Speedometer Drive

If it isn't one thing, it's another thing to do on an older Ferrari.  During a trip to our place in Whistler, BC, the speedometer and odometer quit working.

I first thought that it was a problem in the speedometer head itself.  So I spent some time checking that out.  You can read about that here.  I had undone the cable at the speedometer and didn't feel it rotating when driving back and forth in the driveway.  So I eventually got around to putting the car on the lift and undoing the cable at the transmission.  Then I ran it using a drill and had my wife looking at the speedometer to see if it was working.  She said that it wasn't, hence the work inside the dash.

Once I figured out that the speedometer was working and also worked when I used the drill at the transmission end, I knew that the problem was either in the right angle drive or inside the transmission (ugh!).

I did find that the right angle drive was a little loose, but tightening it didn't help.  I tried to remove it, but there isn't enough clearance.  As you can see, the left end of the drive is right against the fiberglass of the tunnel.

So I had to remove the driver's seat, unbolt the rear and loosen the other bolts holding the tunnel.  Then I was able to raise the tunnel the 2" or so that I needed.

Once I had the drive out, I was confused.  There was a round shaft sticking out of the transmission and a round hole in the drive.  I had expected a square shaft of some sort.  The nice thing about having another car is that you have an example to look at.  So I removed the speedometer drive from the PF coupe transmission.  That had a square shaft sticking out of the drive.  So I went back to the 330 and pulled on the shaft sticking out from the transmission.  It came loose and did have a square end on the transmission side.  Here you can see the PF coupe drive on the left with the shaft in it with the 330 drive on the right and the shaft at the bottom.

The shaft on the PF coupe drive would move around some, but clearly wouldn't pull out.  So I decided to take apart the 330 drive to be able to understand what was wrong.  To remove the parts on the transmission side, there's a small roll pin that you drive out.  Then everything pulls out the side.  I didn't take apart the other gear as it's crimped in place (and I didn't need to).

As you can see, the shaft at the bottom is in two pieces where it is supposed to be one.  Tomorrow, I'll get some 3/16" square or round steel stock (or a long 4mm bolt with 2½" not threaded) so I can make a new shaft.  This is really a job for a lathe and milling machine, but since it's pretty simple, I'll try making it just using a file, grinder and Dremel.  The other interesting thing I found out it that there's an oil hole with a spring loaded cap.  You can see sticking out from the left side of the housing in the picture above.  So I guess that you should put a few drops in it every time you grease your car.

I don't know why the shaft broke.  The speedometer has always worked nicely.  The square part that sticks in the transmission is worn, so perhaps that and the slightly loose fitting let it bind enough to break.

More tomorrow when I try and make a new shaft.

I was able to get some 1/8" square stock.  The actual size of the original was 3mm.  That's .119" whereas the 1/8" is .125", only .006" larger.  My first problem was to figure out how to chuck the square stock as I don't have a four sided chuck.  I ended up taking apart an old tap wrench and chucking it in a drill.  It's designed for taps which have four sides, so it held the square stock fairly well.  Here you can see everything put together with the one end being rounded down to the proper size.

This worked fairly well even though the stock did wobble some.  As you can see, I was able to end up with a reasonable copy.  The slot for the C ring was done using a Dremel tool with a very narrow cutoff wheel.  Note the wear on the square portion on the left.  This is the part that fits into the transmission.  You wouldn't think that it would wear this much, but after 140k kms, there is.

I did have to file each side of the square portions to remove the extra 6 thousandths of material.  I checked the fit in both the transmission and the gear in the right angle drive.  Here's the whole unit ready to be re-installed.

After installation, I lowered the lift to where the tires were still off of the floor and started the car.  Then I put it in first gear and let the tires spin in the air.  Argh, the speedometer still didn't work.  I was in a rush so I had no time to look into the problem then.  The next day, I undid the cover under the dash and found that I had forgotten to re-attach the cable to the back of the speedometer.  Once that was done, the speedometer worked again!

Now I just have to get the tunnel bolted back down, the cover snapped in place, the belly pan and the seat re-installed and I'll be done.