Low Battery Light

[rgloston]

I have an issue with my Yukon ('02 SLE).I'm on my 3rd alternator, and 2nd battery. Both are less than 6 months old. Problem is that the charging system seems to fail once the engine gets warmed up. The voltmeter goes from 14 - 15 volts down to 11 - 12 volts (w/ low batt light shining bright). This has gotten worse over time.

What used to happen (for the past year) is that the battery light would come on (out of the blue) for 10 seconds, then disappear. However, the gauge would stay at approximately 15V. Now it seems as if it doesn't charge once the engine gets hot (water temp over 180*).

The last code I got before I sold my Hypertech PPIII was a P1637. Took the Yukon to the dealer, and had them diagnose it almost six months ago. These guys tell me they found nothing. I'm outta $50+, and no resolution. If I take it back, I'm wondering what they'll find, and how much they'll charge. Anyone seen this before??? I need help desperately. Thanks!

-Ron

[94 Jimmy]

The code is described as:
P1637 - Generator L-Terminal Circuit
Check out:
http://gearchatter.com/viewpost14190...6df8b32619cc29
it gives an excellent description of the circuit and how it works. It's for a Corvette, but alternators are all the same under the skin.
Essentially the L circuit from the Power Control Module(PCM) supplies a small current at engine start to generate a small magnetic field in the alternator to begin the generating process. Once the alternator begins to generate current the L circuit is switched off.
If the L circuit current is not switched off you will get a generator fault code, if the L circuit doesn't turn on prehaps you don't generate the initial magnetic field and start the generating process.
Check the small wire from the alternator to the PCM and make sure that there are no breaks or shorts to ground(chassis).
You might want to take it along with a printout of the code explanation to the dealer or better yet a good automotive electrical shop.
Good luck, let us know what you find.
94

[rgloston]

Lesson Learned...

Don't go cheap, nor try other "brand" electrical components for GM. Stick with AC Delco components. Crap-o-zone's alternators are garbage, and O'Crappy's are not much better. It's a shame that Crap-o-zone has no service individual that truly knows about auto parts & troubleshooting. Where are the ol'heads that truly work on cars.

Anyhow... end result of problem was alternator went bad. These alternators have more computer controlled and diag circuits that could fail internally, but not leave one disabled. I've had that code for months, but it finally decided not to charge anymore. Replaced it with a TRUE Delco part, and problem ceases to exist. Spent less than $200 (w/ a core swap).