What Should Alternator Voltage Be At Idle. Ideally the GM alternator should be putting out anywhere between 135 and 145 volts. Joined Mar 26 2010 3329 Posts.
All too often the alternator is condemned as the source of the problem if only battery voltage is present with the engine running. That all depends on the age of your car the idle speed and the total electric load the alternator is under. Jump to Latest Follow 1 - 20 of 22 Posts.
A reduced output may be a sign that the alternator is failing but the drive belt and wiring should be inspected before replacing the alternator.
I just replaced my batteries not even a month ago. The second alternator normally ran 137-138 read from Edge or 139-140 from Pioneer navigation system at idle. Runs 135 at idle but when I start loading the alternator with lights fog lights AC fan and radio the voltage goes down to 131. That all depends on the age of your car the idle speed and the total electric load the alternator is under.
