Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Quick Test Information
Note: applicable only to vehicles manufactured after week 46, 2004.
The central electronic module (CEM) has built-in functions for regulating the alternator (alternator control module (ACM)).
To charge the battery optimally, output voltage from the alternator is calculated based on the calculated battery temperature. The central electronic module (CEM) calculates battery temperature from outside temperature, which is obtained from the outside temperature sensor.
Depending on the vehicle's ambient temperature and/or if the vehicle has been without power, the calculated battery temperature (read temperature) may differ slightly from the battery's actual temperature for up to 24 hours afterwards.
In the following tests, battery temperature in the central electronic module (CEM) is changed, thereby changing voltage from the alternator.
The test is divided into two parts - an automatic test and a manual test.
Select one of the alternatives below:
- Automatic test of alternator function.
- Manual test of alternator function.
Refer to MANUAL TEST OF ALTERNATOR FUNCTION
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.