Truck loses power at higher speeds

Tiny
LUNDBRECK
  • MEMBER
  • 2005 GMC SIERRA
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 120,000 MILES
Truck starts and runs somewhat okay and does not lose power at lower speeds around town. Took it out on the highway the other night and within a couple of miles approaching 100 km/hr all the lights started fluctuating and dimming. The battery gauge was jumping back and forth until all the gauges dropped, the truck lost power and coasted to the side of the road. I checked the battery and alternator connections and all were secure but the battery was dead and required a boost. I boosted the battery and the truck started right up. I did a load test on the battery last spring and it was under capacity. I checked the eye on the battery today and it shows green. The truck does idle quite low but it has never stalled. Could this be a battery and alternator problem? Should I buy a new battery before also replacing the alternator? Thanks for your help.
Saturday, October 6th, 2018 AT 2:37 PM

1 Reply

Tiny
DANNY L
  • MECHANIC
  • 5,648 POSTS
Hello, I am Danny.

Before changing the battery I suggest performing an alternator test. The battery might still be good but just the alternators output is not to great. I have attached a tutorial showing what is involved in the removal and testing. Once you remove it from your truck most all auto parts stores will test it for free. Hope this helps and ask any other questions if needed. Thanks for using 2CarPros.
Danny-

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/how-to-replace-an-alternator
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Saturday, October 6th, 2018 AT 9:18 PM

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