Mod: Subaru WRX (02-07) Voltage Booster Mod

There is a Nasioc thread over increasing the voltage of the alternator (up to 14.8v) to improve performance. They insert a diode into the ALT-S fuse in the engine bay and it tricks the voltage regulator to put out a bit more voltage.

One aspect of the BRZ EPAS retrofit I was concerned about was the additional load on the electrical system. I don’t run an aftermarket stereo (at the moment, no stereo at all), but I have added HIDs and a smaller battery. On occasion, my power assist would cut out at low speeds; it is inconsistent when it happens, though.

To trick the voltage regulator into putting out about 0.5 volts, we can install a diode with the ALT-S 10a fuse. User 2.5x_sleeper over on has a good write-up if you want to build your own. I did not want to build my own so I opted to buy a resettable diode from Aussieland.

The diode isn’t cheap, but I was looking at it from the stand point of buying a high output alternator. Rather than spending hundreds of dollars I could spend $50 instead.

The diode arrived pretty quickly, tapped inside a piece of cardboard.

Link –

We need to replace it with the ALT-S 10a fuse.

The diode is directional, so when you install it and you get a battery light on the dash, then flip the diode around.

At idle we have 14.6v now compared to the 14.2v before. The car seems more responsive, although it’s hard to say for sure. The steering seems a little more consistent as well, but that could be in my head. I probably need to swap the diode and fuse out a few times to see if there is any difference.

Up next, we are adding a controller to the BRZ EPAS system.



  1. Peter Spinale says:

    Have you measured the voltage at the coil packs? I found on my motorcycle a significant voltage drop at the coils, since primary voltage is directly proportional to the secondary voltage any drop effectively reduces spark voltage. I ran larger gauge wires from the battery and recovered over 1 volt, I suspect that something similar is going on here. I couldn’t apply and performance increase to the change; I was actually trying to “mythbust” the concept floating around the forums. I know the WRX has no larger wire going to the coil packs than the motorcycle did, so I suspect a similar voltage drop.


    1. A.A.Smith says:

      I have not, but that does give me something to consider. I need to test the voltage drop on my trunk mounted battery. I would like to go to a smaller gauge power wire one day, and I would like to compare the voltage drop of different gauges to my current setup.


      1. Peter Spinale says:

        Ugg no way I want to mess around with that routing again. I went big and will leave it! However I should have gone longer so I could have mounted the battery on the passenger side.


      2. A.A.Smith says:

        Haha! I have more things I want to relocate. Plus with the EPAS stuff under the dash, it is getting crowded under there.


      3. Peter Spinale says:

        Well new carpet has to come at some point, but, I thought ahead and ran several multi conductor cables front to back as well as two 1/4 inch tubes, and one smaller one. One of the tubes for the washer tank now in the trunk, the other is for meth (it’s teflon). That’s still in progress, hopefully this winter, I could really use either meth or even water here in AZ not just the heat, but only 91 octane.


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