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Main Relay question

12 March 2020
Hello Prime!

Quick nooby question about the main relay. I was wondering if the main relay controls the ignition of the car. ive read a few posts about the main relay but almost all seem to say it controls a few things such as ignition fuel and starter. Im looking to inspect the ignition line and want to make sure im following the right wire. If anyone has a image showing which wire runs from said relay to the ignition coils that would be greatly appreciated!

The Main Relay does NOT control the Starter. They are separated system.

It also does NOT control the ignition directly and there are no wiring connections between the Main Relay and the IG coils.

The primary side of the IG coil is connected to the IGP (IG1) and the Ignitor Unit.

ECU sends the command to the Ignitor to suddenly cut off the power to the primary side triggering the spark.

There are two mechanical relays inside the Main Relay.
One supplies the power to the Fuel Pump and the other for the power to the ECU and the INJ.

Without the power to the ECU, it can't control the ignition or the fuel injection.
So, one 'could' say the Main Relay is controlling the ignition and the fuel indirectly from a certain point of view.

The Main Relay is more of power supply passive control device for the Fuel Pump, the ECU and the INJ.

Here is the schematic for the main relay:

Main FI relay.JPG

As Kaz explained, the main relay powers up the fuel pump, the injectors and the ECU directly. The ECU controls the ignitor module which ground switches the individual ignition coils. The +12v supply to the ignition coils is off a separate fuse on a switched supply that is controlled by the ignition switch.
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Thank you guys for the responses!
I am looking into getting a rev limiter for more consistent off the line starts. Would you happen to know where i could intercept the +12v power that leads to the ignition coils?
Maybe I'm totally wrong but cutting the power to the ignition wouldn't stop fuel from being injected with potential fatal results for the catalytic system .. maybe the injector power would be a better option?
Yes, you can cut the 12 v supply to the ignition coils and it will kill the engine. An ignition cut is a hard limit and as @Heineken notes will do no good for the cats because it is essentially a misfire. The engine will bang back and forth between on and off at this hard limit. It will also likely trigger a misfire error code.

I presume you are aware that the ECU already has a rev limiter to limit engine damage and that you are looking for something that is lower or settable to deal with a launch? If you have a pre OBDII car you might want to look at what Motor Mouth, Honcho et al are doing with re flashing the settings in the pre OBDII ECU. You might be better off reprogramming the settings on the OEM rev limits rather than doing an external add on hack.
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I was under the impression from some rev limiters such as the bee * R that the cut ignition would allow for me to be at wot while still maintaining a set rpm. I dont have any cats in the car as I am using the taitech gt009 which is a direct hookup to the headers bypassing any form of car or test pipes.
There are some very talented folks on here cracking the ECU code....but for what you are wanting I would think you would need a standalone programable ECM.
Wouldnt a programmed rev limit leave my overall driving rpm limited to whatever it is programmed to? Say if the program limits rpm to 3000 while driving and not trying to launch the car my rpm will be limited to 3000?
You might be better off reprogramming the settings on the OEM rev limits rather than doing an external add on hack.
If you read the Nengun web site information the Bee*R rev limiter is for engine protection. Its not a launch control system and I expect it will have the same bang - bang engine speed control problems if you try to use it as a launch control.

The Bee Racing Rev Limiter cuts ignition when the rev limit has been hit, ensuring protection of the engine in all conditions.

If you are going to hack the existing software limiter on the ECU I presume that you would be keeping a laptop connected if you wanted to adjust the rev limiter setting.

You have a rev limiter feature already with the existing ECU which I think does a mix of hard and soft cuts - I would have to go back a read the service manual to confirm that. But, you definitely already have a built in rev limiter. What it seems like you are looking for is launch control which is much more complicated. Launch control in the implementations I have seen requires a drive by wire throttle and integration with traction control. The DBW acts like cruise control except that it is controlling engine speed rather than vehicle speed and is tied in with the traction control to limit wheel slip backed up by a rev limiter for when you miss that 2nd - 3rd shift.

If you try an external rev limiter as a launch control device, cutting the power supply to the coils will certainly stop the ignition from operating until the engine drops out of the rev limit threshold. If you check the ignition section of the electrical portion of the service manual you will find the wiring diagram for the ignition coil supply. The fuse for the coil supply is clearly labeled and with the harness diagram in the service manual you should easily be able to find the route for the power supply to the coils. My opinion is that I don't think that is going to be an effective launch control system and is probably a bad strategy. However, that is only my opinion and its your car so you get to try what you want to try.
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