I have a 5210 and I think the 5105 is close to it. Your problem is in the cluster, the wiring or the alternator. I would check the wire that goes from the red wire the goes from the alternator to the cluster. Here is what I would do on my tractor.
Disconnect the negative side of the battery (I find it is easier to take out the bolt that connects the negative battery cable to the frame). Take out the 4 screws on the panel that the key switch is on. Remove the 3 bolts that hold the left side cowl and 2 screws on the instrument cluster. You should be able to unplug the small plug on the back of the cluster. Using an ohm meter test the red wire 082 on this plug back to the other end at the alternator on the alternator the wire is 082 or 072.
I have found that if the tach is disconnect the hour meter shows hours, but does not increase hours.
Disconnect the negative side of the battery (I find it is easier to take out the bolt that connects the negative battery cable to the frame). Take out the 4 screws on the panel that the key switch is on. Remove the 3 bolts that hold the left side cowl and 2 screws on the instrument cluster. You should be able to unplug the small plug on the back of the cluster. Using an ohm meter test the red wire 082 on this plug back to the other end at the alternator on the alternator the wire is 082 or 072.
I have found that if the tach is disconnect the hour meter shows hours, but does not increase hours.
