The Stovebolt Site Forums The Shop Area Driveline Speedometer question 49 3100.

Why does my speedometer go towards 0 when I accelerate? As I let off gas or coast down a hill it go to correct speed.

When you drive normally, does your speedometer reach a certain speed (for example 40 mph) and then stop? When you coast down a hill or decelerate, does your speedometer increase speed over the speed it shows when you're accelerating?

No it seems to go to correct speed even above 40. If not on gas. But as soon as I step on gas it goes down to 0.

Is there a possibility something is pinching your speedometer cable? Maybe your accelerator linkage (although that would be a stretch)? It sort of sounds to me like when you accelerate you're doing something that causes your cable to stop turning or to turn much slower. Possibly your rear motor mounts are bad and your motor (and thus your transmission) are lifting up on acceleration? Your motor turns clockwise, so you would be lifting up on the passenger side. If that is happening, your speedometer cable (where it connects to the transmission) could be getting enough strain that it stops turning as well.

The bolts/nuts holding your rear motor mounts could also be loose. I think I'd check them. You might also (a) tell your wife/girlfriend you've cancelled any life insurance you have and have altered your will to give all your worldly possessions to me (you don't have to actually do this...just announce you have) and then (b) slide under the truck while she races the engine to see if it is lifting off of the rear mounts. Then later after a glass of wine or beer, tell her you were just kidding...and prepare for the temperature in that room to fall by 5 or 10 degrees.

I will check that but all of the things under there are new as its a new motor. I did just replace the oaky bushing recently. Had transmission mount and such out. Will give update when get time to check as we are at a show today in this awful heat.

I'll be very curious. Sorry about the heat. We get a lot of that here. Good luck.

I think that the speedo cable can’t “not turn” from pinching and then work again, the core would snap and be done
But it can pull out and then make connection again
If the square ends on one of the ends might be just inside the tiny square drive part that it’s pulling out when there is strain like the engine and tranny moving on the mounts

But pinching the cable would be a one time fail as there is no allowance for it to slip so it will twist off permanently.

I would disconnect the cable from behind the speedo
And watch it as I drive to see if the center stops spinning or not
Trace the issue
Often one of the square ends breaks off, perhaps that has happened to your cable but there is sufficient friction to make it work intermittently still.
-s

I checked the cable and all mounts are tight. Put drill on end of cable and speedometer works fine. It's a new cable so don't think it is the problem. Would it be possible that something in the transmition?

Could be something with the speedometer drive inside the transmission and the rear bearing support. It has been a few years since I worked on that 3 speed, but I recall the speedometer drive gear consisted of a worm gear which was pressed onto the main shaft. The worm gear turns a toothed gear (which is your speedometer drive mechanism). And to remove that speedometer drive gear assembly I think (emphasize the word "think") you have to remove the rear bearing support. Wish I had taken pictures of it...but digital pictures were still 20+ years away at that time. You mentioned replacing the Oakie bushing. I'm wondering if those are connected.

I also think that rear bearing support is held in place by a small bolt on the top of the transmission or maybe on one side of it. I'll look to see if I have anything on this in my old notes.

I have had a chance to pull the driven gear from the side of transmission to inspect it. It shows a couple of miner nicks but nothing major. The teeth look ok. Is the rear bearing support located in the transmission? Not looking forward to taking all that apart again. I haven't driven it since its been inspected but will this weekend. Thanks for all the support. I will figure this out, sometime.

Took truck for drive and speedometer still does the same thing. I think too it has to be something inside that's not correct. It works somewhat ok until everything gets up to temperature. Then as I step on gas speedometer goes to zero. If I let off it jumps up to correct speed. Guess I'm gonna have to take things apart.😜

This sure is an odd one. So when cold, if you step on the gas, the speedo works?

No when I step on gas it goes zero, quits working. As I let off and coast it goes to correct speed.

When cold it works about 1/2.

Well when you see red and blues, let off da gas, and youll know if they're after you.