Battery Light Woes and Hard Starting

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by nates, Nov 1, 2013.

  1. nates

    nates New Member

    Hi all,

    I'm hitting the end of my rope with a problem on my 2005 Silverado 1500 (4.8 120k).

    I've had no issues with my truck for a long time, but a few months ago I had a hard start, and the battery light stayed on after I started up. I managed to get to work and it restarted afterward regularly and with no light. After that (couple days) it would start hard about 2/3 of the time and the light would stay on, but occasionally would be normal.

    I took it into Sears at that point to get the charging/starting system and battery checked, at which point they told me the battery was fine, and that I needed a new alternator. I got a new alternator (BBB brand) from my local jobber supply house. Popped it in, started it up, no light, felt good.... then one start later it started happening again.

    I got frustrated and just drove it as it was for a couple of days, when out of nowhere the problem went away completely. All was well....

    Then it came back a week ago.... BAD

    REALLY hard starting (always does manage to start though), and it actually died once in an intersection (which has never happened before). 8 out of 10 starts the light comes on and stays on, and even when it's not on it still sounds like it's working too hard to start. I took it back into Sears for a diagnostic and they said everything checks out fine.

    My Spitballed Thoughts:

    For some reason I feel like it could be the battery, despite multiple tests from multiple places to the contrary.
    Also concerned that it could be related to the computer control of the alternator shut off from the PCM (anyone heard of issues with this?).
    I know that the new alternator being the culprit will likely come up, but it checked out fine on the retest.

    I'm open to anything here guys. Help.

  2. j cat

    j cat Epic Member 5+ Years 1000 Posts

    with the 2005 the alternator is always working. with a battery light on I would check the voltage at the battery with a voltmeter. should be above 13.75volt to 14.75 volts DC in most all temps/conditions.

    the battery test is load testing this is always the way to be sure that the battery has good cells.

    alternators with these vehicles....... DO NOT INSTALLL RE-BUILT ALTERNATORS .....only use new alternators have a very high failure rate . reason is the windings get damaged from the high heat with this GM design,...

    hard start means the engine will not run ? hard start means poor crank ?

    if the engine cranks good then get a pressure testing gauge and make sure you have 53psi or more of fuel pressure..
  3. nates

    nates New Member

    Thanks for the reply.

    I always understood that the 2005 used shutting off the alternator to save fuel. Mine runs the first 5 or 10 mins at 14-15v (at the gauge), then drops to 12, occasionally coming back up to 14+ for periods when driving. It's always been like this (long before this issue started.

    Hard starting in this case means very slow/labored cranking. The car has NEVER failed to start.
  4. j cat

    j cat Epic Member 5+ Years 1000 Posts

    so the issue is poor cranking on cold start . this can be the wiring. check all connections to the battery starter. clean all connections take apart wire brush and secure. if all these things are done and checked out . and no change then the starter will need replacing. the solenoid has contacts that wear out .

    on the voltage of the dash indicator . this is not to be used as an accurate indicator. using a voltmeter at the battery will determine if you have problems.

    on a cold start the alternator regulator outputs a high voltage of 14.75 volts DC with a temp of 50 degf. when the alternator gets hot usually after 5-20 min depends on air temp / the voltage will drop to the 13 volt range. normally 13.75 volts DC. in arizona in the hot 120 deg f temps 13.25 volts..the hotter the air temp the lower the output voltage.

    with the newer vehicles you will get the lights dimming. also the volts will drop below 12 volts as the vehicle is operated. this is how GM tried to improve MPG'S
  5. nates

    nates New Member


    Hot or cold start makes no difference on the battery light.

    Exploring the starter as the issue.... How would that turn the battery light on?
  6. j cat

    j cat Epic Member 5+ Years 1000 Posts

    as I stated in the last posting

    " on the voltage of the dash indicator . this is not to be used as an accurate indicator. using a voltmeter at the battery will determine if you have problems. "

    the battery light on most all the time means the alternator is not producing proper output.

    now is this new or re-built alternator ? re-built get another alternator this time a new one .

    measure the volts first / WITH A VOLTMETER !
  7. nates

    nates New Member


    I've installed a NEW Delco Alternator. Same problem. Measured Voltage with the light off and the light on, and in both cases it's right around 13.5v (with a VM).
  8. Conlan Rose

    Conlan Rose Epic Member 5+ Years 1000 Posts

    That would be the problem. The voltage while running should be about 14.1-14.5 so that means either the alternator isn't doing its job or power from the alternator is facing resistance on its way to the rest of the electrical system.
  9. j cat

    j cat Epic Member 5+ Years 1000 Posts

    if the voltage across the battery is 13.5 volts engine on then the alternator is bad or the connections to the rear of it are.

    the only time you would get 13.5 volts is if the air temp is about 110 deg f.

    you should get 13.75-14.75 most all the time 99%,,,,.
  10. nates

    nates New Member

    Today was getting 14.2v both with and without the battery light on. I took off the battery cable, and the neg side had part of the boot pinched under it between the cable and the battery side terminal. I cut a little of it away so it would not interfere with the connection. No change. Disconnected my aftermarket stereo and amps (completely from the battery). No change. Checked for parasitic drain... at or under 250ma after 25 seconds, so I don't think any issue there.

    Still at a loss here.

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