With the aid of a DDE function, statements with regard to the status of the engine compression can be made with DDE 4.0 and DDE 4.1.
With this function, the DDE facilitates measurement of the engine speeds for each individual cylinder in non-ignition status, i.e. at starter speed similar to smooth running measurement. The following are the decisive factors for the measured value of a cylinder: the first 60° after firing TDC (power stroke) for a 6-cylinder engine and the first 90° after firing TDC for an 8-cylinder engine. In this range, the cylinder-specific speed is calculated by way of the measured angular velocity.
In unfired status, i.e. when driven by the starter motor, the extent of compression of a cylinder has a major affect on its cylinder-specific speed and thus conclusions can be made as to the compression status.
During the power stroke of a cylinder with poor compression, the lower pressure causes the power to decrease in ”assisting” the starter during compression of the next cylinder. This reduces the cylinder-specific speed in the power stroke of a cylinder with poor compression. The poorer the compression in a cylinder, the lower will be its speed.
For the start of the measurement, a minimum speed of 150 [rpm] must be achieved by the starter motor. The measurement is started automatically once this limit has been exceeded. The DDE automatically prevents the engine from firing.
The end of the measurement is signalled, after which the test results are calculated and displayed. The program indicates the speeds of the individual cylinders as well as the difference between the highest and lowest speed.
Example of a possible display:
Difference between lowest and highest speed: 32.8
Of decisive importance for subsequent procedure is the extent of the determined difference . The program automatically evaluates the difference with corresponding next instructions.
Reference values up to which compression is deemed OK:
No statement can be made with regard to the condition of the engine from the absolute speed levels.
Important:
In the event of ensuing troubleshooting, start at the cylinder with the lowest speed value.