Another speedometer thread, stopping and asking questions before I mess something up.

[Bullet Bob]

I’ve checked my cable and tried a different gauge to verify everything was working from where the cable connects to the transmission to the gauge itself. Went through and looked at every thread I could find to deal with a speedometer, all 18 pages worth. Powerhouse Ranch’s Speedo gear page was the most informative for me. Main questions I have is on the end of my cable where it goes into the transmission. Appears to be an offset gear box of some sort. Was able to unscrew it from the transmission by hand and was what I used to make sure the gauges bounced. Used my phone to look into where it plugs in. I’m assuming this is the housing for the plastic gear, and all I’m seeing is a spring. Should I see a plastic piece? Also see where what I’m assuming is the housing could be removed with a socket wrench. Everything I’m finding looks a little different than what I have. Could anyone tell me what the little offset gearbox is and if that is indeed the housing for the plastic gear. Messed up to much stuff here lately and figured I’d ask before I ended up with going from no speedometer to no transmission.

Truck info
Grandfathers 1980 c20 that has sat in a hay field for 10 years. Supposedly broke the speedometer while pulling cows in the late 80s. Has 454 with 4 speed manual that may have come out of a 77 c30 around 84. Only replaced both fuel tanks, pumps and rubber lines to make it run. Starting to try and figure out mystery wires under dash.

 

[Bextreme04]

I’ve checked my cable and tried a different gauge to verify everything was working from where the cable connects to the transmission to the gauge itself. Went through and looked at every thread I could find to deal with a speedometer, all 18 pages worth. Powerhouse Ranch’s Speedo gear page was the most informative for me. Main questions I have is on the end of my cable where it goes into the transmission. Appears to be an offset gear box of some sort. Was able to unscrew it from the transmission by hand and was what I used to make sure the gauges bounced. Used my phone to look into where it plugs in. I’m assuming this is the housing for the plastic gear, and all I’m seeing is a spring. Should I see a plastic piece? Also see where what I’m assuming is the housing could be removed with a socket wrench. Everything I’m finding looks a little different than what I have. Could anyone tell me what the little offset gearbox is and if that is indeed the housing for the plastic gear. Messed up to much stuff here lately and figured I’d ask before I ended up with going from no speedometer to no transmission.

Truck info
Grandfathers 1980 c20 that has sat in a hay field for 10 years. Supposedly broke the speedometer while pulling cows in the late 80s. Has 454 with 4 speed manual that may have come out of a 77 c30 around 84. Only replaced both fuel tanks, pumps and rubber lines to make it run. Starting to try and figure out mystery wires under dash.

The box is an adapter to adjust the speedometer to match the axle ratio and tire size. The output looks right from the transmission. That isn't a spring, its a shaft with a square hole in it. There should be a long square rod inside the box where it screws onto the transmission that fits inside of that square hole. That square rod usually snaps or the hole gets rounded out in the adapter in the transmission. You can unscrew the adapter from the cable and screw the cable into the transmission directly and see if that fixes it.

 

[Bullet Bob]

Thanks Bextreme04 I’ll take that box off and try it. If the output off the transmission is rounded, is that a part that I can just unscrew and replace altogether or do I have to actually into the transmission?

 

[pduffyd]

The other thing that maybe causing the issue is the nut holding the output yoke to the output shaft has worked loose, this holds the speedo drive gear on the shaft. See items 19 and 20. That nut has to be torqued up real good. This is for a 2wd version.

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[Bullet Bob]

The other thing that maybe causing the issue is the nut holding the output yoke to the output shaft has worked loose, this holds the speedo drive gear on the shaft. See items 19 and 20. That nut has to be torqued up real good. This is for a 2wd version.

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What book/source is this from? Like to figure out what parts I might need to source if the Speedo drive needs to be replaced and how to do it correctly. Since I’m about to replace the U-joints and carrier bearings, I can check the nut on that yoke. Along with anything else I can access from that area.

The box is an adapter to adjust the speedometer to match the axle ratio and tire size. The output looks right from the transmission. That isn't a spring, its a shaft with a square hole in it. There should be a long square rod inside the box where it screws onto the transmission that fits inside of that square hole. That square rod usually snaps or the hole gets rounded out in the adapter in the transmission. You can unscrew the adapter from the cable and screw the cable into the transmission directly and see if that fixes it

When I went to remove the cable from the box it appeared the cable went directly into it and was maybe for cleaning and lubing the cable. Compared the square rod to the extension cable I had and looks about the same and not rounded.

 

[pduffyd]

BulletBob.
That diagram is from the factory manual. Which if needed I can copy and do a pdf link from drop box.
But you can go to places like this.

GM SM465 4-SPD - Rockland Standard Gear, Inc.

tcase.rsgear.com

And see a breakdown of the 2wd and 4x4 box. Item 61 is the speedo gear shown at this site above.
Also that gear can have its teeth stripped, or spin on the shaft, as I said if the yoke nut loosens a bit.
The ideal test of the drive cable to your speedo needs two people one with a battery drill in low speed to drive the little gear box shaft that normally goes into the trans and the other to watch the speedo at the same time. Then you will know where the problem is.
As has been said before, that should be a square peg. You dont need that drill rotating much to get a speedo reading, I think the cable rotates anti clockwise at the gear box end. Its been a while since I tested one.

 

[Bullet Bob]

Pduffyd thanks, for the info. The speedometer would bounce when I hooked the cable to a drill, and the peg looks good and square. So I guess my next step is to check the bolt on the yoke. Which I’ll check when I when I change the u-joints and carrier bearing. Thanks for helping me narrow things down.