Not having a problem, just curious.
On my 70’s Ford truck, the wiring schematic shows a current path from 12 volts through the ignition switch then through something called the “ambient temp switch” then through some kind of vacuum solenoid, then through something called a T.R.S. switch then to ground. (There appears to be an option where the TRS switch is shorted with a jumper wire all the time.) What’s all that about? A little googling suggests TRS means “transmission range selector”, so that appears to be the switch that prevents starting in anything other than neutral and park, right? I’m pretty sure at some point in the distant pass I bypassed the neutral safety switch on this truck. Maybe it stopped working as was preventing me from starting the truck. But what is all the business with the ambient temperature switch and the vacuum solenoid? What do those do?
The terminology “transmission range selector” is fairly new, dating from the introduction to OBD2, a somewhat standardized emissions diagnostic system beginning in 1996. One of the things OBD2 did was standardize the names of certain things on cars. The same sensor on different cars could have been called HEGO sensor, O2 sensor, Oxygen sensor, etc. There’s a sensor on the transmission or linkage somewhere that reports what gear the transmission is in to the computer. Ford called this the MLP sensor, another company the Gear Selector Sensor, another simply Neutral Safety Switch. The standardized name for this became Transmission Range Selector. But all this is a distraction from your truck, which has no TRS in this sense.
I think it’s a Timing Retard Switch.
Carmakers had a helluva time meeting emissions requirements in the 70’s. Emissions on carbureted engines are horrible, especially when cold. One way to curb high NOx emissions is through the use of an EGR valve. Another is to retard ignition timing, or to avoid timing advance when the engine is still cold. I’m pretty sure the vacuum solenoid you refer to is for the vacuum advance in the distributor. When the car is started, the temp sensor and vacuum solenoid would block vacuum to the distributor, keeping the timing from advancing more than a few degrees. This would help lower emissions, but also would keep the car from running well once warm. So once the engine warmed up, the vacuum advance would function normally. Ford also used vacuum delay valves on their EGR systems.
Do some research on something called a NOx Device, which was required in CA for a while. You’ll see how ridiculous some of these devices were.
Thanks @asemaster. There IS both a vacuum advance and vacuum retard line to the distributor on this truck. What you say makes sense, that RS doesn’t mean “range selector”, but it actually means “retard solenoid”. (The way it is configured, it wouldn’t prevent starting the engine anyway, so that would be a poor safety switch!). Anyway, I’ll do a little googling like you say. Thanks again for taking the time to post. Helpful.