This system is used on 1972 vans equipped with the 6-240 (3.9L) and automatic transmission.
The TRS system differs from the Dist-O-Vac and ESC systems in that the speed sensor and amplifier are replaced by a switch on the transmission. The switch is operated by mechanical linkage that opens the switch when the transmission enters DRIVE.
The switch, when opened, triggers the opening of the vacuum lines to the distributor, thus providing vacuum advance.
The switch is also rendered inoperative when the outside air temperature is below 65°F.
So, in short, when the air temperature is below 65°F and/or the transmission is in 1st or 2nd gear, there is no vacuum to the distributor.