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I'm attempting to wire up my remote start in my manual transmission vehicle to ONLY start when the vehicle is in neutral. The way that the transmission works is like this:

  • When in neutral, resistance is >150 ohm and no voltage.
  • When in gear, resistance is <1 ohm and 12 volts.

However, the remote start neutral safety wire works like this:

  • Remote start will start if ground circuit is complete. ("Shows ground")
  • Remote start will NOT start if ground circuit is not complete.

My understanding is that I'd have to use a relay that accepts a positive input, and provides a negative output. I had wired up a 5-pin SPDT relay like this:

  • 30 to ground out (remote start safety wire)
  • 85 to positive input (transmission signal wire)
  • 86 to constant ground (chassis)
  • 87a to constant ground (chassis)

In theory I expected this to provide a normally-closed ground circuit to the remote start indicating that the car is in neutral and able to be started, and when the car is in-gear, it would provide the positive input signal to cause the relay to toggle 30 to 87 (not connected), thus opening the circuit and disabling remote start.

Unfortunately that did not work; the relay did not toggle at all. When I hooked up a multimeter to the positive input (with the ground connected to the chassis), it showed 0.45 volts, as if it were encountering resistance. Disconnecting the positive input from the relay caused the multimeter to read 12 volts again.

I've searched extensively online and there was one video that showed that the above connections should have been appropriate, but otherwise no other help to confirm that is the correct process.

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  • \$\begingroup\$ Those pin numbers are peculiar to auto electrics so you might want to explain their function. You can add a schematic in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Commented Jul 20, 2019 at 21:55
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    \$\begingroup\$ You might want to check the wording of your first sentence. I would want the car to start ONLY when the trasmission is in neutral, or NOT start UNLESS the transmission is in neutral. \$\endgroup\$
    – Peter Bennett
    Commented Jul 20, 2019 at 22:06
  • \$\begingroup\$ What is/was the remote start brain model and the year, make, and model of the vehicle, as both are required to provide an answer to this question. Normally, either the remote start brain's install manual will provide what wire to use, or the data module's manufacturer will (e.g. iDataLink). Usually the wire used is one coming off the BCM or MultiPlex, not one in the engine bay or coming off the ECU. Some M/T vehicles may also require a second wire's circuit to be broken at crank via a relay using a 2nd start output as the trigger. \$\endgroup\$
    – JW0914
    Commented Mar 29 at 12:01

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The issue was not with the wiring flow, but instead that the current coming from the transmission was only a measly 5mA (@12v), which isn't enough to trigger the coil inside of the relay. This makes sense since that wire goes directly into the ECU.

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  • \$\begingroup\$ IIRC, 12V relays require at least 20mA as a trigger \$\endgroup\$
    – JW0914
    Commented Mar 29 at 12:02

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