G4x to control isolation relay?

[Robin]

Following on from an earlier post, I’ve got a G4x (Xtremex) coming for my track car. In the process of sorting out the wiring, but it occurred to me that should be able to control the “kill” relay. Basically, I want the ECU to hold the relay in unless there is a monetary break in a +12v signal – in which case it should kill the engine, and release the relay as soon as the RPM is low enough to minimise the risk of alternator-induced voltage spike.

Appreciate any input…

 

Apologies for the crude diagram attached, but broadly:

• Isolation relay shown kills the main power to everything from the battery.
• Momentary “On” button brings the relay to life, after which power flows to the ECU etc
• 12v is sent through 2x NC pushbuttons to a digital input (say, D1) – this input is always live unless a button is pressed
• (some logic) is set so a momentary break in this input is maintained
• ECU output (say, Aux1) is set to hold the relay in according to the logic block below.
• As soon as the output is released, the relay opens and power is cut to everything

 

The logic controlling output Aux1 would look something like:

Aux1

OFF if

Killswitch

AND

RPM<100

 

Kill switch:

Latch If D1 voltage goes below 10v for more than 0.1 seconds

Shutdown process:

If Killswitch

Shutdown engine immediately.

[Adamw]

In your existing drawing the coil for the isolation relay wont have 12V until the relay contacts are closed so it would never turn on. 

I havent thought through all scenarios in detail but you could possibly have the relay coil connected directly to batt+ if you have a spare ignition or injector output to use instead of the aux.   

The rest of the plan should work ok I think.  For the kill input use a DI, assign it to the stop switch function and enable the latch.  I suspect you would need to set the active state to low which may seem counter-intuitive but this would make it latch off during power up rather than latch on.    

[Robin]

4 hours ago, Adamw said:

In your existing drawing the coil for the isolation relay wont have 12V until the relay contacts are closed so it would never turn on. 

 

Ah, whoops. Foolish. Yes, it should be linked on the other side (to Bat+)

4 hours ago, Adamw said:

I havent thought through all scenarios in detail but you could possibly have the relay coil connected directly to batt+ if you have a spare ignition or injector output to use instead of the aux.   

I'm using all the Ignition and Injector drivers, but why not use an Aux? The relay coil will certainly be less than 3A

[Adamw]

2 hours ago, Robin said:

I'm using all the Ignition and Injector drivers, but why not use an Aux? The relay coil will certainly be less than 3A

Because aux outputs have flywheel diodes so you cant have a live device connected to an aux output when the ecu isn't powered up, it will just back feed and keep the ecu alive.  

If you have a 2 plug ecu you could try aux 5-8 in highside drive mode with the other side of the relay coil connected to ground, but then your "on switch" would need to connect to 12V instead of ground so defeats some of the isolation.    

[Robin]

OK, so that gets kinda messy and complex.

My original plan was to not worry about the run-down (just use a normal latch/reset circuit on the relay and a "Kill" signal to the ECU, which admittedly still involves running battery power to the switches).

To handle the alternator dump, I can use a relay to dump everything into a 3ohm relay (like a mechanical isolator), but a tidier alternative seems to be a TVS diode. Seems to be what the OEM would use for this situation - presents a very high resistance up to a 'breakdown' voltage, where they go to almost zero resistance (IE, keep the alternator from producing a voltage spike by dumping it to earth)

A couple of SLD16-018 would seem to be the correct thing - but I'd really appreciate any guidance.