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Home > TECHNICAL > MANIAC ADVICE SUGGESTIONS & ANTICS > 8/4/2012 Will a high output alternator damage the car's computer (ECU, ECM, or PCM)?


Very Frequently Asked Question

#1 I own a 2003 Mazda Protege5 and I am wondering about buying your HO Alternator but when asking about them on forums and even the dealer I am told I am not able to install it on my car because it may cause problems and or fry my ECU. I am also told that the ECU controls how many amps the alternator will put out in turn making the higher output usless and with time fry my ECU. Is this correct? I just want to be safe about this. Please get back to me.

#2 I really want to get this alternator but I have been reading that the ecu can be damaged by using a higher amp alternator other than the OEM one unless some how the ECU is bypassed because of how it monitors and controls the alternator output, but bypassing causes more issues. Also most shops won't upgrade this alternator because of the type of voltage regulator it has. So would this alternator work fine with my 2002 Protege 5 and not damage my ECU?

#3 I am looking into buying this alternator for my 2004 WRX with a 2.0 turbo. I was told that installing a high output alternator will cause problems with the electrical system of my car. Example: shorts, overloads, ECM problems. Is this true? Will I have problems like this? I donít want to burn the electrical components of my car, but I have a big stereo and I need more power?

And many more questions like this....


This is the most common misconception about how alternators work. This question is by far the most asked. The ECU probably draws about 7-8 amps to operate. It will not try to draw any more than necessary. If you have an alternator capable of 80 amps or 1000 amps the ECU will still only need and draw it's required current. Any electrical component in the vehicle will only draw the current required to operate. The ECU in some cars adjust the field input to the alternator based on the given load of all electronics. On some cars this is all sampled and done 100% in the alternator's internal voltage regulator. And some cars like Chrysler, Dodge, or Jeep the ECU completely controls the voltage and field input. But with all of these systems the loads of the car will be powered by the alternator. All these systems will continue to adjust field input to power loads, no matter the given load. If the alternator doesn't have the capabilities to power the given loads then voltage will not be stable and the alternator will burn out attempting to work above capacity. It is never the amperage capabilities of the alternator that causes damage.

What can cause damage is too low voltage. The voltage in a HO Alternator is regulated exactly the same as the stock unit, keeping ECU and electronics safe. What happens when your stock unit is not capable of powering the demands is the voltage starts to drop as the alternator is attempting to operate above maximum capacity. It is an alternator that cannot handle required loads that can cause damage, not a better more efficient, more capable alternator. Unfortunately, yes, dealers and forums are misinforming you. Of course the forums will probably have those that know and those that repeat what they have heard.

The only way amperage can damage a circuit is if the circuit is not fuse protected and a power wire grounds or shorts out. This causes the massive amount of stored battery current to travel the circuit burning everything in the path. This scenario can happen with or without the vehicle even on. Fortunately most all circuits are protected with fuses in case an accidental short to ground causes battery current (amps) to flow through the circuit.

If bad voltage, too high or too low, causes damage it will normally be the battery that takes the brunt of it. Batteries will buffer most damage caused by out of range voltage issues. Even a stock alternator is capable of massive voltage output, but is regulated to around 14V through the voltage regulator. High Output alternators are regulated in the same manner. You wonít experience anything but an improvement with a more efficient, better capable alternator.

Alternators from the factory are designed to power most all stock electronics and not run at sustained capacity above 80%. If what they say is true, what is keeping your ECU from exploding with the stock 80 Amp alternator when most of your loads are off (blower motor, lights, radio, etc) and your alternator is only required to power a 35 amp demand? What happens to the extra 45 amps? It just doesn't work that way. Alternators work on demand. If only 35 amps is required that is all they are pushing. No circuit, including the ECU circuit is going to draw, or 'get' more current than needed by having a more capable alternator.

Hopefully, We have made a better understanding of this misinformation and one day those that know will drown-out the ones that keep repeating this.






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