My 2000 Mercury Mountaineer's speedometer and odometer just quit working this weekend. How do I tell which cable is it and can I replace it?

If the speedometer is still working, but the odometer does not, most likely the cable is broken (separate drives for odometer and speedometer within a single cable) or the dash unit needs replacing...

its located on transfer case rear end near propeller shaft , have 3 wires plug

There is a little plastic gear that has striped you can fix it but youhave to had a dash to get the part find a used dash and just replace it

Sounds like a broken cable. The cable runs from the rear of the speedometer to the gearbox/transmission. Sometimes the cable can come out of speedometer but not usually on it's own. If you have done any work on or behind dashboard then you may have dislodged it. I would check there first.

The odometer quits working when the speedometer quits working. The most common cause for the speedometer to quit working is the speed sensor in the transmission. You said you replaced it, there is also a transmission speed sensor on your truck that may need to be replaced. I understand your frustration, but there is nothing else that would cause the speedometer to quit except the cable behind the gauges being clogged.

Check the fuses for the speedometer, odometer and cruise control.

You need to replace the speedometer gear that drives the sensor.

Could be a ciggerette lighter fuse, usually cause this when it blows.

it could be a fuse....since a fuse is use just incase you get an overflow in current...and this prevents damage to gauges......

yeah check it first.....

replace a capacitor behind the panel (6.8uF 25V) with the new one.