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June 13

Subaru Alternator Fault Fix!

Modification of ECU for Subaru Alternators

This problem occurs in the following Subaru models with C/S/L Mitsubishi and Hitachi 3-pin Alternators:

Forester 2001 to 2008 (models SG5 SG9)
Impreza 2001 to 2007 (models GG2 GG9 GDA GDB)
Legacy & Outback 2001 to 2008 (models BEE BE5 BHE BH5 BP5 BL5 BPE BLE)

Fault / Symptom

The battery goes flat
Engine check light is on and the alternator appears to under charge (this can be intermittent)
An ECU fault code is logged as “Alternator circuit low”
The alternator charge/warning light comes on while the engine is running

What’s happening and why.

The Subaru ECU has an “Alternator charge Cut-out Function” that is controlled by the electrical load on the vehicle, the vehicle has an alternator with a 3-pin plug (C=computer control: S=Battery sense: L=Warning Lamp). If the ECU detects a “no load” situation anywhere between 20 seconds and 1-minute it will pull the “C” terminal (blue wire) to ground which reduces the alternator charge output to around 12.9v from the usual 14.2 to 14.4volts.

This feature was introduced to reduce engine load by disengaging the alternator, however more commonly it will cause the battery to receive little if any charge, particularly if the vehicle is being used on short drives, starts and stops a lot or if the headlights or air-conditioning system is not used very often.

If the fault code is cleared the system will reset the alternator to supply the nominal charge to the battery of around 14.3volts, however after a short period the ECU function will again drop the voltage to 12.9volts and the engine check light will again come on!

The Fix.

The ECU must be fooled into thinking there is always a load present so the alternator cutout function does not turn on. Subaru Foresters do apparently have an ECU software upgrade available from the Subaru dealer, but for all the other models there is no software upgrade fix so the following work around procedure can be performed.

  1. Locate the main ECU and vehicle wiring diagram
  2. Locate the park light wire in the plug to the ECU, normally thin Black with white traces (but double check!)
  3. Cut this wire and permanently ground it to the large main ECU earth wire, normally thicker Black with white wire
  4. Tape up the wire from the park light switch end

This modification simulates the vehicle park and tail lights are on all the time fooling the ECU and thus causing the alternator to charge above 14-volts at all times.

If you are not sure how to do this we recommend that you take your car into your local repair shop or Subaru dealer and mention this fix, they will be able to do it for you if required.

Of course if you find that the alternator is faulty then it will need to be replaced, we have them available fully tested and guaranteed so give us a call if you do find you need one.