Speedo question

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Chris P
CPL
CPL
Posts: 135
Joined: Thu Dec 08, 2011 12:39 pm
Location: Clemson, SC

Speedo question

Post by Chris P »

I just swapped out my transfer case for an NOS one. The transfer case was technically meant for an M715. I had to change out the three flanges, but my understanding is that everything else is the same.

Before the swap my speedometer was showing speeds too slow by about 10%, and now its showing too fast by about 20-30%. I checked it before and after using an app on my cell phone. (I happened to check it some time ago when I suspected it of running slow.)

Why did this occur? The M37 has an axle ration of 5.83:1, the M715 is 5.87:1 and both take the same size tires.

The NOS transfer case did not come with a speedo pinion drive or retaining nut, so I bought NOS M37 ones. They came with the speedo drive gear, but since that's internal to the transfer case I didn't use it.

It's a new Speedometer and cable with only about 600 miles on them.

Thanks in advance.
Chris P
1954 M37
MVPA
MHCC
Chris P
CPL
CPL
Posts: 135
Joined: Thu Dec 08, 2011 12:39 pm
Location: Clemson, SC

Re: Speedo question

Post by Chris P »

This turned out to be an easy fix. I was able to do it while the transfer case was still on the truck. I'll refer to Figure 128 on page 196 of TM 9-8031-2.

What I had to do was replace the speedometer drive gear (AH), which is different on the M715 than on the M37. First you remove the flange (JJ) and brake drum (KK), which come off as a unit. Then remove the five nuts (RR) holding on the brake support (TT) and remove it. I was able to do this without damaging the gasket (UU). The brake band, etc, come off with the support.

Then slide out the bearing cone (VV), shims (AC-AF), and spacer (AG). The bearing cup (WW) will stay in place. Then slide out the speedo drive gear (AH). This can be done with the speedo drive pinion (C) and retaining nut (A) still in place. I couldn't get my fingers in there, so I reached in with a couple of L-shaped pieces of metal to grab the gear (AH) and pull it out.

Then slide in the replacement gear and put all the other parts back.
Chris P
1954 M37
MVPA
MHCC
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