Speedometer goes no higher than 15mph

[Dennis Detweiler]

My 39 Plymouth speedometer will go to 15mph and stops there. Is this an easy fix?

[greg g]

First thing to do is clean and lube the cable.  Pull I off the back of the speedometer.  Protect your interior with plastic or drop cloth.  Pull the worm out.  It will be covered with oil and crud so act accordingly.  Clean with carb or brake cleaner, re lube with graphite based lube and reinsert.  Needle might be dirty and dragging,you might want to try to pull a dash light bulb and blast some very low pressure compressed air toward the base of the needle.  The cable turns a magnet, the needle has a disc that follows the magnetic field against a spring. If the disc  and spring get dusty, it can cause more friction then the magnet can over come.  

[Don Coatney]

On the back of your speedometer you should find a lubrication port. The one in the photo was not installed correctly at the factory. It should be pointing upwards as gravity makes it work. A couple of drops of light machine oil may get you up to speed.

 

 

 

 

[greg g]

It seems funny that there is a discontinuation of the bead roll to accommodate the lube tube and its at 90° not vertical.

[Don Coatney]

13 hours ago, Dennis Detweiler said:

My 39 Plymouth speedometer will go to 15mph and stops there. Is this an easy fix?

 

Someone has to say it so I will. Next time. you get up to 15MPH try shifting into second gear. ?

[DonaldSmith]

My dad was the always-in-a-hurry type. He told me once of the time he was driving with his father, my grandfather.  Grampa took off, and then shifted from first to second.  Dad suggested that he shift into third.  Grampa responded, "I'm going fast enough."  

[DrDoctor]

Don,

   I looked at the photo of the back of the speedometer, and I was so curious that I went and stuffed my head underneath the dashboard to look at ours (it really wasn’t as difficult as I’m trying to allude, since I took out the vacuum wiper motor, and the wiper linkage, to rebuild the vacuum motor myself, and if it doesn’t’ work, I’m no worse off than I was before . . .)—it doesn’t have that tube as the one in your provided-photo does. Regards . . . . .

[FlashBuddy]

No "tube" on mine. 

 

[DonaldSmith]

My 47 DeSoto speedometer (out of the case) has a felt-lined oiling hole near the end of the threaded part that receives the cable.   

 

[Reg Evans]

17 minutes ago, FlashBuddy said:

No "tube" on mine. 

 

 

Unscrew that tube with the knurled end  and you will find a piece of felt that should be soaked with oil periodically.

[sser2]

From the way speedometer works, the problem of low indication is not in the cable or drive gear. These things are rigidly coupled to transmission, and they either rotate at the right speed, or don't rotate at all. Lack of lubrication in these parts may cause grinding sounds and premature wear, but not low indication. The problem lies in the magnetic coupler. As permanent magnets become weak, coupling decreases and indication becomes incorrect. Old time magnetic couplers used steel magnets. These magnets lose their magnetization faster than any other type. 

 

The only way to radically fix this issue is to take the coupler apart and either replace or re-magnetize the magnets.