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Discussion Starter #1
Hi Everyone,

I'm trying to help out my not really mechanically inclined Mom that lives 1,200 miles away with a frustrating Parasitic Draw issue with 2009 Envoy 4.2 I-6.

OEM options/accessories: Heated Seats, Sunroof, Rear HVAC vents, 6 disc changer, rear TVs and DVD player.

Aftermarket accessories: Remote Starter, Aftermarket Backup Camera.

Here's the run down of Electrical System Issues and Repairs she has done lately.

Aug 2018: Battery Replacement

Oct 2018: Battery Failed, got warranty replacement.

Fall 2019: Intermittent starting problems.

Nov 2019: Starting issues persisted, upgraded to larger CCA battery with better warranty.

Experienced occasional starting issues after truck sat for days at a time, frequent situation since she is retired, doesn't drive daily.

Feb 2020: Battery would not charge with 12v wall plug in battery charger. Replaced under warranty 3 months after purchase.

Took vehicle to mechanic who tested battery, alternator, and ground. Found Alternator was pushing 15v, replaced alternator with reman.

10 Days later: Starting issues continue, will not charge properly, road side assistance jumped battery. Replaced battery AGAIN because it failed testing at Walmart.

Present day: Situation has regressed to the point where the battery will start draining within hours of the car being shut off.

She started hooking up the battery to the charger when parked because it had a trickle charging/battery tender function and claimed that it was showing as low in as short of a time span as 2 or so hours.

I finally got to the point where I told her to flat out DISCONNECT the battery cables when she parked and to leave the cables off until she absolutely had to drive it someplace.

She had another short appointment with the mechanic who claims he tested both the under hood and rear fuse box and that they were both showing some signs of drain. It doesn't sound like he went fuse by fuse to test of Amp drops.

I've told her to go back AGAIN and have him check the Remote Starter and the Backup Camera wiring FIRST then start going thru fuses.

If he doesn't find a definitive source for discovers that it's something more involved like the rear HVAC actuator motor she claims she can hear turn on then I want her to bring it to a dealer and have them deal with it.

Still with me here? Good! Now for my questions!

Is the ECU advanced enough on these trucks that an OBDII Diagnostic Scan could actually show a fault code for an Electrical Issue? i.e. isolate it to something like the HVAC actuator motor?

2) If the ECU is that advanced could the dealer "shut off" a power draining accessory?

3) Are there any known Parasitic Draw issues with these trucks that should be looked into?

Any other advice is appreciated!

333 Posts
I've never seen a voltage draw code - perhaps I'm late to the game on Trail Blazers, but that's never been something that a scanner could see, nor find. LOW Voltage might have a code - but actual DRAW is another animal altogether.

I'd start at basics ----> pull every fuse but to the ECM - and there may be multiple fuses for the ECM. I remember some labeled "A" and "B" - but you get the idea.

You will always 'see' a spark/slight momentary draw if you disconnect the battery and then put it back on - that's usually the ECM taking a gulp of electricity to maintain the KAM - or Keep Alive Memory.
  • This is normal and should not be mistaken as a 'draw' on the battery since it is momentary and very slow to take a battery down far enough to kill it.
  • A long vacation can kill a battery if you don't connect a trickle charger or run the vehicle once in a while.
Then - just replace the individual fuses one at a time until you find a big enough draw to kill a battery. But I may have misspoken here - at least I bet this gets misunderstood.

USUALLY anything that can kill a battery repeatedly and quickly - is the alternator.

It's a 'diode-thing' in that most rebuilts are rebuilt with a paint brush and they are still bad even when reboxed and sold OTC.

Try this - and this is not a test that most stores can accomplish ---> hook up a digital Volt/Ohm meter to the output lug on the alternator.

Test for AC Voltage on that lug. You MUST NOT see any AC voltage over 0.015VAC!

If there's more than that - then the alternator is not rebuilt or at least it is defective. Take no [email protected] from the parts house - this is not something they can usually test for unless they perform the same check, looking for AC on the DC side of the alternator.

The biggest reason why a parts store test misses this is because they hook up to the Positive and Negative battery cables and the battery is a giant capacitor that can keep the AC spikes from showing well.

YOU --- OTOH --- will be testing right at the output lug of the alternator - at least a few inches away from the battery, but it's usually enough to find that ripple!

AC Volts in an alternator's output means that there's a shorted diode and it will drain the battery down to dead every time it has sufficient time to do so.
  • It will ALSO cause a spark to be created when you re-hook the battery cables up.
  • It will create some heat inside the alternator as it gobbles the current - depending on many factors - but there will be heat as the battery is discharged.
  • Shorted diode(s) will APPEAR good on most output tests, but usually it will have a HIGH VOLTAGE output as the AC ripple confuses the regulator and allows it to overcharge.
  • You've experienced an alternator that sent out 15 Volts - right? That was likely just a bad diode trio - or the "sampling diode pack' for the regulation of output voltage.
Now that you've got this under your belt - go and test the actual car, running.
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