigger wheel is bad. I would not recommend installing used camshafts because of the trigger wheels being so sensitive. New camshafts have also been known to cause issues due to storage and handling. I have not done it myself, but some technicians use a piece of paper over the wheel and put metal shaving on it to see if it still has the magnetic coding in it.
Using the scope will help you with many other MRS trigger wheels as well. The FCA 3.0 diesel is currently having issues with a similar looking CKP trigger wheel falling apart, it is not metal but instead a coated plastic type material. The 3.0 engine trigger starts to de-laminate, step 1 is to pull the trans for replacement. There have also been cases of technicians working on the vehicle and placing a magnetic parts tray on the valve cover, the MIL comes on after service work and needs a cam!
I hope you find this write up helpful to diagnose this problem, and hopefully prevent it from happening due to technician error. PSDATA file attached.
Matthew this Is a amazing write up with tons of information. These camshaft reluctor can become incorrect. I was battling proving a reluctor issue or an ecm. But I was able to make the correct call as a bad ecm. If I remember correctly the relector has a small, medium, and large “tooth”. The relector can change and the sensor may pick up the small tooth as a large one. Thank you for sharing
Was the bad ECM not supplying 5 volts on the circuit?
The ecm couldnt process the signal coming on for one of the cams.
I used a lab scope to check the same sensor at the sensor then another channel at the PCM. That allowed me to overlay the two signals to see if I had a drop on from ”A TO B”. I then used a math channel within pico setup within the parameters that the pcm was looking for. That way if I had a glitch the math channel would pick it up. Then I simply let the vehicle run until the CEL come on. I did…
Thats cool Matt. I had the identical problem on a 12 Durango 3.6. Same thing, intermittently the PCM just quit sensing the signal for lack of a better term.
Great diag, Matt. My only other advice would be not to forget about terminal tension issues... ask me how I know :-) I have been seeing so many terminal tension issues and have been burned a few times on a diag, so now every time I condemn an expensive component I de-pin terminals and check tension, in addition to the typical power/ground/wake-up signal checks.
When checking tension are you comparing it to another pin?
I do a drag test, de-pin the wire from connector and test it against the pin it goes into.
Sorry guys. I left out terminal tension. That’s very very important in my case. The terminal was drag tested to confirm it would make contact.
Matt, I figured you tested it. I just wanted to throw it out there as a reminder, that way everyone can learn from my mistakes lol.
Can we check pin fit issues with a micro amp clamp?
I've never bought one, but I am sure you could. Measure current while wiggle testing the connector.
How did you get to run with a different donor car pcm ? Anti theft ? Pcm replaced function on scan tool ?
Thanks Matthew thats a good bit of information. I just had one of these POS in for all timing gears chains etc for wear & my overall view on Chrysler products being rubbish was certainly confirmed,, plenty of traps for the unwary on these engines & the fragile nature of the ckp trigger wheels is ridiculous. Regards
The crappier they make them, the more money we make :-)
Thank you Matthew for the excellent presentation. This is a photo of a 3.0 Chrysler diesel ckp reluctor
yikes, the magnet tray issue/cause is bound to flummox more than a few guys. Thanx Matt.
Yes, but techs often struggle with wiring issues on these engines too. Many techs use a scope and do not see a signal, and it gets a sensor.. or two, or three lol. Most techs do not realize that there must be 5 volts present from the PCM on this type of circuit. If a wire is broken and they do not see a signal, they assume sensor. FCA, and many other OEMs, do not do a good job showing the…
This is a reminder, with 0 volts after you clip the wire close to the PCM, remember to cycle the key so the PCM can re-establish the 5v.
"Yes, but techs often struggle with wiring issues on these engines too. Many techs use a scope and do not see a signal, and it gets a sensor.. or two, or three lol." That's a big problem, that will only get worse with everybody thinking they need a scope for everything, and just hooking up as a first step. Need to know how it works first.
Yes, I agree 100%. When I do scope training I start out with two major points. What are we trying to prove with the scope and what are the expectations? Getting a waveform is the easy part, analyzing it is harder. In all fairness, it can be hard on some OEMs to determine what type of circuit a sensor has... when all you see on a diagram is a wire going to a box.
Luckily, on those modern multi-cam engines, (or with ABS for that matter) there are usually a few NOT-broken sensors we can analyze, if we aren't sure what to expect on the one we are concerned with.
Someone posted a picture a week or two ago, of a mopar trigger wheel with a layer of engine swarf stuck to the magnetic strips. The concern, other than the obvious metal flakes, was lost on me, and I was going to ask for some background. This was very useful. Thanks a bunch.
James, MR sensors are very sensitive, so I am not sure that the metal shavings will affect the sensor operation, but time will tell. The largest enemy to the reluctor is magnetism, but I am sure enough metal shavings building up could potentially be an issue.
The metal is I believe is a sign the needle bearings on the rockers are failing. When they do, the cam will then grind on the rocker arm housing, gets in theoil, and sticks to the magnet. Of course there is usually a tick to the engine as well.
Yeah, I have seen quite a few of those.
Hi Matthew. I had one similar to this recently. The shop called us out to assist them in diagnosing CMP Sensor Circuit DTCs. They had just performed valve train repairs. You can see in my attached image, the irregularities in the CMP Sensor Waveforms. No visual damage could be seen to the reluctors. Not sure if it was physical damage or exposure to a magnet that caused this issue.
Thank you for sharing, this will continue to be an issue so tech awareness is key here. Great capture and use of the scope.
That was great presentation, a lot of great info. Thanks. Seems a bit shortsighted of them to use such a sensitive item, but I guess that is part of the reason they use it.
Yeah, I understand using a MRS for the CKP, there is a need for high resolution and accuracy. My guess is cost or weight. GM has been using MR CKP sensors for awhile, although they used a metal reluctor and hall CMP sensors.
Matthew, I posted about my experience with this in an earlier thread. After I read yours tonight I looked and found the screenshot in my files. Bank one yellow trace. Can't remember if exhaust or intake, but the machine shop managed to do whatever they did pretty good and the ECM was definitely not happy with the result.
Great info! The sensor numbering can be confusing if your not paying attention. We have seen one of these that would set a code on startup and it ended up being the phaser had failed. The cam was able to be turned several degrees without the sprocket turning. Just another ting to look out for!