What is a relative compression test?
Correct functioning of an engine depends on many things like e.g. correct sensor readings and good functioning actuators. But the engine itself also needs to be in good condition. With a relative compression test can easily be determined whether all cylinders of an engine have the same compression.
This relative compression test uses the starter motor current to determine the compression values of all cylinders. The advantage of a relative compression test is that no pressure sensors are needed to check each individual cylinder, all cylinders can be tested at once with just a lab scope and a current clamp.
How do I perform a relative compression test?
Before performing the relative compression test the engine needs to be prepared such it will not start during cranking.
To perform a relative compression test with a Current clamp TP-CC600, first download the file below.
Connect the Current clamp TP-CC600 to the lab scope Automotive scope ATS5004D with a Measure lead TP-C812B and clamp the current clamp around the wire from the battery to the starter motor, see figure 1. The direction of the wire through the clamp must be such that the starter current will introduce a positive output voltage on the clamp.
Open the file in Multi Channel oscilloscope software and proceed as follows:
- Switch on the current clamp and zero the current clamp.
- Start the measurement with the start button or press hotkey S.
- Crank the engine for 2 to 4 seconds to perform the relative compression test.
The relative compression test is now ready and will look like figure 2.
The relative compression test in figure 2 is done on a 4 cylinder engine in healthy condition. Some variations in the result of the relative compression test is acceptable.
By adding an ignition measurement to the graph, measured with channel 2, the exact cylinder can be identified with a current ramp.
- Toyota MR2 bad injector
- A Toyota MR2 is having problems after an engine replacement, fault code P0304 Cylinder #4 misfire detected occurs. The garage swaps several components but does not manage to fix the problem. Measurements with an automotive oscilloscope are required to find out that the fault code is somewhat misleading and the problem is not with cylinder #4