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.

Re: SM420 speedometer
 

Hello,

This would be awesome but as of today, I still can't remove it, I can't find a deep socket. I tried 22mm but to small and 24mm too big....

What is it in inch? 15/16"


I made a lot of researches and I remember having read that for a 4.11 rear end ratio, it was a 12 tooth.

What do you have and how much you want for?

Re: SM420 speedometer
 

Yes it is 15/16". However a 24 mm deep broach socket will fit, I tried it and it fit on the spare unit I have. The only problem you might have is the socket outside diameter might be too big and it is hitting on the case. If this is what happening you will need to grind on the socket to clear the transmission.

In my truck it did have a 4.10:1 rear axle and the speedometer gear I pulled out had 11 teeth with 3 teeth on the gear inside the transmission.

Re: SM420 speedometer
 

OK, I need to see what I can do.

What do you have in stock?

Re: SM420 speedometer
 

I have an 18 and an 11 tooth as well as the matching gear (7 an 3 respectively) that goes inside the transmission.

Re: SM420 speedometer
 

I just removed my speedo cable and tried my 24 mm deep broach 6 point impact socket on my transmission. It fit and there was no room to spare.

Re: SM420 speedometer
 

So I used a 15/16" socket and I could remove it.
Surprinsigly, my gear is not damaged, 13 tooth and it moves freely.

I'm now wondering if the cable inside the speedometer cable housing is long enough to fully engage into the driven gear.

It needs to be deep in the driven gear by something like 0.7mm and ideally 1cm inside.

Is your driven gear also round then only kind of square inside?

Re: SM420 speedometer
 

Kind of hard to tell in my picture. It is round on the outside and .1" square on the inside.

Re: SM420 speedometer
 

If you think the inner cable too short it might be the nut on the back of the dash not tighten all the way. This would allow the inner cable to be short on the transmission end. Reach behind the dash and make sure the nut on the housing is tight and the cable housing does not move.

Re: SM420 speedometer
 

It was tight but i was thinking loosen it a bit to give more room in the transmission.

I'll try a bench test.


Envoyé de mon iPhone en utilisant Tapatalk

Re: SM420 speedometer
 

Found my issue... The driven gear is no longer square inside... I remove the cable and did a bench test... It is round deep inside and the cable is not driven... I need to find a way to get the cable stuck inside....

If anyone has a 13 tooth, it would be great.

Re: SM420 speedometer
 

I made a test today, I used a thermal sleeve around the cable then I inserted it in the driven gear, it creates sufficient resistence to allow to the cable to give me the speed.

I tested it against a GPS and my speedometer is slower than the reality so I don't need a 13 tooth, I'd something with less tooth.

James, I am interrested by your 11 tooth driven gear. Do I need to change something in the transmission as well? I'd like to buy it and to have it shipped to me in Belgium. Can we do business?

Re: SM420 speedometer
 

PM me your mailing address and It will be your Christmas present (Free) (Gotta keep the truck on the road without getting a speeding ticket). I'll PM the tracking information when I get it. I will send you both the 11 tooth driven gear and it mating 3 tooth drive gear. If the driven 11 tooth will not fit your drive gear it will need to be change. The drive gear will require the drive shaft and parking brake removal. Then removal of the bolt that hold on the flange. Then the flange will come off with the drive gear attached. The drive gear is press onto the flange. When you press the new gear on be sure to place it in the same relationship as the original drive gear (on mine I press it on too far. When it was back in the transmission the drive gear was too far aft. I had to pull it off the flange slightly to center the gear up with the hole.).

On mine truck it was not enough correction due to rear axle ratio change (from 4.10:1 to 3.54:1), tires size (LT235/85R16E), and the lack of spare parts. I also had to install a 28% speedup ratio adapter for the final speed correction from PATC https://redrider.us/shop/ratio-adapt...1-888-2012066/. I do not know if there a company in Belgium that can provide this for you. Mine is almost dead on now.

Side note: Do you get much snow in your part of the world? We get very little and sometime its freezing rain.

Re: SM420 speedometer
 

PM sent, thanks again :-)

Re: SM420 speedometer
 

Did you have the chance to see if the parts I sent you would work?