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| | | | Joined: Jan 2008 Posts: 48 Wrench Fetcher | | Wrench Fetcher Joined: Jan 2008 Posts: 48 | Hi.. we've pulled the vacuum control valve from the back of our speedometer on a 1948 FC303 1 1/2 ton truck. This is for the low-high axle. (page 55-58 on the repro gmc maintenance manual)
The diaphram is rotted out, and we're wondering if anybody out there still makes this valve, or if anybody knows what diaphram parts might fit in the original?
Any help would be great!
Last edited by turpentyne; 09/14/2008 8:28 PM.
| | | | | Joined: Apr 2005 Posts: 1,107 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,107 | i tried to make a new one for my 350 from a piece of truck inner tube and it didnt work. i think due to the size (contact area) of the diaphram there wasnt enough movement because it was too stiff because of the thickness for vacuum to actuate it for the required movement to shift gears.
i was going to try some roof rubber and with all the hasle to make a new one i threw in the towell. some pneumatic diaphram material is the right ticket if you can find some. | | | | | Joined: Jan 2008 Posts: 4,962 'Bolter | | 'Bolter Joined: Jan 2008 Posts: 4,962 | It was quite common to replace this valve with an air valve back when these units were still used as trucks. All it does is shift one way with vacuum and a spring shifts it the other direction. | | | | | Joined: Apr 2005 Posts: 1,107 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,107 | yes, but one must have air to do that. | | | | | Joined: Jan 2008 Posts: 4,962 'Bolter | | 'Bolter Joined: Jan 2008 Posts: 4,962 | An air valve can be adapted to shift the 2 speed axle itself, but if you are talking about the part that shifts the speedo gears, I don't know of anyone ever having a fix for that. By the time parts were not available, no one seemed to care how fast they were going. Electric two speeds used an electric unit to change speedo gears. It wouldn't be original, but you could probably shift the 2 speed with vacuum and the speedo with electric. I have a 53 with a vacuum. I have never really paid any attention to the speedo. | | | | | Joined: Nov 2007 Posts: 14 Wrench Fetcher | | Wrench Fetcher Joined: Nov 2007 Posts: 14 | Is this the part you need on the back of the speedo in this picture? http://i6.ebayimg.com/07/i/001/09/ab/515f_1.JPGI pulled that speedo from my 49 coe with 2 speed rear | | |
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