ALT light on, check charge rate

RORY27

MEMBER
2004 VAUXHALL ASTRA
4 CYL
2WD
MANUAL
40,000 MILES

Can any one help? Alt light on, check charge rate, only bat voltage, check at alt still batt volt, check volt drop to earth ok, replace alt an all ok, but after driving for 10 miles light back on an charge rate just bat voltage, it was charging fine but now intermittently not charging an putting light on.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

Sounds like one of two issues. First it is something that is damaging the voltage regulator in the alternator or something in the engine control module is telling the regulator to not turn the field control on.

So, we need to start with testing the alternator.

https://youtu.be/Z-p4dMJUpUw

If this fails, then we need to replace it again.

However, along with this, we need to monitor the field control wire from the control module. If the regulator is being commanded to full voltage, then it may be burning it up.

Unfortunately, we don't have a manual on this vehicle to show a wiring diagram but here is a guide that will help with testing this.

https://www.2carpros.com/articles/how-to-check-wiring

Basically, we need to check the voltage on this wire from the ECM and see if it is being cycled or commanded to 100% all the time.

Let me know if you have other questions. Thanks