Snowmobile Forum: Your #1 Snowmobile Forum - View Single Post

**BC_Dan** · 12-08-2007, 01:54 PM

Most snowmobiles have a mechanical gear driven cable that drives the speedometer head to indicate speed and miles driven. The speedometer gear is usually turned by a pressed wire speedometer key, usually driven by the end of the driveline. The key is inserted in a bushing or square hole in the driveline and a similar hole in the gear adapter.

When the speedometer fails, there can be several reasons for failure.

Failure of the speedometer itself.

Speedometers are pretty reliable, unless they are subject to damage or immersion. If all other checks are ok, it may be the speedometer. Repair to speedometers is expensive. It will probably be cheaper to obtain a different speedometer for the sled.

Failure of the cable

The cable that goes from the gear adapter to the speedometer is subject to wear and tear. Kinks or physical damage can render the cable inoperable. If the cable can’t turn, it can’t drive the speedometer. Surface damage to the outer sheath of the cable can allow moisture to penetrate to the interior of the cable, causing rust, which can also freeze up the cable. Lack of lubrication can also cause the cable to stop operating properly. Sometimes removing the cable and cleaning and lubricating the cable and sheath can bring a bad cable back to operating condition.

The cable is easy to test. Remove the cable from the speedometer gear adapter and attach the cable end to a drill. Run the drill and see if the speedometer moves. If so, the cable and speedometer are good.

Failure of the gear adapter

The gear adapter that the cable is attached to is a mechanical device and is lubricated by the grease fitting on the gear adapter. If it does not get adequate lubrication during maintenance of the sled, it can freeze up.

To test, remove the gear adapter from the sled and attempt to turn the gear with a small screwdriver from where the key adapter inserts on the driveline side of the adapter. As you turn the screwdriver, the gear inside should turn and you will see rotation of the speedometer end of the gear adapter. If the gear adapter does not turn easily or you do not see rotation, the adapter is suspect. Sometimes cleaning and lubrication will be all that is necessary.

Failure of the driveline to gear adapter wire key

The wire key that drives the gear adapter will sometimes shear, not allowing the rotation of the driveline to drive the gear adapter. This is the most common reason speedometers quit working, and the most critical to fix the cause. The driveline of the sled has bearings on both ends. One is in the chaincase, the other is held by two flanges on the PTO end of the driveline. The driveline PTO end bearing is usually a sealed bearing and does not get lubrication when the gear adapter is greased during maintenance. The inside of the bearing is open to the tunnel and is subject to all the snow, dirt, debris, and elements that the sled is driven through. Over time, the bearing starts to fail. The failure of the bearing creates a condition where the hole in the end of the driveline and the hole in the gear adapter are no longer in alignment. The movement of the driveline will shear the speedometer drive key. The speedometer gear adapter is no longer driven, and the speedometer stops working.

The problem is not the speedometer or the cable or the gear adapter. It’s the bearing failing. If the failed bearing is ignored and the sled continues operation, the bearing will fail catastrophically. The bearing race and balls fail, resulting in the ball bearings falling out. Now there is no bearing surface for the driveline. Continued operation will result in the weight of the sled resting on the driveline end, causing damage to the driveline and to the flanges holding the bearing. Eventually, the damage will shorten the driveline to the extent that the driveline can drop completely out of the bearing and into the track. If that happens, the gear and chain on the other end of the driveline pivot out of alignment. Damage to the tunnel, track, gear, chain, and chaincase can be very significant. If the failure happens at speed, serious injury to the driver of the sled could occur.

If the speedometer quits working, make sure what the cause is. Use the failure of the speedometer as an early warning sign of impending driveline bearing failure.