kill switch for 2020 HD gas?

[Hagar68]

I have a 2020 GMC HD gas and want to install a kill switch to prevent it from starting.   Having trouble finding the right fuse in the engine bay fuse panel to break.

 

Initially I considered the starter solenoid but the only starter circuit there has a 30 amp relay.   I could use that but really don't want to wire that many amps into the cabin.   

 

There are a couple of ECM fuses but I would assume killing those circuits would make the computer run in "learn" mode all the time.

 

Can someone suggest the right circuit to break that won't do damage nor set off the CEL ??    Thanks.

[Jsdirt]

You'll need a wiring diagram, but I'd just grab the wire that sends the start signal to the BCM right from the column, cut it, and put the switch in series with that. 

 

I've made kill switches using male and female plugs. Crooks don't expect to see that. A switch is easy to recognize and flip, but if you don't have the other half of a plug, or know which wires in said plug to jump, you're outta luck. Attach the other half to your keychain on a quick release, and run jumpers across the pins you want continuity in. You plug it in, fire it up, then remove it so you don't forget it at your next stop.

 

Even if you can't defeat them, you'll slow them down enough to get caught, or enough to get them to move on to something easier.

[Hagar68]

Thanks, but I don't have access to the wiring diagrams (that I know of).  I have the upfitter wiring diagrams but they don't cover these areas.

 

I had already considered making a switch out of a headphone plug.  Even considered converting one of the power plugs into a switch using a closed circuit male plug as the key.

 

Today experimented with switching the circuit on the electronic e-brake, but the computer just releases it when you put it in drive.

 

The truck has 2 fuel pumps, an electric in the tank and a mechanical driven booster under the intake manifold.  The only fuel pump fuse is labeled "secondary fuel pump" which seems backwards.

 

Forgot to mention the truck is keyless version.  Keep the fobs in shoe polish tins, hood is locked, and obd2 port is in a lockbox.   If I could figure out a kill switch then maybe my OCD could take a rest.

[Jsdirt]

Yeah, I figured you had fobs - anything built after 2011 does for the most part.

 

You have direct injection. The in-tank pump boosts it up to about where a regular FI vehicle's pressure is, then the mechanically driven pump on the engine boosts it up to 500 psi or better before it's shot straight into the combustion chamber. If it were me, I wouldn't go about wiring in a fuel-cut anti theft device, since I would think that starving that high pressure pump of fuel would probably damage it. I could be wrong, but the way things are built these days I wouldn't personally take that chance. I'd rather disable spark, crank signal, or preferably both. 

 

You could try an Alldata DIY subscription for $20 a month, $60 a year, or $130 for 3 years. You can print off everything you need and then some in a months time. 3 years would cover you for any future problems that WILL inevitably arise.

 

With these newer vehicles (Anything built after 1995), without a wiring diagram, you'll be attempting to paddle UP Niagra Falls. Even with a solid understanding of electronics and 12v systems, it's nearly impossible to do anything like this without one. Much less stress having a road map.

 

And just FYI - to disable a 30A relay, you don't cut the secondary side of the relay (The full 30A) - you cut the primary side. The side that magnetically closes the contacts IN the 30A circuit. That is maybe 3A tops, probably less.

Edited July 5, 2022 by Jsdirt

[newdude]

23 hours ago, Hagar68 said:

I have a 2020 GMC HD gas and want to install a kill switch to prevent it from starting.   Having trouble finding the right fuse in the engine bay fuse panel to break.

 

Initially I considered the starter solenoid but the only starter circuit there has a 30 amp relay.   I could use that but really don't want to wire that many amps into the cabin.   

 

There are a couple of ECM fuses but I would assume killing those circuits would make the computer run in "learn" mode all the time.

 

Can someone suggest the right circuit to break that won't do damage nor set off the CEL ??    Thanks.

 

 

Are you keyed ignition or push button?

[Jsdirt]

I'd wager push button on a '20.

[Hagar68]

sorry I left this out, the truck is push button start with the fob in your pocket.

 

jsdirt:. A VERY good point you make on starving the mechanical fuel pump.  Had not thought of that.

 

in initial post I referred to a starter "relay" in the engine room fuse box.    It is actually a 30 amp circuit breaker with just 2 spades.  It seems to be a strange rating though, the starter itself would be 100 amps or so, but a starter solenoid would  not be anywhere near 30 amps.  I could put a relay in that circuit and just pull the low amp relay coil circuit into the cabin.

 

there is start interlock from the brake pedal light switch that would work, but it has 3-4 wires to it so again need  the wiring diagram.  For that matter, there are probably a dozen interlocks that have to give permission to start.

 

the Aldata sub is a good idea, but surely someone has done this before.

[Jsdirt]

People don't seem to tinker these days like they used to. I'd be surprised if anyone chimed in here that has done it. Quite a few people got scared away years ago by attempting to change spark plugs only to find a 6 hour job in front of them on V6's with the rear bank buried under the intake, and then, a check engine light with codes galore after the job is done. 

 

Another possibility to think about, with the complexity of all the modules and their communication systems networks, disabling the starter, or spark, will more than likely result in a code being set. That may or may not activate the MIL, depending on what system the code is triggered in. Body or chassis codes won't, but powertrain and communication codes will. That could be a huge PITA. If you live where you need inspections and have to pass emissions, the hard codes set by this may prevent you from getting a sticker. Just something to think about. If it sets a body, chassis, or any non-powertrain code, you can get away with it.

[newdude]

19 hours ago, Jsdirt said:

I'd wager push button on a '20.

 

 

Depends on trim.  WT and Custom and some super bare bones LT are keyed ignition.  Most all other LT, all LTZ and High Country are push button.  

 

Schematics and the starting operations are different between the two.  

Edited July 6, 2022 by newdude

[Jsdirt]

Ahh, that's interesting. On the car side of things, every single one built after '11 seems to be push button. Got a '16 Passat here for a brake job that has a fob with a switchblade key, but is push-button start.

 

Too much crap today! Impossible to keep up with it all. I want to go back to the 1972 model year when things were friggin normal ... 

[Hagar68]

I posted this last night but some how it did not make it to the forum.

 

With the risk of messing up the computer and the unknowns of setting CEL codes, I decided to change from a kill switch to an alarm.   

 

When I got the truck the first thing I did was add 2 more horns in parallel with the stock ones, with their own power source thru a relay. 

 

I've designed a circuit that will energize these 2 extra horns when the ignition fuse box is energized and my alarm switch is closed.   Bought a wireless switch today to turn the circuit on and off, so nothing will be in the cabin and the computer won't know about it.    Not as good as a kill switch, but probably 80% of the benefit for 20% of the work and risk.    My hood is already padlocked and all the components will be under the hood.

 

With my limited electrical knowledge the horns will shut off when he turns the ignition off.   When my electrical engineering buddy gets back from his motor home tour of the US later this month, I'll get his help to figure out a ratchet relay so the horn will stay on once it is triggered.

[ChevyInbred]

Just find the wire that feeds the fuse to the fuel pump. Run a switch on that parallel and your good. 

Got my wife's truck that way. In memphis you can not chance it with custom cam tahoe lol