Many of you have known me for decades, many of you have had me visit your shops, and many of you have attended seminars or conventions where I have spoken. One of the trends I have noticed over the last several years is the “Graying” of our industry. Most shops have difficulty finding employees as their tried-and-true employees retire and move on to greener pastures.
A couple of weeks ago, I was in a shop where a young man expressed frustration when he attempted to repair a shifting issue on a 1985 THM 700R4. As he put it, “I hate working on this old stuff without computers and sensors. How did the old timers ever fix this stuff?” It dawned on me that we are facing the same issue we did when computers and sensors were introduced into the transmission industry 40+ years ago. The old timers of that era hated anything with a wire attached to it and attempted to avoid it like the plague. Many young technicians in our industry feel the same way about the old technology. When you look at the situation, the reason for the feeling the way they do is the same as it was 40+ years ago: frustration with dealing with something that you are not familiar with.
We sat down for a cup of coffee, and I explained how the shift control systems operated in those ancient fossils of a transmission. I related it to what he was familiar with today, and the light bulb turned on. He recognized that even though the components may be called something different, the fundamental purpose was to control the position of the shift valve for the gear in question.
SHIFT CONTROL
The shift valves in an old-school transmission perform the same function as today; to control a clutch’s apply and release pressure. That is, when and how a shift element (clutch or band) applies. On one end of the shift valve, we have governor pressure, effectively pressure related to road speed. On the other end of the shift valve, we have a spring and throttle pressure.
To make the transmission upshift, governor pressure must exceed the shift valve spring and the effects of throttle pressure, forcing the valve to snap into the upshifted position. This opened the shift valve passages to allow line pressure to travel through the valley of the valve to apply the clutch (Figure 1).
For a downshift to occur, the governor pressure either has to drop (like when you are pulling up to a stop sign), or the driver needs to apply more throttle, which would force the shift valve back into the downshifted position when spring and throttle pressure exceeded the governor pressure. Let’s look at each of these more closely.
GOVERNOR
The governor is basically a mechanical/hydraulic valve that controls its output pressure based on road speed. While several governor designs were used, two of the most common were a check ball design and a valve design. Both designs used a set of ‘flyweights” that reacted to the centrifugal force created by the governor spinning in its bore. The 700-R4 used a valve design governor, so as the weights moved outward due to governor rotation, the valve was forced deeper into its bore, increasing governor pressure.
As the vehicle slowed, centrifugal force decreased, and pressure on the other end of the governor valve forced the valve to move back in its bore, reducing governor pressure (Figures 2 and 3).
As a general rule of thumb, governor pressure would increase at approximately one psi per every one mph of road speed unless it was a diesel. Diesel applications had a different governor pressure curve. The rule of thumb on diesel applications was a 1.5-2 psi governor pressure increase for every one mph of road speed. So, you see, the governor is nothing more than a hydraulic vehicle speed sensor operating at output speed.
THROTTLE VALVES
Two dominant designs were used: cable/linkage operated (Figure 4) or vacuum-controlled throttle valves. The vacuum-controlled valves used a “Vacuum Modulator,” which reacted to the intake manifold vacuum changes. As the load increased, the vacuum decreased, leading the modulator to push the valve further into the bore, increasing throttle pressure. The same was true of the cable/linkage-operated valves. As the throttle opening increased, the valve was pushed further into its bore, increasing throttle pressure.
Throttle pressure is related to engine load, so as the engine load is increased by the operator opening the throttle, the throttle pressure will increase. The system is designed to create a linear increase/decrease in pressure as the throttle opening increases/decreases. Throttle pressure is also used in most old-school transmissions to control line pressure boost to prevent clutch slippage as the throttle opens. This whole system is what we patterned the computer-controlled system we use today. In today’s transmission, the computer uses the MAF/MAP and TP sensors to sense engine load.
Like today, these systems were ultra-sensitive to engine operation, so an engine issue might appear as a transmission problem. In the case of the 700-R4, people who did not understand how the system worked would attempt to “fine-tune” the transmission operation by moving the cable a click or two. Each click of the cable equated to about 10 PSI of line pressure increase, which could lead to 3-4 clutch failure (a common issue).
ISSUES
Like everything we work with today, these systems had their quirks. The governor bores would wear, the governor weights and valves would hang up, and the gears driving the governors would wear. The throttle valves also had their own maladies, such as sticking valves, incorrectly adjusted cables and linkages, restricted vacuum lines, and misrouted and worn or binding cables.
As younger techs, you must understand that working on these ancient units is not black magic. It is simply having a little understanding of how some of these components operate. Younger techs are taking over this industry, so they’ll need to understand how some old-school low-tech systems are serviced, as many are still on the road. There may be some choice words when you start to work on these older designs, but just like the newer vehicles, your comfort zone will rapidly improve. Until next time, remember, “age is just a number, but I would rather not dial that one.”
