on my friends 72 f100, on the door jam under the light switch is a small round thing looks like a thermistat on a drier, has two red wires. My 71 has the mounting holes but that's all. Anybody know what this is? Clint

 

That's exactly what it is Clint. It is part of the emission control system called Transmission Regulated Spark or TRS for short. The Feds mandated it for '71 for most cars and F100 trucks. Basically it allows vacuum advance to the distributor only when in high gear. There is a switch for 4th gear on the manuals and a pressure switch for the autos. Also on the auto vehicles, vacuum is allowed when in reverse. This system is temperature controlled by a sensor located in the driver's door pillar. When the ambient air temperature is below 49* the system is deactivated and the distributor gets ported vacuum all the time. This helps cold weather drivability. At above 65* the system is activated and vacuum is allowed only in high gear. The Feds made everyone discontinue the temperature control switch by March 15, '73 so that model year could have one or not.

Here's how it works, power comes from the ignition switch and goes through the temperature switch. Then it goes through a solenoid valve and down to the transmission switch looking for a ground. The solenoid valve is normally open when not energized and the transmission switch is normally closed under no pressure. So when it is below 49* the temperature switch opens and no current flows through the solenoid, allowing vacuum to the distributor. Above 65* the temperature switch closes allowing current to flow through the solenoid and the transmission switch to ground. The vacuum is now blocked by the solenoid until the vehicle is in high gear. Once in high gear the transmission switch opens under hydraulic pressure and halts the current flow through the solenoid, again allowing vacuum to the distributor.

Hope all this makes sense, it seems like Ford had the most complicated vacuum circuits of the big three, especially for the six-bangers.

Barry

 

Hi Barry,

Can I just leave it unhooked or do I need to reattach the vacuum lines? My tranny engages hard when I first put it in gear in the morning. If I need to reatttach the vacuum lines what goes where.

Thanks Bob

 

Bob if you see this, go here:

https://www.ford-trucks.com/dcforum/DCForumID73/1113.html