Cam and Crank Correlation

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Cam and Crank Correlation

Postby Steve Smith » Mon Oct 23, 2017 6:07 pm

Technical support is one of the many benefits here at Pico Technology. Based on a number of calls taken over the last few months I would like to share a technique here for evaluating valve timing.

Valve timing errors have the potential to generate a world of pain when it comes to diagnosis.
Mechanically checking valve timing can take an “age” where access is limited, and whilst a static check of valve timing can prove fruitless, a dynamic valve timing check (utilising the Cam and Crank sensors) will allow you to make an informed decision on the condition of valve timing arrangement.

I mentioned a fruitless, static valve timing check above as there will be occasions where a “mechanical” inspection of valve timing will reveal no error. However Cam/Crank correlation fault codes will prevail due to whip, flex, stretch and backlash of drive belts, chains and gears.
The procedure below explains how to inspect the Cam/Crank correlation of a diesel engine utilising a crank and single camshaft sensor without VVT

Capture Crank and Cam signals over a sufficient time period to obtain a minimum of two engine revolutions (1 complete camshaft revolution)
20 ms/div is fine for a 4 cylinder engine running at idle speed but 50 ms/div will ensure sufficient crankshaft revolutions during each waveform buffer.

Using the reference signal (missing teeth) of the crankshaft sensor signal we can clearly identify two revolutions of the crankshaft using the Rotation Rulers located in the bottom right hand corner of our waveform. Click on the Rotation Ruler handle and drag to positions on the crankshaft signal waveform that align with the first and third crankshaft reference signals (missing teeth) This will denote 0 – 720° of crankshaft rotation relative to the missing teeth of the crankshaft pick up ring.

Once 0-720°have been identified we can introduce a single Rotation Ruler Partition by clicking on the Ruler button and selecting 2 Rotation Partitions from the “pop up” box. The distance/time between the Rotation Rulers will now be partitioned equally by 2 to indicate 360° of crankshaft rotation denoting a single crankshaft revolution. Whilst this is not essential it will help to interpret the relationship between the cam and the crank with reference to engine revolutions

IMAGE 1a.jpg
Rotation Rulers


Using the Time Rulers we can now drag one Time Ruler to rest directly above the Rotation Ruler denoting 0° of crankshaft rotation whilst the second time ruler can be dragged to align with the first rising edge of our camshaft pulse after the Rotation Ruler denoting 0° crankshaft rotation.

The choice is yours here as you may wish to choose the first falling edge of your camshaft pulse as this may be more convenient with improved definition. The time difference between each Time Ruler is indicated in the Ruler Legend along with the degrees of crankshaft rotation.

IMAGE 2a.jpg
Time Rulers


Using the zoom options the distance between each Time Ruler can be magnified to allow for closer analysis and accurate positioning of both the Rotation and Time Rulers.

The example below highlights how the waveform will appear after zooming. Here we can identify the 0° crankshaft reference point and the first rising edge of our camshaft pulse. The Ruler Legend indicates the first rising edge of our camshaft pulse occurs approximately 41.28°after our crankshaft 0° reference point, or approximately 6 teeth of our crankshaft pick up ring. (1 large tooth and 5 small teeth)

IMAGE 3a.jpg
Zooming


Using the Waveform Library as a reference we can now compare our captured data with an identical vehicle that has been uploaded by other PicoScope users around the world.
To access the Waveform Library, click on File>Waveform Library Browser. Here you will be directed to our cloud based storage facility for automotive waveforms
A link on how to use the waveform library can be found here:
topic10063.html

N.B. In order to access the Waveform Library you will need to have an Automotive PicoScope connected to your PC, an internet connection and a user name and password for the Pico Auto forum. Once you have obtained access to the Waveform Library you can search for the waveform you require using numerous search criteria such as Make, Model and Engine code etc. Here you can then download the relevant waveform and carry out the identical measurement technique described above in order to determine a potential valve timing error attributed to timing chain, gear or belt wear. The links below highlight just how invaluable the Waveform Library has become:

topic18261.html
https://www.picoauto.com/library/case-s ... cold-start
https://www.picoauto.com/library/case-s ... ault-codes

For vehicles with variable valve timing (VVT), it is recommended that each VVT controller is disconnected in order to restore the camshafts to their “rest” or “full” retarded position. Refer to the vehicle manufacturer technical data on how to carry out this procedure and the implications involved.
Be aware upon disconnecting VVT controllers, DTC’s will have been triggered and stored in the PCM so requiring erasure after completion of diagnosis

With all the above in mind we need to be aware of a number of pitfalls using such techniques.

VVT (mentioned above) we must be wary of VVT intervention

Missing teeth (Crankshaft reference signal) may not indicate one revolution where multiple reference signals are used per crankshaft revolution. A manufacturer may choose to include a reference signal every 90 degrees of crankshaft rotation. Here then we must refer to the relevant technical data.

A number of engines only use one camshaft sensor with twin camshafts.
Here we can only confirm the position of the camshaft utilising a camshaft sensor.

We must not assume that the missing teeth of our crankshaft sensor signal indicates TDC of cylinder 1, this is very rarely the case. Often the reference signal may indicate all pistons in line, (safe position) or a specified number of degrees BTDC

We must be aware of the relationship between timing sprockets/pick-ups and cam/crank shafts.
Mis-installed sprockets/pickups will present additional challenges when evaluation cam and crank correlation in this fashion (See the case study below) as we cannot confirm their installation to be correct
https://www.picoauto.com/library/case-s ... -non-start

Be aware of cam crank deviation under load!
At idle speed (no load) the Cam/Crank correlation may be correct, however, during engine start up, acceleration, and deceleration the Cam/Crank correlation may deviate to a point where correlation codes are triggered by the PCM. This may be due to chain stretch, belt whip, tensioner failure (run out), low oil pressure or gear wear/backlash
In this scenario, carry out the procedure above at idle, during free rev and over-run.

Remember, Static and Dynamic valve timing can be correct, but individual cylinder valve timing can be in error. (See the case study below)
https://www.picoauto.com/library/case-s ... fire-p0302

In this scenario we need to move to the WPS500 Pressure Transdcuer for in cylinder pressure analysis to evaluate individual cylinder valve timing, which is a whole new topic. (See the Forum post below)
topic17241.html

I guess one question raised by all the above is "How much is too much deviation?"
This will vary from vehicle to vehicle and will depend upon the monitoring strategies of the PCM.
Once again the Waveform Library comes into its own presenting Cam/Crank captures with and without faults for comparison

Referring to vehicle specific bulletins must never be overlooked as I can remember one such case where Cam/Crank correlation codes were triggered due to the engine momentarily running backward when switching off. Given the PCM would still be monitoring Cam/Crank, a correlation code was stored. The cure in this case was a PCM Re-Flash.

Of course this vehicle has already been treated to a new timing chain!

I hope this helps, take care…….Steve
Steve Smith
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Joined: Sun Aug 25, 2013 9:22 am

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