Testing The Alternator s

Yesterday I had some minor maintenance done to my truck and the shop checked my battery and did a load test on it. I was told the load test was low, 9.8 I believe. I don’t know why a fairly new battery should have a abnormal load test. I thought maybe the alt. is bad so I tested it today. First I measured the battery voltage with the engine off, 12.78, which is fully charged normal. With the engine running I measured the voltage at the battery terminals. It was 12.8 which indicates bad alt. (should be 13 To 15 or so if the alt. is working) With the engine still running I turned on the lights and sound system. The voltage measured 14.2 which is normal if the alt. is working (should measure 13 to 15 when the lights are on or off.) I did it 3 times, same results.
I’m not sure what these screwy test results mean, they make no sense to me. Any ideas?

 

Get someone to help you and watch the battery voltage when the truck is cranked. Should not drop below ~9.5vdc That's a bit better load test on the battery w/o having a full on tester.

Your 12.8 w/no load is awful close to that 13 goal. Confident in the accuracy of your DVM?

I don't know if voltage regulation is in the alternator in these or in the ECU. Your symptoms seem a bit like a regulator issue.

@Jimmyh might have some better input.

 

I have an added voltmeter in my dash. It normally indicates 13.6 volts. In the winter, after starting, it will boost up t 14.1 or 14.2 volts for a short while and then return.You are seeing the action of the voltage regulator compensating for the added load it senses. If you are getting around 13.6 volts, consistently under all factory loads ,your alternator is probably ok. I would clean and tighten the battery terminals first and retest after the battery has regained a full charge. This assumes your drive belt is not worn or wet and slipping under high loads.

 

It was in the regulator up to 2017. In 2017+ it is in the ECU.

OP after starting, the alternator will temporarily run at a higher voltage before settling down. I've measured my hot idle at 13.9v using a multimeter on the terminals (without a load, like headlights). If yours really is 12.8v at idle there is a problem, the alternator should not produce output that low. I would double check the idle measurements.

 

To ClearWater Bills point - I would try another VOM if one is available.

 

Thanks all for the great info. I have another voltmeter and retested. Looks like the alternator is ok. With the different voltmeter I got almost 14 volts with the truck running at about 1000 rpm both with and without the lights on. The battery is not holding a charge as it should and the load test was borderline. I think I need a new battery. At least it is still under warranty and they will replace it no charge.

 

I had to jump a guy last night at work ....about one year old Sears Diehard Gold, dead as a door nail. This is becoming a pretty common thing it seems. He got a new battery.

 

Just for a little data to show how output changes over time, I did some voltage logging:



So you can see that after starting the truck voltage will initially be higher to charge the batteries lost power before settling down to a more normalized output, but even then there are still fluctuations. Peak output in the above chart is 14.09v.

 

I was taught to load the battery to half the CCA rating for 15 seconds, voltage should stay above 9.5V. Depending on the tester used, if a larger load was applied, that would throw off the test.