Flickering lights on '99 Sonoma

Greetings all,

Long-time listener, first-time member. I’ve got a '99 GMC Sonoma that I purchased last summer. Great little rig, runs well and the previous owner took care of it. In the last couple of days the headlights and dashboard display have been flickering at all RPMs. I am guessing a bad battery or alternator, but previous related posts indicate it could be all sorts of stuff electrical. I am a shadetree mechanic with no shade (it’s winter here, the leaves are all gone!). From what I can tell from previous posts, the easier tests are 1) voltage across the battery poles with engine off to test for a bad cell, 2) AC voltage at battery with engine running to test for alternator diode problems (should be no AC voltage, correct?), or 3) something called the A.I.R. pump, which I’ve never heard of and wouldn’t know how to test.

Other thoughts? The last time I had this happen was on a 2000 Caravan, and it was the battery.


Check for clean, tight connections to the battery, alternator and grounds.
Get the battery load tested.

From experience with my 2000 S-10 Blazer with 122k miles, a battery only lasts 3.5 to 4 years. My guess, is that it is heat and a tight engine compartment.

Check the positive battery cable, remove the bolt and pull back the plastic cover. Corrosion builds up quickly on the terminal. I clean the Blazer’s terminals every few months. Check the battery voltage with a small load (i.e. the underhood light). The voltage should range from 11.9V (fully discharged) to 12.6-12.7V (100% charged). Even if the battery is charged a load test should be done. Advance Auto or similar will do the test for free.

The alternator on my Blazer is starting to whine under a heavy load so it may be reaching it’s end of life. Is the alternator on the Sonoma making any odd noises.

Finally, how old is the serpentine belt? I replace mine every 3 to 4 years.

Ed B.


Thanks for the quick reply. The belt on my Sonoma is in good shape, but I have noted a little squirrel chirp recently on cold mornings which goes away within a muinute or two. I’ve replaced many belts in my time, so not intimidated by that prospect. If the battery checks out, the next step is the alternator, which will take someone more proficient than me, since my skills are geared toward cars prior to the age of transistors.

DITWADS (Driving In Traffic With Aging Driving Skills)

When you change your belt, you may want to try turning the alternator pulley by hand to see if there’s any binding going on. 99% of the time flickering lights are a sign of a bad alternator. Combined with the chirp, it suggests the possibility that you have an alternator bearing going out and it’s binding every time the belt goes around.

I checked the voltage on the battery last night, reads 12.6 with engine off, 13.8 with engine running and light load (dome light on). I didn’t have the equipment to check for AC voltage, so will take it in to a competent shop for this test of the alternator. My guess is a failing alternator. Thanks for your help folks, my next question will be on intake manifold gasket replacements, but that is for another day…