The Stovebolt Site Forums The Shop Area Driveline Speedomter ratio adapter questions 1/2 ton, C3100, 3 spd manual

Hi All
I have been debugging my recent failure of my speedometer ( 1955 first series, C3100, 235-6, 3spd manual on tree) and am now looking at the transmission itself, or what appears to be a speedometer ratio adapter on my truck. My speedometer and odometer stopped working last week and I tested the cable itself and the speedometer in place in the dash and they both test OK - here is what I have done so far ( Speedometer and odometer have been working since I have owned the truck starting about 4 years ago)
1) Using a drill, I spun the cable (unplugged both ends) from tranny end and speedo end spins with it - indicating cable OK
2) Screwing cable connector back into speedo back, I spun the tranny end of the cable again with drill and speedo needle works - holds steady reading and follows drill speed as well

So I noticed that the speedometer cable plugs directly into what appears to me to be a ratio adapter at the tranny and the adapter then connects to the tranny housing. I am now focusing on this adapter and how it hooks into my tranny

Q1 - I notice that I can rotate the adapter about 20 degrees around the attachment axis even though everything is tight connection wise - It has same type of attachment as speedo cable to tranny, since it goes inline, but it also rotates in place - is this normal?
Q2 - I am hoping to take it off for testing and was going to try to rotate the input to see if the output rotates on the adapter - If the output rotates, I am then guessing that the problem is in the speedo gear in the tranny housing - is this reasonable?
Q3 - If I can, I was going to remove the adapter and then plug the speedo cable directly to the housing and see if things work correctly - any reason not to do this or other test suggested?
Q4 - Any other tests I should try?

Interestingly, the speedo ( which was working for last 4 years) is off by about 7-9 mph (low) - which I attributed to the speedo itself - I did not measure if the odometer is accurate (my understanding is that the odometer is actually what is driven by the speedometer cable - speedometer needle is driven from odometer movement in the speedo itself) - so even if the adapter is functioning, I am not sure I should replace it anyways

Any suggestions would be appreciated

Joe

Joe,

Here is what I recently learned through trial and error (and getting schooled on here)
1. Odometer is run off the “tit”/ male portion of the cable.
2. There is a shoulder looking part above the tit that connects to the speedo part through a magnet. If the magnet is week, or not seated correctly, speedo will not read correctly due to slippage.
3. Guys here recommended that I tested mine by driving a known distance (Ie 10 miles on interstate) and observe how far odometer travels. Ie mine traveled 8.8 miles so it proved mine was off by certain percentage. (I am purchasing gear to compensate in the future)
4. I am also using cell app for speedo to check MPH so I can accurately guess actual speed.
5. I had issues with my cable not seating correctly. You can either try reconnecting it to ensure it is back correctly or leave one end hooked to your transmission (other end unhooked) and have a passenger observe to ensure it is spinning when driving. (I both)

I know that this doesn’t answer your questions, but might narrow down the issue.

Chris

HI Chris
Thanks for the response. I will add one more test from your list above, and that is to observe the cable while I drive to see if it is spinning with other end hooked - I expect it to not be spinning, since I already tested that the speedo needle moves when I manually spin the cable from the tranny end with my drill

Good luck. I thought same thing and it was my ignorance putting it together. Mine is now working well, just slightly off.

Chris

Those adapters are not the best things around, we used to service them on our school bus fleet all the time. We had a whole box of gears and drive shafts for them at one time. Pull it out and see if the input turns the output. The gears are just pressed on the shafts and when a tooth breaks off, the whole insides come apart. I believe the input side has the removable drive, sort a cable looking piece with a couple drive ears on it. These used to break all the time on us. If the adapter seems right, then one or both gears in the trans is worn or slipping. The adapters can be serviced, just remove the screws and clean out he old grease, just be careful of the gasket. If the housing is turning on the mounting shaft, I think you can tighten them up from the inside with a little ball peen hammer.
Joe

Hi All,
Well, I am officially confused. I ran two tests which should have been mutually exclusive and they both were positive

1) I hooked up the speedo cable to the ratio adapter and into the tranny and disconnected the speedo cable at the speedo end - I then pulled the speedo cable end into the cab so I could hold the inner cable itself between two fingers to see if it was rotating when I did a test drive. I took a 1/2 mile test drive and the cable end I was holding was rotating and increased rotational speed as I increased truck speed - I also applied as much pressure as I could to the cable and it continued to rotate - this indicated that the tranny, ratio box, cable portion (at the tranny end) of the circuit was all working, as it was delivering a a spinning cable to my fingers while I was driving - this indicated to me the problem must be in the speedo - I hooked the free end back into the end of the speedo and took another drive - no speed or odometer moment, indicating to me a bad speedo

2) After waiting for the exhaust to cool, I slid under the truck unscrewed the cable from the ratio box hooked it up to my drill and had a helper watch the speedo needle - much to my amazement, both odometer and speedo needle moved - I was able to get a full needle movement with my drill up to 90MPH -

One of these tests should have failed - first test indicates speedo/odo gauge bad, next test indicates speedo/odo good -

The only thing I can think of is that the end of the cable might be slightly disengaging when I hook everything up and not when I use the drill - I.E. there is something with the speedo cable itself - the other thing I can think of is that the adapter, while spinning, might not have enough torque under load ( although I squeezed the cable hard as I could to try to give resistance in test #1 above ) - I am otherwise at a loss - I was expecting to be here asking how to remove the speedo and buy a new one, but the cost of ~$350 made me retest again - I would sure appreciate some suggestions here

Is it possible that the speedo cable is too short? When you attach the drill you push the cable all the way in so the speedo works. When you are observing the cable turning while driving, the cable is pushed the other way into the adapter, so you can see it turning.

Try this,
1. Unhook everything
2. Ensure you slide the cable all the way into the back of speedo. Need to make sure cable is all the way in. Screw in tight.
3. Reconnect at transmission side. Ensure tight.

I know this sounds stupid. I actually took speedo out of dash to ensure I had it in all the way. If speedo works with drill and transmission is spinning the cable, it ican not be seated correctly. I had the same issue.

Chris

My guess is the end of the speedometer or the cable end, were the cable rides, is worn just enough to not spin the head. If the cable was longer it would engage deeper into the speedometer, which I suspect is happening when using the drill motor. Is the end of the cable still squared off? A new inner cable would likely solve your problem. Do you have a small ball bearing you can drop into the adapter end so the cable doesn't go so deep? This would put more cable at the speedometer head, just be sure the driven end is a blind hole so the ball bearing stays where you put it.

Joe