How to hook up the Neutral Safety Switch on the TR-3550/TKO
The TR-3550/TKO 5sp transmissions come with a Neutral Safety Switch (NSS) pigtail that is on the driver’s side (left) of the rear tailshaft housing. (You can see it in the pics on this page). This switch is typically used for the newer cars with a computer. However, you can also take advantage of this NSS in any
vehicle. I’ll describe how I used it in my ‘69 Chevelle.
The first thing to understand is that this NSS is designed for LOW amps, so you have to be careful and use an electrical relay.
Find your original wire that powers the starter when the key is on. It’s a #12 (or 10) purple wire with a white stripe on my car. Basically it goes from the key to the starter solenoid, when
the key is in the start position, this wire is hot. If you have a clutch safety switch, then this wire goes through the clutch safety switch. If you have an auto transmission, then it goes through the
original neutral safety switch.
That purple wire is big, and carries a lot of amps (too many for the 5sp NSS). This is where the relay comes in. Somewhere near your starter you’ll want to install an electrical relay. Provide
a hot (+) from the battery to the relay. That is the source that will power the starter solenoid. Next, break that purple wire, and connect the end that goes to the starter solenoid to the hi-amp
output of the relay. Next, connect the end that goes to the key to the relay (the terminal that activates the relay). And of course connect the ground terminal to a good solid
ground. See the crude diagram below.
At this point, you should be able to start your car just as you always have. The difference is that you now only have enough amps going through the original large purple wire to activate the electrical
relay. Now you’re ready for the NSS on the 5sp.
If you don’t have, or aren’t going to use the car’s original NSS, then all you need to do is bypass the original NSS. To do this, unplug the purple wires from the original NSS. Splice in
a #16 wire to the hot side, and run that wire down to either side of the NSS on the 5sp. Then run another #16 back up to the other unplugged purple wire. I didn’t bother to try to use the Ford
plug-in that comes with the NSS pigtail. I just cut it off and crimped on a couple of spade terminals.
It’s as simple as that. Now, to make a full circuit to activate the starter solenoid, the transmission must be in Neutral.
But what if you want to use BOTH the original clutch safety switch AND the 5sp NSS? Why? Well, with this setup, you must either push in the clutch, and/or have the transmission in Neutral. I
feel this is pretty convenient, because I don’t always want to put the transmission in Neutral to start it.
Leave the original clutch safety switch in place, and don’t by pass it. AFTER the clutch safety switch, splice into the main purple wire. Run the spliced wire down to the 5sp NSS (either
side). Next, splice into the purple wire in FRONT of the original clutch switch, and run a new wire down to the 5sp NSS.
Voila! You have both a clutch safety switch and a Neutral safety switch that will work independently of each other (or together).
The following diagram is a little crude (ok, a lot crude), but it should give you the right idea.