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When is TDC not TDC - In Cylinder Testing
Posted to Technical Tips Forum on 2/26/2010 49 Replies

Recently, I received the latest edition of iATN Review, good job Scott and staff. This edition contained a couple of articles on transducer testing. The post authored by Kevin from Manitoba caught my eye. Here is the link to the post:

In that post, this is the capture that caught my eye. The notes in that capture stated, and I quote "The secondary igntion pattern has shifted over to the right from TDC (indicating timing out of sync)."

That comment seems to be in error to me, the data is being misinterpreted. If the spark timing is based off the crank sensor and the crank sensor is fired off the crankshaft, how can the timing be shifted (relative to TDC) when the timing chain jumps? The crank sensor and TDC is being controlled by the same item, the crankshaft. While the cam or valve timing is off, the spark should be firing at the proper time, at or near TDC.

If we agree that the timing doesn't shift and that TDC is occuring at TDC (piston position), then the logical answer is that the peak pressure of the in-cylinder transducer is NOT happening at TDC of the piston. Again, the spark timing is NOT shifting versus TDC of the piston.

To help prove the point, I have uploaded a couple of captures. All these captures are made with the new Pico transducer and an inductive probe. I chose to use a Pico scope due to peer pressure. :-)

This first capture is of cranking compression versus spark on a known good cylinder from a 2000 Cherokee with a 4.0 litre inline 6 cylinder. Notice that the spark is occuring at approximately TDC.

The second capture is of cranking compression and spark on a known bad cylinder. Check out the compression pressure and where the spark is occuring. It "appears" to be late. However, this is a DIS igntion system and these 2 cylinders that I am showing are companion cylinders.

Here is a capture with both good and bad cylinders together with spark. You can see the differences a lot easier. Again, the spark is not shifting, the peak compression pressure versus TDC is. This occurs when there is a leak in the combustion chamber.

Because this is a DIS ignition system and I am scoping companion cylinders, the coil firing is the same coil for both of the cylinders. One can argue that there is a possibility the sensor that controls the spark timing could be off slightly and that is causing the variance in the timing versus TDC. For those, I have uploaded this capture showing the good cylinder and bad cylinder with equally spaced cursors. The cursors are evenly spaced between the 2 good firing towers. You will notice that it is offset about the same amount as ignition firing.

My point I am making with this post is: With a leak in the combustion chamber, you cannot trust that the peak pressure from a in-cylinder transducer is indicating TDC. Be careful that this doesn't lead you to the wrong conclusions, especially if you are scoping igntion versus peak pressure. You may see that the timing is late and assume that the timing chain has jumped when in fact the combustion chamber might have a leak.

While the authour of the other post that I referenced, did make the right call even though he misinterpreted the data, he had an obvious case of a jumped timing chain. I can see instances where you may be trying these techniques when the problem isn't so obvious and get burned.

Transducer testing is full of pitfalls, keeps your eyes open for them. Feel free to reply with the pitfalls you have seen with these methods of testing.


Harvey from British Columbia

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