I too out of the blue suddenly had a speedometer that would just stop working while riding. After looking here I decided to give the advice a try. 100% right, now 100% fixed. Here is what I found: There are two fex circuit "boards" (actually Mylar with printed traces). As you look at the instrument cluster with the back removed with the bottom facing you on the bench there is a three "plug" that mates up with a "socket" on the speedometer. A second four pin plug at the bottom of of the instrument that connects to a phenolic circuit board. Both of these suckers are tin plated as well as the receptacles. This is bad as movement and galvanic corrosion will eventually reduce the connection to intermittent at best.
The good news is that this an easy fix. Obtain a bottle of Cramolin "blue" B-5. Remove the pins by gently pulling them out, and I mean
gently and using a cotton swab dunken in the Cramolin gently clean each pin, you'll see where the contact point was because it will be a slightly discolored line on the pin. Reassemble. Go for a ride, bet that fixed it. Did for me.
Below is a photo with red arrows showing where the connectors are. A note about the instrument cluster in the photo an extreme low buck eBay find...it'll never work again, but I knew that when I bought it. It seems the glass had been broken and the bike was parked outdoors for an extended amount of time. Water was allowed to enter to sit in the cluster, also as a bonus there were not one but two spiders nests in there. Junk? Totally. A learning tool? Absolutely! Your cluster should not look like this.
Below is the 4 pin plug and socket. As you can see extreme cheap!
On this, I presume this is an earlier cluster, the plug would be a real Mo-Fo to remove, on the cluster on my K-100 the plug attaches with separate plastic bit, it could be unscrewed and pulled away. Pins swabbed with Cramolin, back together. Good.
Above is the three pin plug, the pins fit through three holes in the back plate and mate up with the three receptacles that line up in the speedometer PCB. Swab with Cramolin, put back together. Done. In my case there were were two foam squares used to stabilize the connection. It seems as there there were some changes/upgrades to the MotoMeter cluster over the years..so YMMV.
I took the speedometer off the back plate on my totally trashed guinea pig, there are three brass screws once loosened the speedometer (likewise the tach) can be pulled away. The metal piece you see through the backplate on the right (not in the photos) the speedo had been removed is the poll piece for the motor that drives the odometer, connected with some very delicate wires, so if you feel this needs to come free be careful. For this repair you don't need to do this.
While you are at it this would be a great time to swap out those burned out bulbs, maybe with LED's? Just watch the polarity (didn't explore to find which is which). Purple would look great! Um, maybe not. I didn't make this mod BTW..I like the retro incandescent look.
Also, pull the odometer reset knob OUT when removing the cluster from the case, this will allow the shaft to clear the instrument card. For this repair you don't need to take it this far apart though.
Hopes this helps someone.
- John.