2005 GMC Sierra, Lights Flicker or Pulsate

This 2005 GMC Sierra came in with the complaint that when the vehicle is started cold, the headlights, dash lights and dome light will flicker or pulsate. As the system runs the problem diminishes but never goes fully away. This is a common complaint on many Cadillac, Chevrolet and GMC full sized trucks and suv’s. I am glad I finally got a chance to test and find the root cause of this issue.

Since I was pretty sure that this was a charging system fault, I connected my tester to the battery and noted that the volts and amps readings were fluctuating as well. From here I took out my 8 gauge jumper wire and connected it to the negative battery terminal and then to the alternator frame. No change. I then connected it to the battery positive terminal and then to the alternator output terminal. No change. This test is a low tech way of checking for a poor ground or power connection condition. It is VERY IMPORTANT that the jumper wire always be connected to the battery first and then to the alternator. If there is a problem there WILL be a spark. The bigger the problem the bigger the spark. A spark near a battery can result in an EXPLOSION. It should be common knowledge that an explosion can maim or kill you! If this does not resonate with you, close the hood and take your vehicle to someone that does understand.

Keep in mind that some minor flickering is normal and is due to the changes in output voltage of the alternator as controlled by the PCM. This vehicle had a swing in voltage of up to .8 (eight tenths) of a volt. That is not normal.

There was no change during the above testing so it was clear to me that there was not an external connection problem. I did however notice that the output stud at the rear of the alternator was quite warm. I decided to take a temperature reading and it was at 162 degrees F.

I then took a reading at the stator laminations and found a much cooler reading of 118 degrees F. The engine had already been running about 5-6 minutes at this time.

After about 10 minutes of operation the stator was at 124 degrees F.

The output stud was now at 181 degrees F.

After fifteen minutes the stator was at 146 degrees F.

The highest temperature reading I got at the alternator output stud was 195 degrees F. Although the highest reading I got a picture of was 192.  Just to be sure, I removed the cable from the alternator and checked for any signs of looseness or corrosion that could build heat. There was no problem with the connection so the issue is internal to the alternator.

Since it is relevant I wanted to point out that the outside temperature was in the low fifties, as indicated by the thermometer in the rear view mirror.

After installing a new alternator I rechecked the readings and this is what I found in the first 2-3 minutes.

After 5 minutes.

After 15 minutes of running.

I also want to point out that in order to maintain the same load on the old and the new alternators, I turned the headlights on, switched the blower to the highest speed and turned on the rear window defogger.

From my nearly forty years of starter and alternator rebuilding, the problem will be in this connection in the positive rectifier plate where the output stud is pressed in. I plan on disassembling this alternator further to see the actual problem but that will be for another day.

From an installer viewpoint, if you are replacing the alternator for the reasons outlined in this article, you will need to make sure that the replacement alternator has had the rectifier fully serviced or replaced.

From a rebuilder’s perspective, if no obvious problem is found during testing of the core (you do test your cores before disassembly, right?), make sure that you either fully disassemble the rectifier to correct any potential problems there or simply install a new rectifier assembly. You may also want to incorporate temperature testing of the output stud with a load on the alternator for 5 -15 minutes. This will prevent comebacks and maintain your reputation.

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