Vacuum Modulator

[joe_padavano]

I wrote the following to answer a question on another bulletin board, but I thought it might also be of interest here. The question was, "what is the purpose of the vacuum modulator on an automatic transmission?"

The modulator on the TH350 and TH400 is effectively a way to control shift points based on engine load. At high manifold vacuum (which corresponds to small throttle openings), the vacuum modulator operates a valve that causes the trans to shift at a relatively low RPM. At low manifold vacuum (which would be at or near wide open throttle) the vacuum modulator valve holds the trans in gear longer and shifts at a higher RPM. On a normal TH400 or TH350, for example, if you temporarily disconnect the vacuum line going to the modulator, the modulator thinks the engine is at low vacuum (W.O.T.) and the trans will only upshift at wide open throttle (or near there at least) at high RPM. (hint: if your TH350 or TH400 ever has the problem of only shifting at W.O.T., check the vacuum modulator and the line to it first).

You can think of the vacuum modulator as a sort of throttle position sensor, since it effectively controls shift points based on a loose relationship to throttle opening. The one thing it won't do is downshifts, which is why both the TH400 and TH350 use a separate downshift system. The TH400 uses an electric switch that works a solenoid valve in the trans while the TH350 uses a cable to work a mechanical valve.

On newer transmissions, such as the 700R4 and the 200-4R, the vacuum modulator and valve is replaced with the throttle valve cable and throttle valve. The TV cable is essentially a mechanical throttle position sensor that controls the valve that replaced the modulator valve. The kickdown function has also been incorporated into the TV cable to save parts and cost. The switch to the cable was done because the 700R4 and 200-4R were conceived during a time when GM was developing engines that did not provide the proper vacuum signal - such as turbo Buicks and diesels. The TV cable was a way to get the same function as a vacuum modulator on these engines. GM did use the TH350 behind some Olds diesel engines and to make the vacuum modulator work properly there was an engine-driven vacuum pump and a variable vacuum switch on the injector pump that modulated the vac signal with throttle position to mimic the vacuum signal of a throttled gasoline engine for the purpose of operating the trans. This was a kluge if ever there was one.

By the way, B&M used to sell a cable-actuated replacement for the vac modulator when these transmissions were used on supercharged or turbocharged engines.