Speedometer drive weirdness

[rtadams89]

Just looking for any input on little things I may have missed.

I recently upgraded to 395 tires and as such wanted to replace the speedometer drive unit to get a more accurate speed reading. I bought a NOS .222 ratio drive unit, crawled under my truck and swapped it in. At first I put it in hand tight. After a quick drive to confirm it gave the right reading, I parked and tightened it down with a wrench. Next drive, I got no speed reading at all. I then removed the drive unit, put it back on, still no reading. I then started from scratch, installing each part from the transmission outward driving a few feet with each part to confirm everything was turning. I finally got to the speedometer cable, installed it, went for a drive and got a speed reading. Drove 8 miles parked. When I went to drive off, again had no speed reading. I got home and just swapped the old drive unit back in, worked right away.

Back on my work bench, I confirmed the new drive unit is turning properly, so I don’t think it is defective. My only remaining thought is a broken speedometer drive cable, but I replaced that only 8 months ago and it has been working fine the whole time, so that seems like a long shot.

Any my other ideas or things to check?

 

[rustystud]

You might be tightening the cable down too tight and it is binding. You only need to screw on the connector by hand until tight then just "snug" it up a bit.

 

[frank8003]

Under the truck chuck up the cable in your pistol drill and slow speed spin cable with drill reversed setting.
That will drive the speedometer if the cable is good.

 

[rtadams89]

I spend another hour troubleshooting before I figured it out. The speedometer drive adapter has a rubber washer in the portion that screws onto the transmission. That washer on mine was either too thick or swollen (though it was NOS still in the package) and was causing the square drive pin in the adapter to just barely engage with the drive shaft coming out of the transmission. For now, I pulled that washer out and now the speedometer works fine. Anyone know what size the washer is supposed ot be so I can get a replacement?