The Saab VDO speedometer is a good quality instrument. But like all of us, some parts get tired and gummed up after many years, and...uhh...some parts quit working. My drawing shows a dual register VDO speedo with the internal gubbins phantomed in.
MILEAGE recording--on the odometer registers--is accomplished through a double set of transfer gears. My phantom drawing shows the SIDE set of transfer gears. Interestingly, SPEED indication is done magnetically--there is no physical connection from the speedometer cable input to the speed indicating needle. The input shaft is connected to a small magnetic drum that rotates inside a sort of cup that is connected to the needle. The faster the speedometer cable turns, the faster the magnetic drum, and the farther around the speed dial the needle moves. A tiny hairspring insures that the needle always returns to rest at the "0" peg. So now you know, generally, how the thing works.
Next is how to get it apart so what DOESN'T WORK can be fixed/replaced.
I designed and built a fixture [clamped in my vise] that holds the speedometer firmly in place. The ring removal tool [with the green handle] has a specially ground tooling tip that just fits under the stainless ring, that, as I move the tool around the speedo, gently stretches the stainless ring until it pops off the speedometer. CARE and PATIENCE are vital here, but the ring does come off with NO DAMAGE!
I've got the ring off--NO DAMAGE! Below it is the glass, and below the glass a painted trim ring. Now I can get on with the work on the speedo's "innards".
A look at the special fixture that secures the speedometer. Without this fixture it is REALLY EASY to booger up the very soft stainless steel outer retaining ring. I've completely rebuilt a number of these VDO speedo's now--for Saab, VW and Porsche--so if yours is AWOL in the function department, I can rebuild yours, too. Give me a shout---541-499-0246 or email me at email@example.com.
Next---I take you into the VDO "innards". I know---you can hardly wait....
City Car Design
6 years ago