The Stovebolt Site Forums The Parking Lot Big Bolts High/Low Range Vacuum Switch Question

Hydraulic clutch repair work on my 1966 GMC 5500 truck first led to replacing the master cylinder, and has now expanded to the vacuum system that ends up connected to the hydrovac. This inter-connected vacuum system includes the rear end over-under (high/low range) switch that is controlled by the shift lever button. In addition to connections on the switch that go to the hydrovac and the rear end, there is a line that ends up at the left side of the dashboard near an obviously replaced vacuum gauge (salvaged from dairy equipment) that is not working. This line is unconnected (open) at the dashboard end, where I am guessing that it is supposed to be hooked up to the vacuum gauge. I am hoping that someone can tell me if this guess is correct or not? It just seems odd to bring a line from under the truck to measure vacuum.

Right now, I am trying to track down other vacuum leaks, including those coming from the rats nest of old hoses around the hydrovac. And I am replacing the brake booster check valves after breaking an end off one of them. This gave me a sinking feeling because parts have been hard to come by, but it looks like Napa has what I need.

I also have a story about finding a new tandem (clutch and brake) master cylinder to fit this truck that I will share after I know that what I ended up with works.

JRMunn

Back in the day, vacuum gauges were standard equipment on my fleet vehicles.

The small vacuum line coming from under the truck was probably connected to the 2-speed rear selector switch located on top of the trans. The other end of this line would be connected to a 2-speed reduction unit located somewhere in the speedometer cable line (maybe mounted directly to the back of the speedometer)...it keeps the speedometer calibrated in both Hi and Lo range. If not used or needed it should be plugged.

Mike B

Mark and Mike,

Thank you for the replies.

I am guessing that the current vacuum gauge was put in to replace a gauge that came with the truck, but it is not hooked up.

I had not thought about an adjustment for hi- and low-range speedometer readings - perhaps because the truck was stuck in low range when I bought it, hopefully because the button cable was frozen (and now works after much treatment with penetrating oil). The vacuum line I am asking about definitely comes out of the hi/low switch, but could still be headed to the speedometer. I will start by capping it off so that I know it doesn't interfere with tuning the engine, then connect my vacuum gauge to it so that I can tell when, and to what degree, it is active.

If I remember right the vacuum switch on the trans has 4 lines attached to it...Main Vacuum Supply, Hi to the rear, Lo to the rear and small line to the speedometer reduction unit.

I've had a few of these vacuum switches apart over the years and most of them were heavily corroded inside. The housing is pot metal and collects moisture causing it to corrode and fail.

Mike B

Mike,

Thank you for the insights.

Corroding and failing is not encouraging news about the hi/low vacuum switch. Can these be purchased new or otherwise replaced?

From the looks of the mounting hardware and steel tubing bends, the small vacuum line coming out of the switch appears to have been headed for the dashboard when the truck left the factory.