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