SM420 speedometer
 

Hello,

My speedometer cable was toasted so i replaced it, i also cleaned the speedometer and greased it and it works perfectly on the bench.

Now, having driven the truck, the speedometer does not move so i wanted to understand if the issue was coming from the speedometer or the cable so i removed the speedometer cable from the back of the speedometer and i drove it again and it does not move so i suspect something is wrong in the transmission.

Have you ever faced this issue?

I need to jack the rear end and to put the truck in gear to see if it moves at the transmission itself.

Thanks;


Dylan

Re: SM420 speedometer
 

I believe there's a little plastic gear in the transmission that the cable goes into. That thing sometimes cracks (or disintegrates!) and keeps the tranny from turning it.

Re: SM420 speedometer
 

Strang enough, I was also expecting that but the cable end is just inserted in the transmission.

I did not see a drivegear....

Re: SM420 speedometer
 

Dang it, I have to go check now... :)

Gimme about 20 minutes

Re: SM420 speedometer
 

Well I stink at looking at titles, I guess. I run an SM318, not a 420. But I didn't see that before I went and tore mine apart so here's what that looks like. Surely yours is similar. Heck, this style was used up to the 80's or so in one form or another! :teehee:

This is probably way more detail than you need, but if someone else needs it at least it's posted. Here's a look up inside the hole where the speedo cable goes. Notice that it's square so it can hold the cable and turn it.


http://i66.tinypic.com/160e9ud.jpg









You take off the hold-down bolt on the side and that releases the clip holding the cylinder in.


http://i64.tinypic.com/4tocnm.jpg









Once the clip is out of the way the cylinder just pulls out. You might have to twist it back and forth a little if it's rusted or painted on.


http://i67.tinypic.com/v8ekn7.jpg









This is the cylinder all the way out. That yellowish goopy looking thing sticking out of the cylinder is the plastic piece that busts up sometimes.


http://i65.tinypic.com/huelo1.jpg









That plastic piece just pulls out. It's not screwed in or anything. Here's what it looks like.



http://i68.tinypic.com/e71lbr.jpg









And that's it. If yours doesn't have a plastic thingy at all it has probably been tossed by someone that didn't want to put miles on their truck.No plastic, no odometer, truck never drops in value...It was common in the older days. Or it just disintegrated one day and nobody bothered to replace it.

Re: SM420 speedometer
 

I don't recall having seen the hold down bolt but i'll have a look, for sure.

Thanks a lot from Belgium :-)

Re: SM420 speedometer
 

Oh yes, seen here:

http://6066gmcclub.com/showpost.php?p=65272&postcount=5

Re: SM420 speedometer
 

Looks like on the SM420, it is different and of course, I don't have the right socket....

Re: SM420 speedometer
 

After some researches, it looks like this.

http://talk.classicparts.com/attachm...ear1-jpg.8726/

Does anyone have some in stock?

I've got the 4.11 rear end.

Re: SM420 speedometer
 

How many teeth on your gear? I have a small selection.