If the alternator is showing an output of 13.2 volts is it suspect?

JamesC

 

If you're doing this as a quick check by measuring the voltage across the battery with the engine running, it depends on the state of charge of the battery in order to interpret what the 13.2 volts actually represents.

It's kind of like blood pressure. Your doctor measure yours and it reads high. Is this because something is wrong or is it because you just climbed two flight of stairs to get to the office.

Are you trying to diagnose an undercharged battery condition?

The correct way to check alternator output is by loading the system, maintaining the test voltage above a certain value and measuring current output. The current output is supposed to be within the rated output of the alternator by "X" amperes to be confirmed "good".

Electrical testing requires that the battery be capable of holding and be fully charged.

 

The voltmeter gauge is acting a bit wonky, so I had a local battery shop test both the bat and the alt. I was told that the bat was fine and that the alt was at 13.2. I read online that anything below 13.5 was cause to believe that the alt was having issues.

JamesC

 

I'm surprised the local battery shop couldn't tell you straight up the alternator was bad.

 

It depends on the condition of the batt, the load on the system, etc.

If the car is running, the post on the alt should be at a slightly higher voltage than the + batt terminal. Something in the range of 50 mVDC up to a couple of tenths of a volt. Put your meter probes on those 2 spots (red on the alt, black on the + batt term) and see if the alt is at least making current go the right way... i.e. INTO the batt.

If you see a relatively high difference in voltage there, like more than a few tenths, then the batt may have a shorted cell, and the alt may be alting its happy butt off, but the batt can't get up to full voltage... with one cell shorted, everything will tend to be around 2V low.

Next, see if there's any AC volts in the same spot. Reason being, the alt is actually a 3-phase generator with a bridge rectifier; and it can have a problem (any of several problems actually) whereby not all 3 phases work. When that happens, the output during the fraction of the time that it IS working, will be correct; but during the phase(s) that do NOT work, there will be no output. As a result a DC meter will read some sort of average of all that; so you might have 14.2V but only for 1/3 f the time for example. This will show up as ALOT of AC mV at the output, where there should be essentially none (should be only a small fraction of the DC mV developed in the resistance that exists between the alt and the batt).

The batt should be no lower than 12 V just sitting there, should go no lower than 10 volts or so while starting the car, then once it is running, the voltage should increase to some value above whatever the batt was at before starting it up. That's the crudest yes/no test you can do for batt & alt function.