82 300sd odometer not working
 

I brought the car home 3 days ago and it was busy counting the miles as I drove the 70 miles home. It sat the next day, and last night I took it for a little spin and the trip counter and the odometer were not counting! The Tach and speedometer work ok as do the other gauges on the dash.

Maybe I hit a time warp and did not really travel the 20 or so miles I went last night :)

Any ideas on where to start?

odometergears.com
the SD has plastic gears that disintegrate over time... your time is up.
it's also possible that the transmission sensor failed, but then your speedometer would not work either.
it's pretty easy to pull the cluster... just grip the edges of the "glass" and pull it out. reach behind it and unhook the huge speedometer connector, and then note where the lights go, pull the other wires and you have it out. open it up, and get to the gears, count the teeth on the damaged gear, and order new ones.

wonderful. Do you have a link or know of a write up for how to take out odometer and replace the gear in there?

it's not difficult. there is a writeup on the 123 D's, but not on the SD's. it's just so simple. pull the cluster, (if you have problems with your turn signals, now's the time to change the flasher...) pull the cover off, and pull out the speedo, then change out the gears that are garbage.
some advise using coat hangers pushed in around the cluster, but you can just grip it with your fingers and pull it out.

you can start reading threads here...
http://www.peachparts.com/search.htm?cx=015238323195528004774%3A_wneywo5nvm&cof=FORID%3A9&q=SD+ODOMETER+repair.&sa=Search#1098

HA! the second one in the search is ME!
wow, amazing to see how much I didn't know back in 06...

Alrighty then. Thanks for your help. I plan to spend a bunch of time on this thing over the weekend, guess I have another thing to add to the list!

The light that lights up the cluster at night is really dim. Its on, just not bright. I guess Ill replace that too while I am in there

if it's dim, odds are good it's your dimmer switch... they are difficult to fix. most just bypass them for full voltage to the bulb.
you could also have a melted prism where the bulb mounts... try alcohol to clean it.

there is a dimmer switch?!? haha. ill probably just bypass it.

yup. your odo reset switch twists to dim the light...

Check for the correct bulbs as over-wattage bulbs will melt the prism, reflector and holder.

Also they are not very bright to begin with so your rheostat may be fine. The optic prisms are extremely inefficient and waste a lot of light. I've replaced my bulbs with LEDs....it took 6 Ultra-high-intensity LEDs on each side just to make the gauges visible at night. Bulbs were nearly unilluminated.
All I have heard is the rheostat completely failing, making the entire gauge cluster and center console fail to illuminate. Although since it is often the springs failing inside, I suppose it could easily get stuck on half-dim. The rheostat is very easy to jump, solder a wire in between the only two soldered joints on the instrument cluster board behind the unit.