Voltage drop headlights flicker while in D during hot weather

[jgazdag]

Hi there, I am trying to sort out some issues with my 08 Silverado crew cab 5.3L. When it's hot outside 90+, my voltage drops to 12ish or below, my headlights flicker rapidly when I am at a light or idle for even a short period of time, it also seems like my AC output gets weaker- not as cold as when actually driving. When its 75 or below, I do not have any issues.

 

If I put the truck in P the voltage slowly creeps up. When I start the truck it is 14ish and it drops from there as I drive. At highway speeds it builds up enough so that the AC seems normal and my headlights are no longer flickering.

 

I replaced my battery- positive battery post came out of the battery, and had the system tested. Advance Auto tested everything OK.. the truck was turned off so the battery was back to 14 when it started but it did drop slightly and came back up because the truck was in park. At home I pulled up to park and had my foot on the break and saw the headlights flickering into the garage. As soon as I put it in P, the flickering stopped and the voltage restored. It has completely shut all electrical off before and in the past there was times it wound not start unless I jiggled the positive battery cable..sorry lots of issues recently in the past but I have not been able to isolate anything since they are testing ok, but I don't believe my headlights should flicker and the only thing I see when this is happening is my voltage gauge shows 12ish.

[Jsdirt]

I'd be giving both ends of both battery cables a good look. Sounds like a loose connection.

 

If that all checks out, it could be a belt tension or belt problem, a wiring problem between the alternator and BCM, or the alternator is failing.

[jgazdag]

35 minutes ago, Jsdirt said:

I'd be giving both ends of both battery cables a good look. Sounds like a loose connection.

 

If that all checks out, it could be a belt tension or belt problem, a wiring problem between the alternator and BCM, or the alternator is failing.

Thank you for the suggestions. I get back home later this week. I’ll crawl under and double check the cable terminations. 

[Doug_Scott]

You may also want to check the voltage at the headlight as well.  Make the check with the bulb connected.  I know that makes it difficult but if you check with the bulb disconnected you will get an erroneous value of battery voltage due to what I call "stacking of voltage drops".  You will not get a proper reading on an open circuit. Check the voltage drop of the ground at the bulb with the positive meter cable and the negative meter cable on battery,  I don't know if GM is running a slightly lower than battery voltage feed to headlights in order to lengthen headlight life, like motorcycle manufacturers, have been doing for years, or not.  

When I moved to 100w Xenon HID headlights I also moved high/low beam headlights to relays that will get around any interference from the manufacturer.

[mjm-1957]

What kind of vehicle are we talking about? Year, engine etc, what is the idle speed in gear and with accessories on and operating and what is it in neutral?

[Doug_Scott]

On 7/18/2019 at 9:44 AM, mjm-1957 said:

What kind of vehicle are we talking about? Year, engine etc, what is the idle speed in gear and with accessories on and operating and what is it in neutral?

The first post says "08 Silverado crew cab 5.3L".  Idle speeds are controlled by ECM so not really a variable anymore in gear or park.

[Colossus]

may have already been mentioned- too lazy to go re-read thru everything, but:

if you have over 100,000 miles, your alternator may be getting tired.  Always buy a new one and NEVER a re-manufactured one. 

Saw a post regarding battery cables- you said the positive terminal came out of the battery when you went to change the battery.  I'd suggest examining the battery cables very closely, both positive and negative.  12+ years ago on here I got some great advice regarding my GMT-800 (99 silverado) regarding battery cables being messed up under the shielding so we can't always see it, and to replace the complete cable assemblies.  I did so and it solved 2 issues I was having at the time. 

My next truck, an 07 Sierra GMT-900 I would notice occasional dimming of the headlights.  Figured it was characteristic of the vehicle as others complained of the same issue.  Also- grounding issues were VERY common with the earlier GMT-900s.  I had one issue that would kill the power to the vehicle completely for minutes.  The dealer never did figure it out and I traded up to a Duramax, but I believe it to be a grounding issue based upon others feedback on here later on. 

[jgazdag]

Well, I have not had a chance to measure my grounds but I noticed that when I am in D, and driving for about 10 minutes my voltage drops and my headlights go into a strobe light effect but once I go back to park, the issues go away and my voltage returns to 14V. This is more common (nearly all the time when its gets warm outside 80 degrees+) so it explains why the tests at Advance Auto shows normal when in park. There is no voltage drop, only when driving... any suggestions?