Replacing an alternator
Charge Light On :

Replacing An Alternator On A 1999
GMC Jimmy, Chevy Blazer or S10 Pickup

May Also Apply To Full-Size GM Pickup, Tahoe, Yukon, Suburban Etc...


In This Article:

The serpentine belt is removed, electrical connectors are disconnected, 3 bolts are removed and the alternator is taken out. Installation is the opposite of removal, mostly.

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Skill Level: 3 (Intermediate) Time Taken: About 2 Hours

By Bruce W. Maki, Editor


Signs Of Trouble:

The charge light (A) appeared on this 1999 GMC Jimmy.

The voltmeter (B) was reading quite low, around 12 volts. When the car is running, the voltmeter should read around 14 volts.

Using a digital multi-meter, I measured the voltage at the battery, and it was around 12.5 volts. (I wouldn't trust the voltmeter on the instrument panel... it is not a precise instrument.)

If the alternator was not working, the car would run just fine... for a while, until the battery completely drained down. The alternator simply recharges the battery by applying a voltage higher than the battery's 12 volt output.

(Actually, car batteries are normally very close to 12.6 volts... there are 6 cells wired in a series circuit, and each lead-acid cell produces 2.1 volts)

Dashboard showing charge light on due to alternator malfuntion.


Notes On Diagnosing A Faulty Alternator:

The first thing I checked was the tension on the serpentine belt. On cars without an automatic belt tensioner it's common to experience a slight loss of belt tension as the belt ages and stretches. I pushed on the serpentine belt and watched the automatic tensioner to make sure it swiveled back-and-forth slightly. Belt tensioners sometimes seize up and fail to apply force against the belt.

If the alternator drive belt is not tight enough, the belt can slip on the pulley, and the altenator won't spin fast enough to generate enough voltage to recharge the battery. Belt slippage usually makes a high-pitched squealing sound.

I also examined the battery cables to check for corrosion on the cable ends and the battery terminals. Corrosion can increase the resistance of the electrical connection, which can prevent the battery from charging properly. Corroded battery connectors can create so much voltage drop that the car's starter cranks slowly or not at all.

Since the serpentine belt tensioner was working fine, and there was no corrosion problem on the battery connections, I had to conclude that the alternator was faulty.


Alternator location in GMC Jimmy or Chevy Blazer.

The engine compartment. The red arrow points to the alternator.

Note the series of slots on the top of the alternator. Many alternators have slots like these, which I assume are to assist with cooling.


The alternator is easy to reach on this car. On some vehicles the alternator may be near the bottom of the engine, but it will always be driven by some sort of fan belt.

(Except for hybrid vehicles... I understand they are completely different.)

GM alternator on 4.3 liter V6 engine.


First Step: Disconnect The Battery

Disconnecting the car battery before working on electrical system.

As always, I removed the negative battery cable first.

ALWAYS remove the negative cable first. Read why...


Then I removed the positive cable, which really wasn't necessary.

This used a 5/16" socket.

Disconnecting positive battery terminal last.


Second Step: Remove The Serpentine Belt

(Also known as a poly-V belt)

Serpentine belt tensioner, GM. This is the automatic belt tensioner. The red arrow points to a 3/8" square hole in the metal casting.


I inserted a 3/8" breaker bar (a ratchet would've worked just fine) and rotated the tensioner until the serpentine belt became loose. Loosening automatic belt tensioner.


Removing serpentine belt from idler pulley.

Then I slipped the belt off the idler pully.

There's always one pulley where the serpentine belt slides off the easiest, and it's usually a small idler pulley.


The main alternator wire (red arrow) comes out of the back. This wire (which is a large red wire) is covered by a black rubber boot.

Alternator output wire hidden behind rubber boot.


Removing insulating rubber boot from electrical terminal, GM alternator. Using a curved pick tool (red arrow), I pried the rubber boot away to reach the connection terminal.


I had to use 2 wrenches to remove the terminal nut (arrow), since the post tried to move when I turned the nut.

These were both 13mm wrenches.

Alternator output connector lug, GM.


Removing positive cable from alternator. Then the cable just slipped off the terminal post.


I wasn't able to remove the other connector, so I decided to leave it connected until I had the alternator removed. Electrical connector on GM alternator.


Front bolt locations on GM Delphi alternator.

Front bolt locations, 2 places.

These bolts required a 1/2" socket, though I think I used a 13mm 6-point socket by mistake, which worked fine.


Back bolt location.

This also used a 1/2" socket or wrench.

Rear mounting bolt on GM alternator.


Front bolts on GM alternator are very long. The front bolts were really long.


Once I had the 3 bolts removed, I had to use a pry bar (arrow) to lift the alternator up out of the bracket. Prying alternator loose from mounting bracket, GM truck.


Mounting bracket for alternator, GM truck. This is the bracket arrangement... it's a tight fit.


After some more prying, I had the alternator free from the mounting brackets. Lifting alternator from engine, GM.


Electrical connector on back of alternator, GM.

I turned the alternator around so I could reach the smaller connector.

There was a release tab (arrow) that was difficult to see, let alone reach, when the alternator was attached to its mounting bracket.


New and old alternators.

The new alternator (rebuilt, actually) cost about $125 at my local NAPA store. There was a core charge of about 50 bucks.

New alternator next to old unit.


Alternator mounting location, GM truck.

Tight Fit:

The alternator needs to fit between these two points.


So I tapped on these plugs with a small hammer to back them out a bit. Tapping on metal plug or grommet to loosen the tight fit when installing GM alternator.


Plugging in electrical connector, GM alternator. I plugged in the electrical connector.


Then the alternator fit into place without a fight, and I replaced the bolts and tightened them. Installing alternator bolts, GM.


Cleaning alternator terminal with emery cloth to remove corrosion.

Before re-installing the large electrical connector, I used a piece of emery cloth (arrow) to clean up the mating surface.


Notes On Re-Installation:

Make sure the rubber boot over the alternator output wire is replaced properly. This boot keeps water away from the electrical connection and therefore should reduce corrosion problems.

Re-installing the serpentine belt uses a procedure similar to removing it... swing the tensioner aside and slip the belt into place.

It's a good idea to clean off the battery cables, and battery terminals, with a wire brush. Corrosion can increase the resistance of the electrical connection, which can prevent the battery from charging properly. Corroded battery connectors can create so much voltage drop that the car's starter motor cranks slowly or not at all.

When re-connecting the battery cables, always install the positive cable first and the negative cable last. Read why...



After installation was complete, I started the car and measured the voltage at the battery... 13.59 volts. That's about right. Measuring alternator voltage at the battery.


More Info:

Tools Used:

  • Basic Mechanic's Tools
  • Digital Multi-Meter
  • Sockets, 3/8 Drive:
  • 3/8 Breaker Bar
  • Pry Bar, Medium
  • Mechanic's Pick, Curved
  • Emery Cloth

Materials Used:

  • Alternator, Remanufactured

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Written July 24, 2007