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Discussion Starter · #1 ·
Until I get RPMs up to 1500-2000, either in neutral or while driving. Takes getting up to 20mph for needle to move back up & will start to drop at idle at a red light unless I shift to neutral to rev engine to get it back up

Normally, gauge used to show needle at center mark of 14 or slightly above with engine running

Gauge now shows same - 14 or slightly above only when I have revs up while driving or raising revs at idle to get back to 14 or above

I have to assume that it means the battery is discharging when it is below middle gauge and have had to jump it a few times recently even tho battery reads right at 12v when tested

Battery is fairly new & has been reliable. 12.45 is highest reading I've ever recorded with engine off right after shutting it off. Van has been sitting for 2 days and has been reading 12.36 with checks both morning & night both days

Looking for some guidance where to look for my problem

As always, TIA

Bill
 

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Have you checked it with a meter at the battery above and below 2,000 rpm? Could be a poor connection or a failing alternator (bad diode in the rectifier). The battery should read higher right after shutdown.
 

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Discussion Starter · #3 ·
Have you checked it with a meter at the battery above and below 2,000 rpm? Could be a poor connection or a failing alternator (bad diode in the rectifier). The battery should read higher right after shutdown.
 

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Discussion Starter · #4 ·
-Battery read 12.36 before starting today, temp low was 36* last nite-
-Battery read 12.00 at idle
-Battery read 14.7 @2000+ RPM
-Battery read 12.36 at shutoff
-Moved Van to driveway and filled tires with small compressor
-Battery read 11.93, engine off, while using cig lighter plug to put air in all 4 tires
-Drove van 1 mile before putting in garage
-Battery read 12.70 after shutdown
 

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2000 Lifted 4x4 Astro 92 V8-350 Shorty
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Battery reads lower running at idle than it did sitting with engine off.
This is a good clue... drawing more running than you are charging.

You have one several possible likely issues:
(assuming battery is good)
  • Bad alternator. By this I mean it is weak or partially bad... with possible some good and some bad internal diodes. Only takes one diode to fail, while the others still work to cause weak output. This means it will charge, but no where near it's full typical current output. Often times high revs will bring a "weak" alternator to just above flat rate.. producing only slightly more than you are drawing.

  • Excessive draw. But unless you are drawing more than normal (and these readings are not typical), this isn't likely the case. My "stock" alternator always showed negative at idle, but improved with rpm. I was always drawing more current than it produced at idle (problem got worse when unit got weaker) and a higher output unit solved that problem

  • Poor power connections somewhere along the path, or at back of alternator. This isn't uncommon either.
One thing you definitely want to do is take a reading at the back of the alternator itself (not just at the battery). I've seen a poor power connection at the alternator main output bolt do exactly as you describe. But in a sense, no different than a weak alternator doing the same thing.

You MUST measure output of alternator (at the alternator output stud itself) to know for sure where the problem is coming from. You'll want to compare this reading with your battery readings. This will immediately tell you if it's a bad alternator, or a wiring issue.

Of course a weak battery won't hold a good charge, but this doesn't sound like your trouble. Evan at a weak battery, the charging voltage should still be good. My money is on a weak alternator (provided it was properly installed and no bad connection). My most recent stock unit did the same thing when it failed. Upgraded to the GM AD244 HO unit.. be great since.
 

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Unless you have an ungodly amp draw somewhere, the brushes and/or regulator in the alternator are on their way out. On many alternators, we have seen the "varnish" buildup on the stator contact rings and brush condition being the culprit. At lower rpm, they don't provide sufficient contact for adequate voltage and current. When you increase above idle, voltage goes up. If you are on a budget, you can clean those rings and even the brushes (if enough meat left on them) with scotchbrite and see if that solves your problem. If not, then the internal regulator is probably dying. Simple solution is that after checking the voltage at alternator under load - with headlights and a/c on, if low, below 13.5v - replace the alternator. Usually cheap enough today. Other posts are correct, check voltage at back of alternator. We have seen positive cable to junction/fuse block with powdered wire syndrome. Cable looks good, but corrosion is turning the copper wire to dust and swelling inside the jacketing.

As for ".fairly new and reliable" batteries, stop by an AutoZone (of similar) and ask them to do a battery test for you. None nearby? Old tech school test, when you know your battery is fully charged, 12 volts or above, turn on your headlights for 15 minutes with the engine off. If your battery is good, your van will start. If it's dead or the headlights look like candles when trying to start it, time for a new die hard. But look into both, could be time for a new alternator and battery as well.
 
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