How to install a shut off switch for anti-theft?

Being a custom installation, you're asking for answers to questions we should be asking you. Where would you like to hide the switch? How would you like the switch to interrupt something? Some factory systems turn off the electric fuel pump. Some turn off fuel and ignition. An easy circuit prevents the starter motor from cranking the engine, but if defeated by jumping the starter relay, the engine can still be started, but with a little inconvenience. This would be necessary if the switch went bad, for example, but a thief would have to know to do that and take the time to go under the hood. He's likely to just move on.

To interrupt the fuel pump, ignition system, or starter, you can do any combination with one toggle switch that must be switched to the proper position to turn those systems off. This can also be done with multiple switches that must all be set to the correct positions. A common location is under the steering column on the left side so it's easy to reach just before you jump in. The drawback is it might get hit by your kneecap. If you're happy just disabling the starter, my preference would be to use a push button. Unlike the silly push buttons used the last few years by the manufacturers, this would be a switch that must be pressed with one hand while you turn the ignition switch with your other hand. That's a less common method and one a thief wouldn't think to look for. If you want to mount that switch under the dash where it's hard to see, that could imply to a thief that it's not meant to be seen or used while driving, so it's related to anti-theft. Instead, mount it up higher where a driver would have normal access to it, and label it "Train Horn". That accessory would also use a momentary switch, but it's not likely to be pressed by a thief experimenting with random add-on devices.

Another option for any combination of systems is to use three, four, or five switches in a row. Each one must be set to the correct position for the engine to crank and run. You can have one switch for each system, or all of the systems turn off if only one switch is set wrong.

If you want to get really complicated, consider a row of three or four push button switches, and only the proper one(s) must be pressed while turning the ignition switch, for the starter to crank the engine. I'd incorporate a relay with that setup and include a warning light that says something to the effect, " Locked in theft mode. Try again in one minute". The one-minute time-out would be hard for me to design, but instead, it could just be a false warning to discourage further attempts to steal the truck.

Let me know what you'd like to do related to these options. I'll find the appropriate diagrams with wire colors and connector views. Depending on how involved this gets, I may need to do this at home over a day or two. At a minimum, you'll need to be able to cut and strip wires and crimp on electrical terminals. Better is if you are good at soldering. Keep in mind some of what you add might have to be removed in the future if things must be disassembled for other repair work. Trim panels under the steering column, for example, need to be set off to the side, out of the way, but that's harder to do when a switch is mounted to it with wires that are soldered on. Removable terminals work better in that case.

I actually did this kind of stuff to a dozen cars. Each car had up to 50 switches to create defects I called "bugs" for my students to diagnose. Some symptoms, like the inoperative starter, could have up to five or six switches in different places within the circuit. The kids just had to narrow each one down to between the closest available test points, meaning connector terminals, relay socket terminals, and things like that.

You also must decide whether you want any modification to be permanent or easily removable. Leaving extra wires can cause a lot of confusion for the next owner of your truck if they have to find a problem in that circuit.

This first diagram is from my notes to myself for the two switches I put in the starter circuit of a '98 GMC that was donated to my program. It's drawn a little different, but it's basically the same as for your truck. Your diagram is in the second image.

For reference, all starter circuits can be broken down into three parts; the low-current circuit, the medium-current circuit, and the high-current circuit. In the third diagram, I traced the low-current circuit in purple. Current will be very low, as in much less than one amp. Switch contacts can easily handle that, and the wires can be a small diameter. This is the circuit where you'll want to put the added-on switch.

To complete the story, the ignition switch in "crank" sends current through the neutral safety switch, then the coil of the starter relay and turns it on. When that relay turns on, the medium current flows through the relay's contacts, then through two coils inside the starter solenoid on top of the starter motor. In the fourth diagram, that circuit is in blue. The electromagnetic fields are needed from both coils to build enough strength to pull the drive gear into engagement with the ring gear. Current through the "hold-in" coil goes to ground. Current through the "pull-in" coil continues on through the starter motor itself which has extremely low resistance, so it's like it's just a piece of wire. Total current flow through the two coils is around 10 to 15 amps. That's too much for the contacts in the ignition switch, but the relay can handle that.

Once the solenoid has engaged fully, a contact switches on the high-current circuit shown in the fifth diagram in red. This can be as high as 300 amps to get the starter motor spinning. Once it's up to speed, that will quickly drop to less than 200 amps. Due to that very high current, connections must be near perfect, but it's where we'll find most problems. It's also the least complicated part of the system. One note of interest here, once the solenoid switches on, battery voltage is applied to the starter motor right at the point where the pull-in coil is connected. That puts full battery voltage on both sides of that coil which bypasses it. Once the drive gear has engaged, it takes much less energy to hold it in place during cranking. By bypassing it, that few amps becomes available to go through the starter motor too. That little bit extra can be just what's needed on a real cold day to get an engine started.

Finally, in the sixth diagram, I outlined the section where you can cut in and add a switch. That can't be done in the part between the battery and the ignition switch because that feeds everything else controlled by that switch, such as the heater fan, radio, wipers, and power windows. That section can have rather high current, in the area of five to ten amps. You'd need a pretty beefy switch there.

My preference would be to stay inside the cab. Look for connector C266 under or near the steering column. The diagram shows terminal "D1" as a yellow wire for this circuit. That's shown in the seventh drawing. If you plan on making this permanent, just cut that yellow wire on either side of the connector and extend them to the new switch. I like to be able to put things back the way they were, so the way I'd do this is to visit a pick-your-own-parts salvage yard, and pull out that wire with both of the mating terminals from the connector. On your truck, pull one of the terminals out of your connector and plug it into one of those you just harvested. Plug the other one into the connector. If you were able to get enough wire with each terminal, they might reach to the new switch, otherwise you'll just have to add some additional wire. Either solder the wires to the switch terminals, or you can use crimp-style universal terminals, but I always solder them too.

If you use a toggle switch, you'll have to remember to switch it to "No-Crank" mode every time you want it to do its thing. If you use a momentary push button, it's set automatically, but you'll have to press it every time you want to start the engine.

I'll work on the fuel pump circuit next, but that one will require going outside the cab, either under the hood or in back under the box.

By the way, if you need to make the diagrams bigger, copy and paste them into a typing program such as MS Word where they can be expanded. I can help with that if necessary.

Here's the condensed version for the starter cutout. Connector C100, (blue arrow), is the bulkhead connector on the firewall. Everything above that connector is inside, under the dash. Look for C266 near the steering column. The wire to cut is the yellow one in cavity D1. I pointed it out with the orange arrow in the second drawing. There will be multiple yellow wires for other circuits, so be sure you find the one in this location in the connector. It doesn't matter which side of the connector you cut that wire.

The fuel pump circuit is a little trickier. I'm gonna explain how it works so the rest of the description makes sense. You don't have to remember this later. The entire circuit is shown in the third diagram. The low-current circuit, shown in green, sends current through the fuel pump relay's coil, then to ground, to energize that relay. Normally this would be the ideal circuit to put the switch, but GM added a clinker, plus, this circuit is usually very short and hidden inside the under-hood fuse box. Best is to leave that alone, if possible. When the relay turns on, the higher current, shown in red, flows through the relay's contacts, the fuel pump motor, then to ground. This is exactly the same as how Chrysler and many other manufacturers do it. A cutout switch capable of handling ten amps or more would do nicely in this circuit, but it will require a lot of wire and a hole to run it inside the cab. The gray wire is buried within a large bundle, making it difficult to access anywhere except near the tank.

The problem with GM's circuit is they added a second feed circuit that I traced in yellow. With the engine off, fuel pressure should hold for days or weeks, but in case it bled down, all manufacturers run their fuel pumps for one second when you turn the ignition switch to "run". That's to be sure pressure is up for starting. After that, the relay gets turned on again when the Engine Computer sees engine rotation, (cranking or running). Once the engine is running, it will stall if the relay is removed.

With the yellow circuit that GM added, the fuel pump is also powered through a tap on the oil pressure switch. Here, if the fuel pump relay is removed, the pump will remain running as long as there's oil pressure, meaning the engine is running. If that relay fails or is left out on purpose, the symptom could be a long crank time because it can take a few seconds to build oil pressure while cranking, but the pump will run. To say that a different way, the cutout switch has to be after that part of the circuit so the 12 volts can't reach the pump from the relay or the oil pressure switch. Those two circuits join at splice S100. Where that splice is located is not shown, but most likely it's near the back of the engine. The best place to find the gray wire is back by the pump, terminal "B" in connector C103.

Alternatively, you could cut the pump's ground wire in terminal "D" in C103. That's down at the bottom of the diagram, a black / white wire, (black with a white stripe, or "tracer"). The connector is shown in the fourth drawing.

I looked at adding a cutout switch to the ignition system, but decided that is not a good idea. With Chryslers, there's a lot of places a switch can be added that will disable the fuel pump and ignition systems together, but on your truck, most of those two systems are separate, meaning they have little in common. If you disable only the ignition system to prevent theft, the injectors will still keep squirting fuel during repeated starting attempts. That fuel will wash the cylinder walls of their oil film, and dilute the engine oil. That often leads to spun bearings and an expensive engine repair or rebuild. Losing fuel doesn't cause any problems other than the no-start condition.

Other people can view this conversation if they're researching a similar issue, so we have to include enough detail to help as many people as possible. Without the detailed explanation related to the fuel pump circuits, a person may not get the results they're expecting. I got very specific as to which color wires to look for and where to look for them. I even copied and formatted for uploading the connector views so there would be no confusion as to which wire to cut. You don't have to know or remember the circuit explanations I included. You still haven't told me what type of switch you want to use, so I'm imagining the most common switch a person might choose, then going from there.

If you want to disable the starter system, cut the yellow wire in the 48-pin connector under the steering column, cavity "D1".

To disable the fuel pump, cut the gray wire or the black / white wire at the connector for the fuel pump. This will likely require dropping the tank.

There are a lot of other ways to disable these circuits. Some require running a lot of wire. Some require a lot of explanation to find the right wires. Unfortunately, when you tell me there's a bundle of eight wires, they don't show that on the diagrams, so I have no way of knowing what each of those wires is for. If you find a gray wire in there, you can take your chances and cut it to see if that disables the fuel pump. There are many wires of the same color, but I'm pretty confident there are no tail light wires running up by the fuel pump connector. That's why I feel safe sending you right to that connector. There are only four wires going to the fuel pump, two for the pump motor and two for the fuel level sending unit. If you can follow that bundle of four wires to the harness you found with eight wires, I'd be a lot more confident you can cut the gray wire, if there's one in there.

Let me know if one of those works for you.

This diagram is for your starter circuit. I added arrows to show the best wires to cut. You should find the yellow wire in that harness, but to be sure there aren't two or more yellow wires, it might be smart to use a test light at a connector terminal to be sure it's the right one. You should find 12 volts on it only when the ignition switch is in the "crank" position.

Outside the cab, the neutral safety switch and the starter relay are in the circuit. That relay draws very little current so you won't need much of a switch. It can even be a momentary push button switch that you have to press and hold while turning the ignition switch. If you use a toggle switch, you only have to switch it off when you want it to do its thing.

The yellow wire goes from the ignition switch through the bulkhead connector, then down to the neutral safety switch. The purple wire continues the current path from the neutral safety switch, up to the coil of the starter relay. Adding a switch to the purple wire would do the job, but you'd have to run the wires through the firewall to your switch. Common sense says use the yellow wire instead. It's already right there under the dash.

Starter circuits can be broken down into three individual circuits. The low-current circuit includes the ignition switch, neutral safety switch, and the coil for the starter relay. The contacts in the starter relay switch on the medium-current circuit that sends current to the starter solenoid on top of the starter motor. The solenoid switches on the high-current circuit that runs the starter motor. The typical relay coil will draw much less than a single glove box bulb, perhaps one or two tenths of an amp. The smallest of toggle switches can easily handle that. They're typically good for at least three amps, and most can handle six to ten amps.

Nope. Stay away from any purple wires under the dash. This is one area where GM has standardized wire colors. They've used a purple wire from the neutral safety switch to the starter relay for decades, but that wire runs from the transmission to the under-hood relay / fuse box. It is not inside the cab. Only the yellow wire from the ignition switch is under the dash.

You should see connector C266 near the base of the steering column. It looks like the first drawing. My orange arrow is pointing to terminal D1. That should be the yellow wire to cut. You can cut it on either side of the connector.

I can't tell what the connector looks like, but I do see a lot of wires, and a yellow one at the end, like in the drawing.

If you still don't believe me, poke a test light into that connector by back-probing right next to the terminal. You'll see it light up when you turn the ignition switch to "crank".

Look back at the drawing I just posted with my orange arrow pointing to terminal "D1". Right under it are "A1", "B1", and "C1". Now look at the next drawing that shows the first half of the list of wires. "A1" is listed as "white". "B1" is not listed, so it's empty, but "B2" is right next to it and is black. "C1" is listed as another white wire. "A1" is shown as a much smaller terminal than those other three. If that's what you see in your connector and they're right by the yellow wire, that yellow one is the one to cut.

This is just like reading a blueprint, except instead of reading dimensions, we're reading shapes and locations.