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1974 W100 speedometer cable. WHAT???

8667 Views 6 Replies 3 Participants Last post by  GLHS60
I've been working on this truck from my dad, almost have the transmission ready to install, there is no drive pinion in the housing of the transmission tail. The cable is routed to a little black box along the frame and another cable comes out of the box and is routed to the transfer case. Has anyone ever seen this? Also, the speedometer wasn't working last time I drove it, it stayed at about 15 no matter what speed you were going, once in a while it would rise to about 22 and stay there but eventually it would settle back at around 15 while driving. Can I purchase a new pinion, install it and just route my cable there? I don't like not having a working speedometer, especially if my wife is also going to drive this truck. Also, the truck has 33 X 12.5 15 tires, is there a special pinion I would need to purchase for those size tires since they aren't stock to have an accurate reading? Thanks!
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The speedometer head sounds like it is stuck or binding if there is little pointer movement. Most greases may have hardened into candlewax during this amount of time. I have also found paper labels fallen into the mechanism causing interference with movement. These mechanisms are delicate, like watch repair, so use care.
The speedometer cable can be removed from the sheath and lubricated while pushing it back in. Vaseline may be better than wheel bearing grease. There was a special speedometer cable lube we used at the dealer:
Does the speedometer cable black box also have wires coming out of it? It may be an early emissions check light switch to turn on a dash EMR light? California emissions truck?
There has to be a pinion drive gear. It may still be in the tailshaft?
There is a chart to select a pinion gear tooth count according to axle ratio and tire size. It is in the service manual. Oversize, non-stock tires may not be included in the chart and would have to be calculated. There may not be a part number for that gear, but get as close to the tooth count as you can.
An example here:
Mopar Speedometer Gear Selection Guide -
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I agree that the Speedo Correct is aftermarket and probably a step-up gear to get a more accurate speedometer reading with the larger tires. The EMR distance counters were mostly kept inside, under the dash and out of the elements.
Many later vehicles had a 2-piece speedometer cable, so that speed control could be more easily added.
The lower cable usually got well-lubed with ATF and the upper one was the one that got dry.
If you don't already have one, the 1974 Dodge truck factory shop manual would be helpful. Chiltons and Haynes are pretty useless for the little things that need fixing and can be inaccurate or just plain wrong.
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