See Figure 1
All 1972 models equipped with the 3.9L 6 cylinder engine or the 5.8L V8 engine and an automatic transmission use a transmission regulated spark control system.
The Transmission Regulated Spark control system (TRS) 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 activated by a mechanical linkage which opens the switch when the transmission is shifted in high gear. The switch, when opened, triggers the opening of the vacuum lines to the distributor, thus providing vacuum advance. So, in short, the TRS system blocks vacuum advance to the distributor only when the outside temperature is above 65°F (18°C) and the transmission is in First or Second gear.