I'm going to learn something today (I hope).

[BButturff]

I've been working on cars and trucks for about 50 years but what happened yesterday is the kind of thing that keeps me up at night.


About a week ago I was driving my Model A and the speedometer stopped working. I crawled underneath and disconnected the cable from the housing and, sure enough, about 3" of the cable had twisted off. I don't know why it broke, but the cable had plenty of lube and the speedometer spins freely so who knows? Maybe it was just time.


Fast forward to yesterday and my new, complete speedometer cable arrived. I measured it and found it to be the same length as the old one. After properly lubricating it, I installed it without a problem. However, it was raining here again and I didn't want to drive my A in the rain to test the new cable,


Then I remembered reading here on the Ford Barn that you can jack up one wheel, chock the other three, start the engine and spin the jacked tire to test the speedometer. Seems reasonable enough so that's what I did. The result? NOTHING - The speedometer just sat there and stared back at me.


Something MUST be wrong! So I disassembled the cable, spun the speedometer with my fingers, spun the speedometer cable with a drill, watched the speedometer drive happily turning with a short piece of cable inserted..........everything worked as it should. Then I reattached the cable to the drive and speedometer and performed the jacked tire trick again. The result? Nothing - The speedometer just sat there and stared back at me even harder. Something MUST be wrong! So I decided the speedometer drive must be the culprit. But all this jacking and spinning and disassembly and reassembly has made me tired and it's late so I decided to give it a fresh look in the morning.


Fast forward to today. It wasn't raining (rare here this year) so I decided to postpone my speedometer troubleshooting adventures until tomorrow when it's going to be raining again, lowered my A from the jack stand and took it for a drive. The result? The speedometer works PERFECTLY!


Does anyone have a explanation for this?

 

[TMarsh]

My speedometer was rebuilt and work as it should. I takes a few revolutions on the drive train to get the speedometer to respond.

 

[nkaminar]

Maybe the end of the cable was not seated in the drive down my the transmission and your driving added enough vibration to get it seated. Otherwise, I have no idea how this could have happened.

 

[BButturff]

Maybe the end of the cable was not seated in the drive down my the transmission and your driving added enough vibration to get it seated. Otherwise, I have no idea how this could have happened.

Yes, I thought of that too and the square cable end appears to go into the drive a good 5/8" of an inch. Several times I was able to verify this from the speedometer end and could feel with my fingers that it was engaged. I also considered that it might be going too far into the drive so that the square end on the drive was unable to engage the round cable. I confirmed that this wasn't possible by probing the drive with a short section of the old cable. The only thing I have not done yet is to remove the speedometer drive to see if there's any way it could operate intermittently. It'll be a week of two before I'm able to do that.

 

[Big hammer]

When I’ve tested a vehicle for noise, vibration, etc, I’ve jacked up the rearend and put it on sturdy stands both wheels off of the ground. It’s a bit scary so I would be ready to push the brake and clutch encase the vehicle would come off the stands. By chance did you push the clutch down and not then spin the one rear wheel ? The other thought did you have it in reverse, I don’t think the A speedometer works in reverse. One other possibility is with only one wheel off the ground does it take twice the speed of the wheel to get the speedometer spinning ?

 

[BButturff]

When I’ve tested a vehicle for noise, vibration, etc, I’ve jacked up the rearend and put it on sturdy stands both wheels off of the ground. It’s a bit scary so I would be ready to push the brake and clutch encase the vehicle would come off the stands. By chance did you push the clutch down and not then spin the one rear wheel ? The other thought did you have it in reverse, I don’t think the A speedometer works in reverse. One other possibility is with only one wheel off the ground does it take twice the speed of the wheel to get the speedometer spinning ?

Of course, when something like this is tried the tires must be chocked both in the front and rear of the tires and reliable jack stand(s) must used. I can't stress this enough but DO NOT try this without jack stands and proper wheel chocks. The amount of vibration and shaking the vehicle does is surprising and much, much more than expected.


To add to your thought, yes, I started in first gear, shifted to second and then to third on two different occasions. Both times I estimated the wheel was turning for about a mile. I'm not sure about the wheel speed needed to get the speedometer spinning but the speedometer is driven off of the drive shaft inside the torque tube, right? I'm no expert but it seems to me it shouldn't matter if one wheel or two. I hope someone that knows for sure will answer that part for us.

 

[katy]

The only thing I have not done yet is to remove the speedometer drive to see if there's any way it could operate intermittently. It'll be a week of two before I'm able to do that.

Be happy it's working, leave it alone.

 

[BButturff]

Be happy it's working, leave it alone.

I'm happy that it's working, well, kind of. It's not in my nature to not understand the "why" of things. I think its possibly a form of OCD but I don't consider it a disorder at all. Just a need for knowledge and understanding. As a young boy it drove my parents crazy. I would get something new as a present and immediately take it apart to see how it worked. Sometimes I even got it put back together so that it worked again.

 

[Benson]

1. 28 and 29 and some 30s (oval speedo) cables are .120 inch square one end and round with tab other end.


2. 30 and 31 (round speedo) cables are .104 inch square end! Square both ends

So if you put 0.104 cable on a 28 -29 speedo you have a problem as you describe.