Underhood - Service Repair

Search Autoparts/Motorage/Underhood-service-repair/

Diagnosing variable camshaft timing systems

Sunday, April 1, 2018 - 07:00
Print Article

It is crucial to point out at this point that if you are going to capture a “known good” synch waveform there must be no cam phasing occurring when the pattern is saved. There are vehicles that can phase their camshafts at idle and if you are not familiar with the cam phasing strategy of the vehicle you are working on you would need to verify with a scan tool that the cams are in the home position, or better yet just un-plug the cam timing oil control solenoids before you capture a synch waveform. It would be a serious mistake to decide to tear down a motor to replace a timing chain that you think is off because you compared a waveform you captured to an incorrectly captured waveform uploaded to the Internet by a tech not aware that the supposedly known good waveform he was posting had the cams moved 8 or 10 degrees from the base position. The best “known good” waveforms are the ones you capture yourself and you know exactly what the conditions were when you saved the waveform.  

Motor Age Magazine Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.

SAVE 20%

On Automotive Underhood Training Videos, ASE study guides and more.

There are a few items you need to know when analyzing a VCT system waveform, the most important being the design of the crankshaft sensor trigger wheel. Scopes that have rotation rulers make it easy to determine how many degrees of engine rotation each tooth on a trigger wheel is valued at. For instance, many engines use a 60 minus 2 tooth trigger wheel for the CKP. This means there is room for 60 teeth but 2 are removed to create a synch for the PCM to identify. If you divide 60 into 360 you get 6, which means each trigger wheel tooth is displaying 6 degrees of engine rotation. You can now line up a CMP waveform signal edge to the CKP signal and determine very accurately how far off an engine is from correct valve timing. I simply pick a point in the waveform where the two signals have a transition and start counting teeth.

This known good synch waveform from a BMW 3.0 N52 engine will be zoomed into to look at CKP\CMP correlation.
Here you can see the relationship between the CKP missing tooth region and the leading edges of the intake and exhaust CMP sensors. The oil control solenoids were not connected.

Once you determine the correct CKP/CMP relationship you can continue testing the system by applying power or ground to the oil control solenoid with the engine speed raised so the engine does not stall and capture a waveform of the cam sensor with the cam phased to its maximum travel. Count the number of teeth the CMP sensor has moved from its synch position and multiply by the number of degrees per CKP tooth to see if the camshaft moved its full published range. Remember while viewing the signals on a scope, the exhaust cam signal will move to the right because it retards and the intake cam signal will move to the left because the cam advances. This testing routine can be employed on any engine and can be used when the vehicle does not supply scan data PIDs for cam timing.

Grounding the intake cam VANOS solenoid moved the cam to the fully phased position. The scope measures 69 degrees, service information states a 70-degree range for intake cam travel.

A real-world example 
Putting this testing technique to use in the shop is straightforward yet many techs are intimidated by scope diagnostics or are unwilling to spend the time to capture these waveforms. A shop called concerning a problem they were having with a 2006 Nissan Altima with a 2.5 engine. A code P0011 was setting and their scanner read the intake valve timing PID as -26 degrees and they were not sure what that meant. Some scan tools read Nissan cam timing data incorrectly and this was one of those cases. The shop had replaced the cam and crank sensors but the code returned quickly. I mentioned they should scope the cam and crank sensors along with the ignition trigger but they decided to send the vehicle to me for a diagnosis. Having seen these vehicles before I have a known good synch waveform but I decided to look at service information to see if the data was available to the other shop to make an accurate diagnosis. Upon looking at code repair information for a P0335 I found a hand drawing of the correct CKP\CMP relationship from Nissan training information.

Nissan training book depiction of correct CKP\CMP signal relationship. I have added the callouts shown.
Article Categorization
Article Details

< Previous
Next >
blog comments powered by Disqus