Different wiring for the inertia switch

[Bitsyncmaster]

This change lets you run a hard ground on your fuel pump. And it actually shuts off your engine faster when the inertia switch gets tripped then what the stock wiring did.

After you hard wire the ground side of your fuel pump (I used frame ground), you cut the black/purple wire at the inertia switch (leave enough on the switch side to splice a new wire (use 16 AWG) running it back to your relay compartment. That new wire replaces the ground wire on your AUX relay (the one will all the white wires). What that does is turn off that AUX relay when the inertia switch gets tripped and therefor turns off power to the ignition ECU and stops your engine (same as turning the key off). When the engine stops, the fuel pump stops because the RPM relay sees no ignition pulses. Now your inertia switch only conducts about 100 ma. rather than the 6 to 7 amps of the fuel pump. You also don't need to run a new ground on the inertia switch to make your gauges read better.

Test your change by verifying the engine shuts off when you trip the inertia switch.

Now you also have an extra wire (the black/purple wire that did ground the fuel pump) running up front if you need to power something new.

[DMC-Ron]

Thanks Dave. This seems like a really good idea. Just to clarify, the relay that your talking about grounding thru the inertia switch is relay 36 which is called Aux Relay - Ignition Position -2 in the workshop manual and listed as relay 33 (Main Relay) in the DMC-12 Relays and Fuses diagram? As opposed to the Aux relay - Accessories Position -1.

Ron

[Bitsyncmaster]

Thanks Dave. This seems like a really good idea. Just to clarify, the relay that your talking about grounding thru the inertia switch is relay 36 which is called Aux Relay - Ignition Position -2 in the workshop manual and listed as relay 33 (Main Relay) in the DMC-12 Relays and Fuses diagram? As opposed to the Aux relay - Accessories Position -1.

Ron

The relay that drives fuse #1 has the white wires. Not the relay with the white/blue wires. Who knows if someone has swapped the position of the two AUX relays so looking at the wiring is a better idea. You would cut that small black wire and attach the new wire to that relay side of the cut wire.

[David T]

The only thing I see that could be bad about this mod is if the relay is stuck and the inertia switch won't shut it off. If you test it once in a while by popping the inertia switch you should be OK. In fact it is probably good to move the inertia switch occasionally so it can't get stuck either. Some owners do that as an anti-theft device. Make sure you have the inertia switch with the white splotch of paint (recall) and it has been moved to the footwell, not the Lambda counter if you have an early car.