The Stovebolt Site Forums The Shop Area Driveline speedometer acting up on my 1941 1.5-ton

Hi, I wasn’t able to find anything in the forums quite like this. I have a 1941 1.5 ton, the speedometer is acting up. It had traditionally worked fine and when we got to say 4pm or 50 mph, it would wave back and forth between about 25-40.. now, it is working up to around 30 just fine. When we go faster, it pegs to like 80-90 and sits there. Odometer still racking up miles.

Is it the cable or internal to transmission?

Remove the cable from the sleeve and lubricate it with lithium grease.

Speedo cables like graphite for lubrication. Grease could tend to solidify and make things worse.

Let’s start with simple test. Drive a mile at 25 by using a gps phone app. See if it’s a mile on the truck od. Then do it at 50. If it’s a mile both places then you have a gauge itself issue. If it off at 50 but not 25 then going to say cable. If both off. Lots of things could cause it. But let’s do the easy test first

I've always been told that graphite is absolutely NOT supposed to be used on speedo cables. White lithium spray-on grease is preferred.

Checking on a number of forums, it seems that it's about 50/50 between white lithium and graphite based lubes. Nowhere did I see NOT to use graphite. The 47-51 Service manual calls for AC speedometer cable lubricant, whatever that is. All of the stuff I saw said not to gob it on, and the service manual says lube only the lower portion of the cable. One guy mentioned having a pile of grease under his dash as it got pumped up the cable.

That's just what my high school auto mechanics teacher told us and it stuck in my brain all these years. It's possible or likely that at that time he was talking about the powdered graphite. Certainly lubricant technology has changed over the years and even in the last 20 years. Rules of thumb that used to be gospel no longer apply.

The 3 month/3000 mile oil change interval is an example. I still go by the 3000 mile even though I know today's oil can easily last 5000 or more miles.

Gocart, If you have checked the condition of your cable and lubed it as needed, and your speedo still does this erratic movement, give a read to this reply from member "Jon G" to another Bolter on a way to check and fix your possible speedo problem. Dried up/crusty internal lube in the speedometer head can give the same erratic action results you speak of.

Before you you lube the cable, remove the cable wire and spray some brake cleaner through the cable housing and blow it out with compressed air. Repeat until clean brake cleaner comes out the other end.

I'd buy a new cable.

I was just getting ready to mount my cab to the frame (scheduled to happen tomorrow) and thought about checking the speedo cable. Stuck the end into the tranny and spun the cable. It should do that, so I dug out a new cable that had been hanging in my dad's garage since about '96 (that was to fix a speedo issue on the car I bought for my daughter for college - which turned out to have an electronic sender and speedo. ) So it got put to use today on the '51. Stuck the end in the tranny and no spin like it should have been.

A new cable like Wally suggested might be a good idea.

You might try putting a drill on the cable while connected to the gauge to see if it gives you a consistent reading using different speeds.
On my 1940 truck, the spedo needle is pressed on a tapered shaft. Sometimes it comes loose a bit are higher speeds.

Mike