I have a 2007 Chevy HHR that needed a new alternator at 163,700 miles. I replaced it with a NEW OEM Delco alternator, not a used or rebuild. Output voltage is normal now. The battery is 2 years old and has load tested good, also.
Now I get the battery warning light coming on after about a mile for a couple seconds, and then again in about 2 more miles, and then it goes out after about 15 seconds. Output voltage is good when this light comes on, I have a VOM plugged into the cigarette lighter. I checked with a scanner and there are no codes.
Connections at the alternator and battery terminals are good.
My question- does the ECM have to go through so many start-stop cycles before the ECM recognizes the new alternator? There is nothing wrong electrically with the car at all. Is there a re-learn process?
Maybe this will stop happening in a few days, I don't know.

 

It appears the replacement alternator could be faulty, have the battery load tested and the charging system tested, Amy local parts palace can do that for you, usually for free.

 

The battery load tested fine and the alternator is putting out 14+ volts when the light comes on. Then it goes out and can stay out for a long time. It's a strange gremlin. I don't like the idea of a faulty alternator and the hassle of getting it replaced, but that could be the issue.

 

Broken diodes are difficult to diagnose. Especially intermittent ones. Or, maybe just the wires at the connector, it is pretty tightly bent.

 

Wire from the + terminal at the front of the under hood fuse box, nice , clean and tight?


This one under the red safety cap

that wire down to the starter, clean and tight?



That larger one in the starter solenoid
Then of course the alternator wire connection , clean and tight? You just installed the alternator, so it should be clean, but just check it.

 

I am going to check all connections again today, including the battery-to-frame connection in the back where the battery is.
I will also get the old alternator bench tested to verify whether it is bad or not. My Fluke said it was no good, but if it is good-then the problem is in a bad/loose connection somewhere or the ECM.
The car starts and runs fine in the meantime.
Question- don't you think the headlights and instrument gauges would blink if there was a major ground connection going bad? Just thinking out loud.

 

Not necessarily, unless it was the headlight circuit ground wires.

 

Here's what I came up with this morning- the battery tested at 12.8 volts with a Fluke 87 with the key off. With the key on, engine running, and with every possible electrical device on (A/C, fan, high beams, radio, turn signal), the voltage at the battery was 14 volts at idle.
I connected the scanner, it showed no codes, and the battery light was off. I set off for a 5 mile test drive with the scanner connected and reading the ECM voltage. The voltage never dropped below 12.7 and went as high as 14, but normally stayed at about 12.8-13 or so. The wave constantly fluctuated about .2 volts. This is completely normal. I drove with the headlights on and A/C going all the way, fan at MAX.
The light came on briefly after about a mile, and then again in about 2 miles for about a minute. Then the light went out and never came on again. Voltage was good with the light on, according to the scanner.
I verified the connections at the alternator and the frame ground, all good.
My conclusion- there is a glitch in the ECM, possibly limited to the trigger that turns on the battery light. Otherwise, the car runs normal. I will have to get used to the annoyance of the light coming on until the glitch clears, if ever. I am NOT replacing the ECM.

 

Perhaps a full battery lobotomy is in order! Just disconnect the battery negative cable for a full 30 minutes, this will rest the computers and possibly get them to recognize that voltage is within normal parameters of the BCM and ECM , it can’t hurt to try it.

 

I will do that today and let it set for a few hours, I have another car if I need to go somewhere. It can't hurt to try it again for longer.
This car has given me 120,000 trouble-free miles in a Southwest climate where it really gets hot. Bullet-proof so far, to say the least!