The Stovebolt Site Forums The Parking Lot Big Bolts Speedometer Adjuster Repair

I have another mundane repair question. Does anyone know where I might get a tiny replacement rubber boot for the two speed adjuster connected directly behind the speedometer on my 1966 GMC 5500 truck with a vacuum operated two-speed rear end?

I have been on an under-the-dash repair journey that started with needing to replace the heater core. Now, I have pulled back the instrument panel to replace the defroster ducts and found that the vacuum line to the speed adjuster had been disconnected , so I removed it to figure out why. After cleaning off the grime, it was obvious that the small rubber "boot" that I think holds vacuum in the adjuster was torn (with a spring poking out). I am hoping that the only repair needed is to replace the rubber boot, and that the internal gears are not corroded as mentioned by Mike B in an earlier thread. Any help in finding a replacement rubber boot would be appreciated.

I am putting the under-dash parts back together, so it is time to give up on finding a new vacuum diaphragm for the speedometer reducer. I patched the underside of the old diaphragm with PVC tape and hope it holds for a while. If this doesn't work, I will see if a speedometer repair shop can fix it. I am disappointed by GM engineering in this case, because it appears that spring pressure from the vacuum inlet caused the problem by pushing on the rubber diaphragm. This could have been easily solved in the original design.

I am also surprised that no one could tell me if a diaphragm that fits can be found or not. This speedometer reducer design seems to have been used by GMC and Chevy for many years starting in the 1950's and continuing into at least the later 1960's, so many trucks must have them, and replacing the diaphragm is easy once the reducer has been removed, which I also found easy to do after pulling back the instrument panel.

Since this all started with trying to replace heater/defroster ducts, I can also report that the basic ducting is relatively easy to work on once the instrument panel, heater/defrost controls, heater box, and glove box are moved out of the way. Because of openings in the cab (such as a deteriorated steering column boot or gasket) and with the old ducts either missing or falling apart, the entire heater box and other openings were filled with mouse nests (and mouse bait had been stored in the heater box), along with more nests in other hard to reach under-dash nooks and crannies. So this project was really needed. However, it was not easy to install the new ducting because of some odd shaped inlets and because the Gates brand 2 1/2" diameter duct tubing is a bit smaller than the inlets, which made it very hard to push on. I finally had to cut (but not remove) the embedded spiral plastic cording at the duct ends to stretch them over the inlets - then I applied a few layers of PVC tape and a zip-tie to prevent leaks and hold it all together. Now I am hoping not to have broken any of the many electrical connections while pushing things around and that it will all work without further repair.

JR,

The speedometer adaptor Control Assembly you are looking for is Chevrolet part number 3375765. This unit was not serviceable, so the diaphragm was not sold separately.

Hope this helps.

Mike B

Mike B,

Thank you! Now I, and others who read this thread, know to look for the whole assembly (with a good diaphragm) instead of only the diaphragm. Too bad that GM didn't figure out that the diaphragm would go bad and is easy to change. It seems like an after market parts producer could have figured this out considering the many years that this particular speedometer reducer was used.

Thanks for the input,Mike! i posted the same thing,this Spring,when I found mine has the same problem.,('55 6500 T.F.). The funny,(NOT)!! part is,that the 1955 Chevy Truck Service Manual,(for that year,anyway),DOES NOT cover anything pertaining to the vacuum-shift 2-speed switch,adapter for "speedo",or the "pancake"!! Very little info.,on the hydrovac,vacuum system,although the BRAKE system is pretty thorough. G.M. must've thought that the vacuum systems would out-last the trucks?? The reason I purchased the shop manual in the first place,was for info. on the 2-speed configuration,(on my truck),since the "PO" removed the switch from the transmission,and had a hanger wire attached to the "pancake",to change gears! Is 3375765,the one used on the T.F.'s as well?? THANKS!!

Willy,

The 55-59 TF Trucks speedometer adaptor Control Assembly is part number 3715172 (55-59 Ser 4-5-6-7-8 w/ 15000 lb. 2-speed axle (8.72-6.40 axle ratio).


Mike B

Thank-YOU,Mr. MIKE!! One step closer!