Speedometer Adjustment

A few weeks ago, Lowell Brown was over and we looked into his speedometer issue.  When he got his car, the trip portion of the odometer wouldn't reset.  Quite a while ago, we took out the speedometer and determined that the trip odometer cable was broken inside the speedometer.  Lowell went ahead and found a used speedometer and we installed it.

I had borrowed his old one back when I was having my speedometer problems as I thought I might need some parts.  I didn't, but I took it apart and figured out where the trip cable was broken.  There's a stud that the cable screws onto and that had sheared right at the base.  Probably unfixable, particularly since Lowell already had a replacement.

Lowell and I had talked about how the mechanism worked and in particular how the trip portion was a separate part and easily removable.  So we decided to swap the trip portion with the old odometer so the car would show the right mileage again.

After removing the speedometer in the car and taking both apart, it turns out that they aren't built the same.  His original had a metal geared trip odometer while the other one was plastic.  Of course the parts weren't interchangeable.  We did go ahead and set the mileage (ahead, not back!) to the proper number on the good speedometer and re-installed it.  However this wasn't the adjustment I'm talking about.

In order to take the odometer section apart, you have to remove the needle.  In my usual mindless way, I just popped it off without regard to marking the position.  The needle moves because of an induction caused by the speedometer cable rotating a magnet.  I'm sorry that I didn't take any pictures so this might not be too clear.  Anyway, the needle shaft and induction plate are spring loaded to return to zero.  But zero isn't where the needle goes since there's a pin at that stops the needle at 10 mph.  I didn't know how far to turn the shaft for 10 mph.

After thinking a while, I got out a reversible electric drill and hooked it up to the speedometer.  The speedometer needs to be driven in reverse and the drill at full rpm was able to run it around 26 mph.  We determined the speed by timing how long it took the odometer to move 2 miles.  Then we adjusted the needle to read 26 mph with the drill turning full speed.

I guess that it would be possible to measure the angle the shaft moves when moving a delta of 10 mph and then preset that angle from the resting position.  However, I think the method I used is more accurate.

So if your speedometer reads quite a way off, I bet that someone did the same stupid thing I did but didn't go to the trouble of setting the speed correctly.

Here's one trick that I figured out.  Getting the trip odometer cable back through the dash is quite difficult.  I found that making a tail with duct tape works.  You make it long enough to go through the dash hole before you put the speedometer in its hole.  Then once the speedometer is in place, you can pull on the duct tape and the trip cable follows right into place.

It's possible (and even desirable) to put the washer on and start the nut before removing the duct tape.  This prevents the cable from slipping back while you are trying to get the nut started.