New Alternator or Not?

Yesterday when I turned on my truck the red battery warning light came on. The battery volt indicator gauge showed a charge of about 12. I tried driving it to an auto parts store, and the battery slowly died while I was driving. Ended up getting a new battery last night. Drove home, but the warning light stayed on, and the gauge only went up a little. This morning, I did some looking around, and figured it was probably the alternator was bad. So I went to the auto parts store and bought a new alternator. The whole trip there, the red battery warning light kept blinking. On the way home, driving down a bumpy road, I noticed that the battery volt gauge was finally reading 14 volts like it should. So now I’m left wondering, did I still need to replace the alternator? Was the low reading this morning with the new battery simply because it needed a bit of a charge? Is that normal? I haven’t returned the alternator yet, so I could still replace it if I need to.

You didn’t have the parts store test the starting and charging systems before you bought the battery and alternator? Most will do it for free.

They did test. Last night, when they tested, they said it was the battery. So I bought a new battery. I had them test again this morning, and they said that since the battery was under 12v and it being a new battery, that it was probably the alternator. But now I’m wondering if it was just under 12v because it had been sitting somewhere slowly discharging prior to installation, and I just hadn’t driven around enough previously to get it up and charged.

You may have not needed any parts.

Try checking this.

Battery connections are the first place to begin when you have a “No Charge" situation. Even
if you have a new battery, if the connections are loose, dirty or corroded, you will not be
allowing the full flow of current to pass thru the connections. The connection may be
enough to turn on the lights, but not enough for the huge flow that is needed to operate the
starter. This is where many people say that they know the battery is good….”because the
lights come on”. This is no more a battery test than licking a 9volt battery. It only tells you that there is electricity…not how many volts or the amperage that flows from the battery.
Jump starting may have wiggled the terminal just enough to allow the current to pass and start the engine, but tomorrow you have the same problem.

First remove the cables from the battery and use a wire brush to remove any corrosion and dirt from the battery posts and the cable terminals. There is a tool with a round wire brush for this purpose, found at any auto parts store for less than $10
Before connecting the cables, apply a coating of di-electric grease to the battery posts this will keep oxygen away from the connection so that it will not corrode as fast.

It is just as important that the other end of the cables also have a clean connection. Remove the positive cable from the battery again so that you do not short anything out. Follow both cables to their far ends, remove this connection and wire brush the connection and the cable terminal clean and retighten these connections.

If there was work done recently, there may have been an “engine to body” ground that was not installed following the work. These grounds normally run from the rear of the engine to the firewall and are uninsulated and most are a braided wire. If any of these are found unattached…reattach them.
Remember….this is not a “Sherman Tank” don’t over tighten the connections.
Tight…tight………………too tight…broke!!!


My first suspicion would be the connections to the alternator itself since going down a bumpy road caused it to start charging again and you don’t have starting problems until the battery becomes discharged.

Try this: have somebody jiggle the alternator connectors while you watch the voltmeter. If it bounces back and forth between 12 and 14 volts your connections need cleaning.

We don’t know the age of this GMC Yukon, so I don’t know if my advise could be accurate. But, I had a similar problem with my Toyota. It had over 250,000 miles on it and approx. 14 years old on the original alternator. The alternator light would go on and voltage would drop to 12V, but I removed the alternator twice and it would bench test Ok. Put it back in, and everything ws fine for a week or so, then the light would return and voltage dropped. 3rd time, it finally croaked for good. Curious, I took it apart and found the brushes worn almost completely out, just very small nubs left. With the age and mileage, I opted for the reman with a lifetime guarantee.

Don’t waste your money purchasing one of those battery terminal brushes. It’ll do you no good as the battery in your vehicle has the side mount terminals.

What year is this Yukon?


One way to see if you needed a new alternator is to open it up and check the brushes. If they’re less than a quarter of an inch long, you needed new brushes, maybe not an alternator. When they don’t make contact the alternator doesn’t work unless you hit a bump and suddenly have contact.

Don’t worry about putting the thing back together. Once it’s open you will see where to insert the paper clip to pin the brushes back for assembly. A drill bit will do.

It may be a hindrance but I would advise never driving a car when the alternator light is on. In the old days a car could be driven clean across a large state with the alternator lying in the trunk and the radio blaring.
The electrical drain of modern cars means you won’t get far at all.

My suggestion would be to clean not only the battery cable terminals and posts but also the junction terminal connector is the car has this. It should be located near the battery positive terminal. Just follow the cable to what appears to be a stud or bolt.

To do a backyard check on the alternator turn the key to the RUN position (engine not running) and touch the alternator pulley with the tip of a screwdriver.
If you feel a magnetic attraction through the tip of the screwdriver the alt. should be good; assuming the red lamp in the dash is functional.

"Don’t waste your money purchasing one of those battery terminal brushes. It’ll do you no good as the battery in your vehicle has the side mount terminals.

What year is this Yukon?"

Around the 2007 Model year, GM came to its senses and went to top post batteries in its full size trucks and suvs

Group 48 battery, actually