Speedometer Jumping Around....Why?

[69sixpackbee]

Heres the scoop. In my '72 I have the speedo jumping around and I am trying to isolate the problem. It is not the speedo itself or the cable as I have swapped them both. The speedo reads well while the truck is under way and is using power to do so but when I am at, lets say, cruising speeds, the speedo starts to drift downward consideralby but as soon as I tap the gas pedal it straightens out and reads correctly. This condition only occurs during very light throttle cruising or deceleration. I have a 4-speed in it and it has one of those offset little "boxes" that screws onto the speedo pinion on the trans and the cable attaches to that. I am assuming it is a ratio adapter. This has been on the truck forever. I hav a Dana 60rear w/ 3.54:1 gears and 16" rims so maybe this is what it is for but I am not sure it is the problem. Could this be the culprit or is it possibly the pinion gear itself?
The cable is well lubricated and no kinks whatsoever.

If it was just a speed indicating thing I could care less but since I am putting EFI back into it and will be utilizing a VSS module that attaches directly to the cable assy. I sure don't need this malady to affect my EFI function.

Any suggestions??
Thanx,
Bud

[70rs/ss]

Shoot some lithium spray or good old WD40 and it'll run smooth as glass. Over the 40 or so years the cable dries out and grabs as it spins in the sheath, so a little lube'll do you good!

[71swb4x4]

Before reading your whole post my first suggestion would have been to lube the cable. Now that I have read your post, my first suggestion is to try another cable, and second suggestion is to try another of the ratio boxes that the speedo cable screws onto.

Something I have learned the hard way, just because you have replaced a part doesn't mean the replacement part is functioning correctly.

[70rs/ss]

Crap responded to the title, but didn't read it, sorry. Honestly a gear either works or it doesn't so the little box might be the culprit, but ya never know till ya try?? Usually it is the cable and it just needs cleaned out and lubed up, did you buy a new one or just clean up one?? It could be the speedo, but you said you replaced it (new or used??) the movements in these old guages isn't great, so that could be a part of it as well?

[Brad]

It sounds like your cable has been replaced and is brand new? Then I would pull the convertor box and see if it still happens.

You already tried that and it didn't work? I dunno man.
I know that I recently had to use brake cleaner on my used housing to clean it out thoroughly then used graphite lube and lubed the actual cable with lot's of grease. While it still seems to take a split second to 'catch up' from a complete stop, it's working much better than before and I think that's just because it's such a long cable (longer than stock).

If I'm not mistaken, the vss sensor attaches to the transmission and I suspect it'll work just fine.

[Huck]

Common problem--replacing the inner cable only addresses 1/2 half of the cause. What is happening is the outer calbe housing is caked up with kaka dried out grease. replacing the inner cable only puts a new cable in a old crappy, sticky housing. The problem is more significant as the weather gets colder and the cable lube gets thicker. Best solution is to replace the outer and inner housing---another fix is to flush the housing with thinner, WD40, goof-off, and soak the inner cable in a pan of Mobil 1 synthetic oil. This doesn't get thick under normal US weather. I have done the above on multiple occasions and it has always worked. Huck

[69sixpackbee]

Thanx for the help guys!!!

[Stocker]

But.... is it fixed? What was the problem? I've been watching this but had no suggestions other than what had already been said.....

[69sixpackbee]

I got as far as pulling the cable out on Saturday. Hopefully, I can get it lubed and put back in. When I removed it from the trans I spun the cable with my fingers but did not "feel" any type of resistance but that was with a few finger spins. I am going to clean the cable (inner) with Stoddard solvent then keep it wet and put it up in the housing and hook it up to the drill and let it spin a few minutes. I will take it back out and clean it, dry it and spray some Lithium grease on it and shove it back in. If this works, great!. If not then the whole cable assy. comes out and I will either just buy a brand new one or see if the one I have is salvageable. I checked out the ratio adapter and it seemed to be okay so it must be either the cable or the speedo head.

I'll keep ya posted!
Thanx,
Bud