Tachometer and odometer not working

I assume this is not the digital dash since the tachometer has a pointer. That means the odometer is a mechanical unit with rotating number wheels. There is a small gear for the odometer that breaks. Many years ago I read a post where the person found a source for those gears, and he bought about ten of them. Unless you can find that source through searching the internet, your best bet is to find a good one in a salvage yard. You also might experiment with gluing the pieces back together if you can find them.

The dash gauges are no longer mechanical with spring-loaded pointers. The pointer sits on the armature of a "stepper" motor. Those do not have brushes and they do not spin like regular motors. Instead, computer circuitry pulses two pairs of electromagnetic coils with varying voltages and polarities to place the armature at the desired position. When the circuitry is turned off, the pointer stays right where it is. If they return to "0", it is because the circuitry is designed to pulse them to "0" when the ignition switch is turned off.

Since stepper motors do not have spring-loaded pointers, when they are commanded to go to "0", the armature is going to rotate in whichever way it is shortest to do that. If the ignition switch is turned off while the pointer is rotated over 180 degrees, the shortest way to "0" is clockwise. The problem is there is a small mechanical stop peg, and the pointer gets stuck behind that. Now, the next time the engine is started, the armature wants to move clockwise to the current rpm, but it can't because it's on the wrong side of that peg.

There's three ways to return the pointer to the correct side of the peg. The hardest is to remove the face plate, then push the pointer counter-clockwise by hand. The next is to use a scanner to perform the gauge test function. That will command all the gauges to move to 1/4, 1/2, 3/4, full, then back to "0". After reaching full scale, all the gauges will follow the command to go back to "0". The easiest way to reset the pointer is to simply run the engine momentarily faster than halfway around the dial, then let it return to idle. For example, in my sad drawing, half scale is 4,000 rpm, but if you look at the pointer, 180 degrees is close to 5,000 rpm. At the instant the engine reaches that speed, plus a little more, the shortest way to the correct reading is to move the pointer counter-clockwise. It will jump there, then follow engine speed back down like normal. Any of these three procedures will work for the speedometer too. You have to reach the reading that is straight across from "0", plus just a little more.