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| | | | Joined: May 2010 Posts: 5 New Guy | | New Guy Joined: May 2010 Posts: 5 | I have a 71 C-50 with a vaccumm operated 2 speed rearend. How does the speedo know what rearend gear the truck is in? Thanks.......................Flat67 | | | | | Joined: Jun 2004 Posts: 8,597 Riding in the Passing Lane | | Riding in the Passing Lane Joined: Jun 2004 Posts: 8,597 | There is a little transmission in the speedo cable that shifts at the same time.
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing. 1972 Chevy c10 Cheyenne SuperIn the Gallery Forum | | | | | Joined: May 2010 Posts: 5 New Guy | | New Guy Joined: May 2010 Posts: 5 | Mr. Wrenchbender,
Thanks for your answer, I'll have to check out the "the little transmission" because I don't think it is working. I can shift the rearend but the rpm/speed doesn't change accordantly.....................Flat67 | | | | | Joined: Mar 2004 Posts: 6,214 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,214 | Flat 67,
The speedometer speed reduction unit should be atached to the back of the speedometer. It has a small vacuum diaphram that shifts the speedometer from high to low range to match the rear. All 2-speed rears have the same 1.36 differential, so all of the reduction units are the same. I'm betting yours has dry rot and that's why it no longer works.
Mike B | | | | | Joined: Jun 2010 Posts: 384 Shop Shark | | Shop Shark Joined: Jun 2010 Posts: 384 | I have the same issue in my truck..
Hope I will just be able to fix it.
My Chevy Viking 1959Ulf Hedlund. Eskilstuna. Sweden [email protected]Also: Ford Galaxie 4 dr 1959 Ford Custom 4 dr 1964 Ford Mustang GT 2005
| | | | | Joined: May 2010 Posts: 5 New Guy | | New Guy Joined: May 2010 Posts: 5 | Mike B,
Thanks for the advice. Have you been successful in repairing the speedometer speed reduction unit? Sounds as if you have to be a watch repairman to fix one of these...............lol.........Flat67 | | | | | Joined: Mar 2004 Posts: 6,214 'Bolter | | 'Bolter Joined: Mar 2004 Posts: 6,214 | Flat 67, I haven't repaired one yet. I was able to find a NOS Diaphram a few years ago, but I'm afraid to install it for fear it's to old and will tear as soon as it's put to use. I'm thinking about sending it out to see if I can have repro's made new. Mike B  | | | | | Joined: Sep 2006 Posts: 1,276 Shop Shark | | Shop Shark Joined: Sep 2006 Posts: 1,276 | Take a look at 70s emissions carburetors you will find a plethora of small vacuum diaphragms and linkage that with very little effort can be adapted to shift that little transmission.
Another little trick is to use the transmission from a truck that had an electric shift rear end and do away with the vacuum one.
The electric ones are usually remotely mounted with a speedo cable to and from where is the vacuum ones are direct mount to the back of the speedo.
| | | | | Joined: Sep 2006 Posts: 1,276 Shop Shark | | Shop Shark Joined: Sep 2006 Posts: 1,276 | BTW my 64 GMC has an electric shift speedo and a vacuum shift rear end and that is the way it came from the factory so you may even be able to adapt the whole system from a later truck. This would give you the electric shift transmission, the electric/vacuum switch and the needed cables | | | | | Joined: May 2010 Posts: 5 New Guy | | New Guy Joined: May 2010 Posts: 5 |
Thanks for all the tips guys...
HevyHauler,
Found the speedo reduction unit and it is like yours, electric shift speedo and vacuum shift rear end. I will start trouble shooting it, wish me luck........Flat67
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