It's fairly often around here that we get a call from someone asking about the best way to handle a neutral safety switch install in their project, especially since many older cars and trucks were not equipped with them from the factory until it became a requirement in the early 1980's. Each transmission handles this a little bit differently so it's important to know the limitations of your particular transmission before deciding the best way to approach this task. I'm only going to touch on a few options here to just give you a quick overview of a few different examples, but the possibilities are only limited by your creativity. We really just want to stress the importance of having a good quality set up that works well rather than risk the potential of an accident.
We'll start off with an example for an automatic transmission. The GM 4L60E and 4L80E series of transmissions have been extremely popular transmissions due to their wide availability and access to performance upgrades. This also makes them one of the easiest to add a proper neutral safety switch (NSS). Depending on the case style you have, you can simply add on a factory MLP switch that was used in OE applications. The MLP switch contains the wiring for engaging the starter only in the park or neutral positions. It also gives you a reverse light switch, and the ability to illuminate a display for the gear position you're currently in, if so desired. Just keep in mind that in order to utilize this particular switch, you will need to verify that your transmission case has the required threaded bungs and the long shift shaft.
The picture below is one way we would advise against setting up a NSS on an automatic, especially this particular 4L65E. With the option of having the proper MLP switch available there's no reason to set it up in this manner. Yes, this version works if it's your only option, but be mindful that this type of solution only provides starter engagement in the Park position, and in this instance the installation was not as good as it should be. The bracket used is made from a thin material that easily flexes, is only secured by one bolt, and has been repeatedly coming loose which has been causing starting issues for the owner. It will be replaced with the correct option during the re-installation.
There are also many other newer automatics that utilize a similar style of MLP switches for operating the NSS, but there are also a lot of other automatics that did not use a MLP switch, but rather a micro switch on the gear shift selector, or maybe even nothing at all. In those cases you may decide to use an aftermarket gear shifter such as one from a company like Lokar. Their shifters will have a park/neutral switch built into the shifter that can be wired up to the starter relay for a simple, but effective solution, or utilize one of their add-on micro switches as seen below. As you can see in the photo it is similar to the set up we discussed above, but has a much more robust mounting system to prevent issues down the road.
When it comes to a manual transmission the need for a NSS becomes even more important to avoid a potential disaster. Some manual transmissions will have a built in NSS and others do not, for example, the Tremec TKO and new TKX both have a built in NSS, but the Magnum 6 speeds do not. This is mainly due to the fact that the Magnum is based on an OE design that uses a clutch pedal switch for starter engagement which negates the need for a transmission mounted switch. We get a lot of questions about adding a NSS to the Magnums, but there really is no good way to work that out so we always recommend installing some type of clutch engaged switch. This is where that personal creativity can really help you out. I've seen several effective ways to pull this off and there are a couple examples below to help you visualize a method to make it work.
The TKO 5 speed pictured above is equipped with an onboard NSS (circled in red). This can easily be connected in line with your starter solenoid trigger to eliminate the need for a clutch pedal mounted switch. The only downside is, the connector used on the harness no longer has a mate, this means that the end gets clipped off, and you hardwire it in or utilize a 2pin weatherpack connector instead.
The top and bottom pictures here show some examples of a clutch pedal mounted NSS. These are a universal type part and may require some customization to make them work, but an easy solution to get you starting safely.
You may even consider a switch set up like what is shown in the picture below. You can use something as simple as a brake light switch to get the job done. It can be mounted to the firewall or on the pedal hanger depending on what may be easier for you.
