So I have a EMU Pro 8 that I am running on my racecar, and I am currently running it without a main EFI relay. I have the B15 switched voltage pin, injector 12v, coil 12v, and O2 heater 12v all wired to one side of a switch. The ECU’s 12v supply pins B8 and B14 are both wired to permanent 12v supply, not switched. This setup works for the most part, except if I go to turn the engine off the ECU stays on since the ECU pulls the negative side of the O2 sensor heater to 12v when the engine isn’t running which backfeeds over to the positive side of the heater which is connected to the same switch as B15 switched power. If you manually ground B15 to turn the ECU off everything stays off since the ECU is no longer pulling the negative side of the O2 heater to 12v.
Is the way this is wired supported, and if so how would I fix the O2 sensor backfeed to B15? If that configuration isn’t supported, would also switching B8 and B14 be a supported configuration instead? Would you recommend wiring it this way in this application or is it generally always recommended to run an EFI relay?
Also I was wondering what the minimum activation voltage is for pin B15, and is there any way to change this? The backfeed I am seeing in my current setup is only 2.3v but that seems to be enough to keep the ECU on.
That’s an interesting idea. That should work since it would decouple the positive side of the O2 heater from B15, but I have my harness heat shrinked and sealed up so I might save this as a last resort haha.
Would there be any disadvantage to just having the ECU permanent power pins B8 and B14 go to the ignition switch as well as just cut all power to the ECU? My switch is rated for 30 amps so I’m guessing it has enough room to power everything, and reading the ECU pinout dock it does say this is supported. Only thing it mentions is that delayed turnoff would no longer function, but I don’t see any settings for delayed turnoff. What advantages are there to having delayed turnoff?
Another workaround I just thought of is if I turn on the “enabled heater without RPM” setting the heater would get pulled to ground when the engine stopped so I wouldn’t get backfeed and the ECU would turn off. The downside is that this would also turn the O2 heater on when the ignition is actually on which would be really hard on my sensor, especially since I am running a really rich methanol mix at start.
You are right. The delayed turn-off has not yet been implemented in the software. It’s a feature that lets you keep the ECU on after switching the ignition off. For example, to cool the car before actually turning everything off.
I have some other ideas you can try.
Add a series diode on the positive wire for the O2 heater. The diode should allow the current to flow from the 12V switch to the heater but not the other way around, which should stop the back feed of the switched input. It should be a standard silicon diode with a 2A rating or more.
Add a pull-down resistor on the 12V switch output (connected to ground). When the switch is turned off, the resistor should keep the voltage close to ground level. Less resistance means stronger pull, but you must keep the power rating in mind. When the switch is on, that resistor will be connected to the battery voltage. A 100 ohm pull resistor can have a power loss of 2.25 W (15V*15V/100R). You can start with a 1k resistor, which might be enough, and won’t get hot.
Okay that makes sense on the delayed turnoff stuff. Now that I know what that is I don’t think that it’s a feature I need, so would you have any objections against just running the ECU 12v supply pins off the ignition switch as well?
And those two other ideas seem promising as well. I like the idea of the diode since it doesn’t really draw/waste any current like the resistor although that isn’t a big deal. And really this application is what a diode is made for.