What is an Alternator and How Does It Work?
The alternator is a part on the engine that uses the spinning of the crankshaft to provide electricity to the car as well as keep the battery charged. More details about how the internals of the alternator work can be found by clicking here. It's a great in depth read on alternator basics! For the purposes of this post we aren't getting into those details.
Subaru Alternator Types
There are a few different alternator styles, but generally speaking you want to use the alternator that came with your chassis even if you’re installing a new engine. They all function the same, the differences just lie in the connection points between the chassis and the alternator so it's important to keep the alternator matching the chassis and not matching the engine. The older alternators do not have fuses between the battery sense wire and the rest of the car so you could potentially damage the voltage regulator (part that keeps the power output steady) by using a newer alternator in an older car if something goes wrong. Below are links to the different connector types.
- Green Alternator plug - most common
- 2 Wire alternator plug - 98 and older
- Black Alternator plug - found on newer models
- Battery Sense wire - found on newer models
How Does the Alternator Wiring Work in my Subaru?
With an alternator plug there can be either 2 or 3 wires plus the main power cable depending on the model and year of the car. We’ll break down what each wire does below.
Wire 1 – This wire is the trigger to excite the electromagnet (device that creates the electricity) within the alternator. Most manufacturers run this circuit to jump start the alternator through the charge warning light on the dash. The reason for this is two fold.
- First is that the amount of current required to excite the alternator is very small.
- Second is that while the alternator is not producing electricity the warning light will come on. This circuit flows to ground because the alternator is grounded via the case when the it's not producing electricity thus completing the light circuit to ground and illuminating the lamp.
Wire 2 – This wire is the battery or voltage sense wire. It is the wire that the alternator uses to figure out how much current it should supply via an on board part called the voltage regulator.
Wire 3 – For the cars that have this wire it’s for an external device (like the ECU) to help regulate how much power the alternator is outputting. Certain electrical systems, like the headlights, require more power so this gives an external module a little extra say in how much electricity the alternator is supplying.
Main Power Cable - This is the large cable that is bolted to the alternator itself. Subaru typically runs 2x 8 AWG (approximately 6 AWG combined) wires that connect up to the fusebox and the battery. This is the conduit for the alternator to recharge the battery and power the car.
Alternators and Grounding
Alternators are grounded by the metal case that holds it. Be careful if you are attaching the alternator to something that is painted/power coated/anodized/etc. or if you have coated the alternator itself. If there is no direct metal to metal contact, the alternator is no longer grounded. This lack of grounding can lead to a build up of resistance which can cause the alternator to over heat, spin out of control, and/or fry the battery cables.
You Removed Your Gauge Cluster For a Digital Dash and Now the Alternator Doesn’t Work
What we would recommend in this scenario is to install a basic incandescent bulb as your warning light or replicate it with resistance. When replicating resistance you need be careful because resistance generates heat. To do this safely you'll need to generate the appropriate resistance for the application and the resistor has to be able to dissipate heat well enough so it doesn’t melt. You should also include a diode because you don't want the alternator to accidently back feed your fusebox. This would be especially important if the car has a battery cut off switch because the possibility of the alternator sending power to the fusebox via the alternator trigger wire is a real concern when cutting power in an emergency.
We've used 6 AMP 68 OHM resistors with a 6 AMP Diode with good success so far but we're sure there are other combinations that would work just fine.
You do not want to supply a non resisted voltage to the alternator trigger wire because it could potentially cause serious damage.
If you look closely inside the alternator you'll see that there are built in fins. These act as fans to the keep the alternator cool while it's running. When the key is on but the engine is not running there is a possibility that if there is too much voltage being supplied to the regulator via the excite wire, it could overheat and fry the alternator. This would lead to an alternator outputting voltage out of control thus frying battery cables and potentially causing very serious damage.
Damaged Warning Light
If you have a burnt bulb or damaged cluster you may not see a warning light on the dash for the alternator but in this scenario the alternator will not charge. If you have a charging issue, first check that with the key in the "on" position the warning light turns on. If it doesn't turn on, double check the power that feeds the light as well as the bulb or LED itself.
This is a basic alternator circuit for a 2004 Impreza chassis (RS/WRX/STi). You'll see on this diagram that for STi models only, there is a tie in between the ECU and the alternator. WRX and RS models of this era do not have a wire that connects the ECU to the alternator.