Low System Voltage

[Aster]

I have a 2008 Astra 2dr XR.

When driving, my system voltage is low and varies with load.

• With A/C blower on high, I have 12.8 V.
• With A/C blower off, I have 13.1 V.

I thought it was the alternator; however, I put in a new GM alternator and the voltage readings didn't change.

I connect a smart charger to the battery whenever the car is not in use, so the battery resting voltage stays normal (12.7 to 12.8 V).

I don't have access to a service manual, so I am asking what things I should check to find the problem.

[AstraFasta]

Are you reading the voltages with a DMM under the hood? Or using the secret menu on the interior screen?

[Aster]

I am reading with a meter on the cigarette lighter while the car is being driven.

I've not heard about the secret menu.

[AstraFasta]

We did some voltage drop tests. Then cleaned the battery terminals and a bunch of grounds. That improved charging performance. You can search my posts for starting tips and warnings.

Get a DMM and measure voltage at the battery terminals.

[AstraFasta]

See these pages for what we did to improve charging. This also shows some of the important grounds. Reducing resistance across the system just a little can make a big difference:

http://www.saturnfans.com/forums/sho...=208132&page=4
http://www.saturnfans.com/forums/sho...=208132&page=6

Cleaning the positive terminal at the starter might help; you can check the schematics to see if that is worth doing. I can't remember.

We disconnect both terminals of the battery, wear goggles, etc. for safety.

**Removing fuse 36 (OnStar™ Module/OnStar™ InterfaceModule/Display) resolved our overnight drain issue

A few people speculated that a dead OnStar backup battery could be causing the radio to wake periodically and call OnStar. That battery is one-time use and could be drained from an accident, a mechanic disconnecting & starting the car in a certain order, etc. Page 6 of above thread gets you started.

I suppose testing & replacing the OnStar backup battery could solve this issue. Disabling OnStar is not that easy; you can see some schematics and photos of the OnStar system in that thread (Battery is behind the glove box, some equipment is also under the steering wheel.)