Volts low for a few seconds after a cold start

jd8181

Original poster
Member
Jan 8, 2012
22
I have noticed lately when I start my 05 trailblazer (4.2 68,000 miles) the volts are around 10 or 11 when I start it (been doing this since it has been about 30 degrees or below outside), but after about 3 seconds they jump up to 15 or 15.1 and after 30 seconds or so they settle back down 14 or right around there. I have a scan gauge so I always watch them. Is this normal or is there something I should check out?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
It's totally normal and expected behavior. If GM cared, they would have mentioned it in the Owner's Manual but they were goofs and failed.

The PCM suppresses the alternator's output for a few seconds on a cold start to allow the RPMs to stabilize a bit on the cold engine with cold oil. Then, later, it allows the alternator to light up, and put its load (1-2 horsepower) on the serpentine belt to recharge the cold battery that's been a bit more depleted during the cold start. If the full load of the alternator were dumped on the freshly-started engine, it might cause a stall.
 

Rubberman

Member
Dec 14, 2011
117
yeap i notcied that after a cold morning start. the headlights and dome lights dim somewhat for a bit.
 

jd8181

Original poster
Member
Jan 8, 2012
22
Ok another question or 2. What is the average volts a trailblazer should be running?
Also every once in awhile at night if I am driving and have the heater on high when I hit the brakes the lights will dim and the heater will lower for a split second. Normal or not?
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The alternator's voltage regulator varies according to how discharged the battery is, and the temperature. I've seen from 13.6 to 14.8 on a good voltmeter (not the useless dash gauge).

The alternator can sometimes take a split second to readjust when the load is already high (blower plus headlights) and you hit the brakes adding another few amps. Not as much as it droops when the SAIS performs its run-time diagnostic test (3, 30 Amp pulses a few seconds apart). Again, unless you have a loose set of fat wires, it shouldn't happen too often or droop too much, but a few droops are totally expected with this platform. So much so that GM put out a service bulletin to help service advisors placate the nervous owners who reported droopage.
 

jd8181

Original poster
Member
Jan 8, 2012
22
Thanks for the insight roadie. The headlights only seem to do it when it is cold and just a couple of times at the beginning.
My scan gauge reads between 14.6 and 14.9 volts when driving.
 

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