Scott,
I'm going to take a wild guess here as I'm not familiar with Smiths gauges but many temp gauges are based on thermocouples. Basically what you have within the tube (sheath) is two wires surrounded by a ceramic filler. The wires are of different alloy and where they connect at the tip, the dissimilar metals generate a small voltage (as temp changes so does the voltage..that's how the gauge works). What has likely happened is that one of the wires within the tube has broken. We used to repair our thermocouples at work on occaision but it's often impossible to find the break. You should get continuity between the two leads, if not I would replace the thermocouple (the tube).
What color are the wires going to the gauge? Thermocouples are usually coded by an international scheme according to type. The most common is "K" type which has red and yellow wires. If I remember correctly these are what Autometer uses. If your gauge supplier won't replace just the lead or is asking outrageous $$ you may want to check with an industrial supplier like Omega Engineering (
www.omega.com), they can get any type of thermocouple in any length and sheath size.
Hope this is of help, (I'll soon be in the process of wiring my car up with a 10 channel digital gauge with "K" type thermocuples myself)
Regards,
Francis "Turbo II"