There are two potential causes of this condition. If it is a mechanical starter drive (Bendix) and sticks intermittently, it most likely needs to be lubricated. This type of starter drive is initially lubricated to allow smooth operation. The drive gear must move in and out and also rotate. In use it can become dry and may begin to stick and fail to engage or disengage. We recommend lubricating this type of starter drive every fifty (50) hours of operation.
The other likely factor for intermittent engagement would be a faulty starter contactor. If you want to check a contactor, it is best to do so with a load applied. Resistance in the contactor, even a fraction of an ohm, is enough to produce poor or no operation due to the high current field. In a 12-volt starting circuit drawing 60 amps, for example, the addition of 0.2 ohms of resistance due to an oxidized connector or carbonized contact may drop the current to the load to 36 amps. This is a 40-percent loss in starter operating power caused only by a 0.2-ohm resistance.
Checking the circuit without a load would require that a lot of resistance be placed in line. The best method of checking the contactor is to disconnect the lead from the starter and connect it to a battery load-meter tester. Activate the contactor and check the voltage on each side of the contactor. A 0.1-volt drop is normal for a new or nearly new contactor. If you find any more than 0.3-volt drop, replace it.