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Sync for Relative Compression Testing
Posted to Technical Tips Forum on 3/15/2014 29 Replies

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 test.

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 vehicle.

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 graphing"

Harvey from British Columbia

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