You can test the tach/hour meter with your cordless drill and a round file. Use the squared end of the file handle to put in the drive unit of the hour meter and the file in your cordless drill chuck. Can't remember proper direction of drill Fwd or Rev. If RPM needle goes in wrong direction then change direction on drill.
On the 4240 it is a German Diesel and the drive unit is up front behind the fan. IIRC there is a fiber washer that keeps the driven centered on the drive gear. If the fiber washer wears the driven gear pushes off the drive gear.
A similar problem occurs with the BD-154 Diesel engine with the tach drive gear assembly.
If you are in a climate where you get freezing temperatures it is very important to keep the tach cable lubed with a light grease so moisture does not collect and freeze the cable causing the cable to twist off or the tach drive gears to strip.