Electronic diagnostic compression testing reliable?

[hedges88]

I'm aware of how easy compression testing is on an engine but I'm just wondering if compression testing through diagnostic software is reliable or accurate?

I'm just getting used to Mercedes-Benz Xentry/DAS which has a compression test cycle (I'm sure other manufacturers have similar systems) where it disables fuelling while you crank the starter for 10 seconds and then gives a measure of any RPM differences between each cylinder.

As my spark plugs that are nigh on impossible to reach I'm hoping someone will know if this is at least a vaguely reliable indication of compression compared to doing it the old fashioned way? Despite having a brand new battery it's advised by the system to use a booster to maintain power for cranking and diagnostics during the test so I don't want to invest in a booster if software based compression testing is unreliable or inaccurate

[stevieturbo]

Depends exactly how it is performing the test, and what measurements.

But in general, external relative compression tests via a current clamp and scope, can yield very good info and is quite common.

But as always, is one of many tests.