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There's some idiot stealing similar cars to mine -- mid to late 90s Subarus -- in this part of town. The police are pretty confident that he has a jiggle key (the cars have been mostly recovered, dumped but unharmed; apparently he's just using them to drive around for a while).

I'd like to do something that will make my car annoying enough to start that if he picks it out, he'll likely give up in a few minutes. A poor man's starter interrupt, in other words. Something I can disconnect when I park and then reconnect when I'm ready to go again. I'm looking into it for the future, but at the moment, I'm not going to buy and install any relays, or a real SIS system.

I don't mind spending a couple of minutes each trip with a wrench if I have to. I'd really rather not disconnect the battery and reset my ECU every time (plus that's a bit obvious). I'd also rather not pull the fuel pump fuse or relay, because that seems like a recipe for coming back to a dead battery should the car be targeted.

This is a manual transmission -- hydraulic clutch -- and I've been trying to come up with something sneaky that would prevent it from disengaging, or more likely using the clutch interlock switch.

I have an electronic ignition, not a distributor, so as far as I know moving those wires around won't prevent starting, just screw up my engine.

Is there a good option for non-obviously disabling the car that I've missed here?

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  • 7
    This question reminds me of how Mr. Bean foils a car thief by taking the steering wheel off :)
    – Zaid
    Feb 13, 2015 at 8:03
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    Not a bad idea at all, @Zaid, except that I'm scared of the airbag.
    – jscs
    Feb 13, 2015 at 8:10

7 Answers 7

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I used the switch that is connected to the clutch pedal. It is used to ensure the clutch is depressed when starting the vehicle. It's a great option as it's simple to access and it's low current. Just put another switch in line with the existing switch. I'm taking it a step further and adding an RFID reader that enables the switch for a short period of time when the correct code is read. I'm hoping this will be even more difficult to find and defeat.

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  • I've never seen a car having this switch, though I know there are some. Modern cars may use it for the start - stop automatic, so if you add an extra switch, it should be on the entire tour.
    – sweber
    Sep 18, 2015 at 6:14
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    I should have also mentioned I have a Subaru. @sweber - I selected the switch because from the wiring diagram in the service manual it seemed to be a good low current option that ensured the starter relay never engaged. I had a friend that added relays and such to disable higher current systems which I shied away from as I didn't want to complicate the system.
    – The Babbster
    Sep 18, 2015 at 14:18
  • Similar to @sweber's comment, my Honda Civic has a feature that depressing the clutch cancels the cruise control. I would not be surprised if this uses the same switch. So if you implement your RFID plan, you'll want to ensure that when you authenticate to close the switch, it stays closed as long as you are driving - otherwise the cruise control would not work.
    – Nate Eldredge
    Sep 18, 2015 at 16:53
  • Yup, the clutch interlock switch. This had the best combination of ease -- it's simple to find and not too bad disconnect -- and non-obviousness, and it's what I ended up doing.
    – jscs
    Mar 25, 2017 at 13:46
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The other day while I was searching for information on electronic relays, I ran across this solution to your problem:

pulled from 12vplanet.com

(I found it on this site.)

Here is the write up which goes along with it, which I find pretty cool:

This is a clever little circuit involving two relays and a momentary switch and is more a of a 'logic' circuit than one used to switch a high current with a low current. Once the ignition key is in the IGN position, you press and release the momentary switch and then turn the key to the START position and fire the engine as normal.

The button press momentarily energises the coil of Relay 1 which allows +12V out of terminal 87 and into terminal 86. This has the effect of keeping the coil energised after the button is released (note that whilst the button is pressed there is 0V between terminals 86 and 87). Terminal 87 also sends power to the coil of Relay 2 which enables the starter motor solenoid connection, ready for when the key is turned to the START position. When the ignition is turned off the power to the coil of Relay 1 is cut which cuts the power to the coil in Relay 2 and breaks the starter motor solenoid circuit, so the engine cannot be started again without going through the above routine. The momentary switch can be mounted out of sight and acts a simple starter inhibit security device.

Here is what is connected to the different posts on the relays:

RELAY 1

  • Terminal 86 - From one side of momentary switch.
  • Terminal 85 - Connect to a suitable earthing point on the vehicle chassis.
  • Terminal 30 - From +12V ignition switch IGN position. This supply also feeds the other side of the momentary switch.
  • Terminal 87 - To terminal 86 and Relay 2 terminal 86.

RELAY 2

  • Terminal 86 - From Relay 1 terminal 86.
  • Terminal 85 - Connect to a suitable earthing point on the vehicle chassis.
  • Terminal 30 - From +12V ignition switch START position
  • Terminal 87 - To starter motor solenoid.

You could work the momentary switch one of two ways.

First, you can hide it the best you can. If you take this approach, it really needs to be out of the way. Someplace you can find it, but not somewhere which is obvious.

The second way to deal with it is to put it out in the open ... completely in the open. If you make it look like a NO2 switch, it would easily fool a would be thief. If it was incorporated into a gear shift or some other out in the open means, it would totally confuse and confound the perp.

Whatever you decide to do ... DON'T TELL ANYONE. I am even saying to the point if you use this method, don't even check it as the answer. You need this to be as private as possible so the would be vehicle assassin will not have an inkling of what you have done to disable your vehicle.

Good luck with whatever you do. I hope they never get your vehicle from you.

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  • Thanks, this is good stuff; I ran into it too, or something very similar, in my research. I suspect it's what I'll end up doing, but like I said, I'm really looking at the moment for the "no-install" option.
    – jscs
    Feb 12, 2015 at 23:32
  • @JoshCaswell ... The reason I like this is because it should be relatively cheap and easy to install. It would also be hard to detect or to understand what is going on even if someone did detect it. Good luck to you, anyway. I'm glad you are being proactive about it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 13, 2015 at 0:50
  • You could also do as Paulsters2 suggests in his answer, but substitute in a regular on/off switch instead of the momentary switch, which then would have to be hidden. When not using the vehicle just flip the switch off. Looking at the diagram everything will be the same except for the switch. Just my 2 sense:)
    – HasH_BrowN
    Feb 13, 2015 at 7:51
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    @HasH_BrowN ... I don't think you are getting the purpose of how the above diagram works. The momentary switch negates the need for a rocker switch by using the relay as a switch that works only while the ignition is on and only after the momentary switch is pressed. Once the ignition is turned off, relay 1 shuts off and no more power to the ignition. Your rocker switch addition would negate all of that. You could in essence utilize one relay at that point.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 13, 2015 at 11:05
  • Yes, I jumped ahead of myself. I guess another way to skin a cat.
    – HasH_BrowN
    Feb 13, 2015 at 16:10
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If you really want a no-install option, and even @paulster2 answer seems like work then why not use old school locks like gear/steering locks? Or better, this - Wheel Lock

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  • Not a bad idea ... I would just hope the popo didn't come along and tow it thinking it was illegally parked! :D
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 13, 2015 at 11:06
  • If I put it on the curbside wheel it wouldn't be too noticeable, @Paulster2.
    – jscs
    Feb 15, 2015 at 19:34
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Just so you are aware most clutch assemblies (at the pedal) have a pin with a cotter key holding the clutch cable to the pedal and if you just remove that it will not operate! Also doesn't cost anything.

Bit of a pain crawling under dash but cheaper.

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My dad found a way to disable the starter on a 1988 Dodge Caravan we no longer have by using a 1/4 inch(?) headphone plug & jack installed under the dash. I believe he wired the female headphone jack into the starter relay circuit & took the male headphone plug & soldered a wire across the plug's terminals so that when the plug was inserted, it completed the circuit enabling the starter relay.

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There are wires from the key block going to the ECU and to the starter motor.

Expose some of those wires, cut them in two points, add proper connectors.

When you park, disconnect this "extension" and bring it with you, so that there will be a 30 cm gap and unless he has the connectors and an extension, he won't be able to start the engine.

A switch can be bypassed, but MISSING wires cannot easily.

Of course, the next day he'll be back with the extension cord, provided he understands what's going on.

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Disabling the starter on a manual trans doesn't do much good. My driveway has a gentle slope - and if I put my '96 Ranger 5 speed in reverse and let it roll about a foot and pop the clutch, the engine starts. You need to disable either the ignition, fuel pump, or fuel injectors. A good option for a momentary switch, totally hidden, is a microswitch. It can be behind plastic ANYWHERE and all you need to do is pass a magnet across it to energize it. Cutting the positive line to the injectors with a relay is simple and effective - the computer grounds the injectors to fire them - in pairs or sequentially - but the power is common to ALL the injectors - so only a single point to interrupt.

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