Posted to Technical Tips Forum on 3/15/2014
Sync for Relative Compression Testing
For those that don't know what relative compression testing
is, it is where we use a labscope and an inductive current
probe to measure the current it takes to spin a starter.
Each compression stroke will cause the current requirements
of the starter to increase during the compression stroke
versus the other strokes. The higher the compression, the
more current it needs. The lower the compression, the less
amperage it needs for the compression stroke.
We can use this to "see" if we have a cylinder or cylinders
that are low in compression. Because we don't measure the
actual pressure, we typically compare one cylinder to the
next. Hence the term "relative".
For those that perform relative compression testing, we know
how quick this test is to confirm if there are any
mechanical issues or not. If I have a misfire and perform
this test and find low compression, I'm almost done my
diagnosis. It can be a good quick first step. And it should
be the first step before any transducer testing.
We also like to have some sort of "sync" or marker to
indicate which pulse is which cylinder. Typically, we like
to use the coil firing as the "sync" as that indicates, not
only which cylinder is on it's compression stroke, but we
can also judge approximately where TDC is.
In order to prevent the vehicle from starting, and still
have spark to "sync" off of, we have to disable the fuel
system. Different vehicles have different methods to disable
fuel. Some we just fully depress the accelerator pedal and
achieve a "clear flood" mode. That's the simplest, if it
works. Some have fuel pump relays that we can remove. Others
need slightly more work to disable fuel, I've disconnected
all the fuel injectors in some occasions.
I had a Dodge 4.7 liter V8 in the shop today that I wanted
to perform the relative compression test on. This should
pretty easy to disable fuel as there is a relay box under
the hood with a fuel pump relay. I removed the relay to
perform the test.
For those that have done this test and unplugged the fuel
pump (or relay), you probably have encountered where the
engine will try to fire up while cranking as there is
residual fuel in the lines. Once we crank long enough and
use up the fuel in the rails, then we can perform this RC
test. On some of these Dodges, that seems to take forever.
And once you think you've used up the fuel, you let it sit
for awhile and the darn thing will want to fire up again.
When this happens, it makes the cranking test invalid, its
time consuming and frustrating.
This is how I deal with it on these types of systems.
First, I unplugged the ASD relay. When I unplug the ASD
relay, it will remove the 12 volt power supply to all my
injectors. This will not allow the injectors to fire, which
makes any fuel in the rail moot. The problem is, it also
disconnects my 12 volt power supply to all my coils. But I
still want to "sync" off my coils to ID my cylinder for the
How I overcame this problem was by taking a normal test
light, and connecting one end of the test light to the BAT
(+) terminal. I then disconnected the coil connector of the
cylinder that I wanted to "sync" off of, and connected the
other end of the light to the COIL CONTROL terminal. I then
connected the lead of my scope to the same side of the test
light as the coil control signal (coil primary).
Sync for Relative Compression
With the IGNITION ON, I cranked the engine. In this
particular example, I used a remote starter switch.
This is the relative compression result from this
My sync pulse is the green trace. When the signal is high,
the coil is not being energized. When the signal is low,
that is when the PCM is grounding the coil. So the upward
pulse after it goes low is when the coil would be firing.
Remember: "if its worth measuring, it worth
Harvey from British Columbia
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