I want to be be able to immobilize a vehicle. The use case is for theft prevention.

All along I have been thinking in terms of disabling the starter relay, as discussed here, Current from key to solenoid of starter motor relay , however a relatively high current carrying capacity is required which makes the control difficult.

What if I were to disable the fuel pump instead. Does anyone know what are the pros and cons of immobilizing the fuel pump versus the starter relay?

2 Answers 2


Using the relay to disable key elements of a vehicles ability to run give me pause. Any misstep in the implementation process could lead to a failure of the pump or device critical to the vehicle continuing to run could proved to be dangerous.

I would not recommend using a switch or relay due to the hazard of the vehicle failing to run while driving. A plethora of disastrous situations come to mind that I will not illiterate due to relevance.

Disabling the starter motor through the starter relay with a switch/relay device seems considerably safer. If the device you insert fails, your car will not start. Since it will not be in motion during the failure, it seems like a considerably safer route.

This should be technically simple as you would disable the low voltage side of the starter relay and the voltage will be 12.6 volts, roughly.

  • Thanks DucatiKiller, so basically, failure by the relay located at the fuel pump could cause the vehicle to cut out while in motion?
    – SeanJ
    Commented May 12, 2015 at 8:11
  • I know from an installer, with whom I work, that a lot of immobilizers are installed at the fuel pump, any ideas how they get around potential cut-outs?
    – SeanJ
    Commented May 12, 2015 at 8:17
  • The fearful part of me that has seen so many mods done by people fail, says, "don't do that one." :) Not that it's a mod, more that it's so DIY. Commented May 12, 2015 at 8:28
  • I'm sure they do quality work. I'm not saying my beliefs are correct. I'm just sayin. This question turns out to be very opinion based rather than fact. I've shared an opinion. Commented May 12, 2015 at 8:30

If you immobilize the starter motor, then the start motor wont run, so even if you put the vehicle in gear there will be no movement.

If you disconnect the fuel pump, the starter might start and run, if you engage the gear (depending on the vehicle) it may move a little distance too.

Could you please provide little more details on the use case for immobilizing... based on that we can suggest better.

  • Thanks Dinesh, just editted the question, use case is theft prevention. When you say little distance do you mean a few feet or kilometers? Also, which would typically be easier to install?
    – SeanJ
    Commented May 11, 2015 at 14:50
  • @SeanJ it depends, but mostly it will be few feet. For Theft Prevention, we need to have both the starter motor control (with the use of relay) as well as injection control. If you dont cut off the fuel and if it is mechanical injection, then by just pushing the vehicle also we can start it, so disabling starter relay alone wont help. For more information look into [passive anti-theft system ] Commented May 22, 2015 at 11:27

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