TEST 2: Checking The Continuity Of The Bat (+) Cable
The alternator, on your 2.6L equipped Isuzu Pick Up (Rodeo or Amigo) has two electrical connections.
One of them is a heavy gauge wire that connects to the alternator with a nut. The other end of this heavy gauge wire connects to the under-hood fuse and relay box.
All of the current that the alternator produces, to charge the battery and to power any of your Isuzu's electrical needs, passes across this heavy gauge wire and then thru' a 60 A mega fuse (called the Main Fuse in the Isuzu service literature), before finally reaching the battery.
Although rare, this 60 Amp Main Fuse sometimes gets blown or a problem develops in the heavy gauge wire.
So, before condemning the alternator as bad, we need to make sure that this heavy gauge battery wire is not open on your 2.6L Isuzu.
NOTE: This 60 amp mega-fuse is located in the under-hood fuse and relay box. See TEST 3 for the illustration/location.
This is what you'll need to do:
Disconnect the battery negative (-) cable terminal from the battery negative post. The positive cable remains connected.
IMPORTANT: Don't continue to the next step without first disconnecting the negative cable from the negative battery post.
With the battery negative (-) cable disconnected from the negative battery post, turn on your multimeter and select Ohms mode.
Locate the stud that the battery current output wire connects to on the back of the alternator. This wire is attached with a nut to the stud. Place your multimeter's RED test lead on the stud to which this wire is connected to.
The orange arrow in the photo above points to this stud.
With the black multimeter test lead probe the center of the battery positive post on your Isuzu's battery.
You'll get one of two results.
1.) If the battery wire (circuit) is ‘open’, for example the 60 Amp Main Fuse is blown, your multimeter will indicate a reading of OL (which means over limit). In other words there will be NO continuity.
2.) If the battery wire does have continuity, your multimeter will register an Ohms reading of 0.5 Ohms or less. In other words, there will be continuity.
Let's take a look at what your results mean:
CASE 1: Your multimeter registered continuity. This is the correct test result and it indicates that the entire circuit (including the 60 Amp Main Fuse) is OK and that if the alternator were charging the battery then this charge would actually reach the battery.
There's one more test to do. This one involves checking the condition of 3 fuses. For this test, go to: TEST 3: Checking The Alternator Circuit Fuses.
CASE 2: Your multimeter DID NOT register continuity. This result tells you that the 60 amp mega fuse is probably blown or a problem causing an open in this wire. Either way, this means that the alternator's charge is not reaching the battery.
Your next step is to go to TEST 3 and check the 60 Amp Main Fuse in the under-hood fuse/relay box.
TEST 3: Checking The Alternator Circuit Fuses
There are two fuses in the under-hood fuse/relay box that need to be checked. These are the 60 Amp Main Fuse and the #5 10 Amp fuse (see the illustration above). NOTE: If you have done TEST 2 and your multimeter registered continuity in the battery output circuit, then you have confirmed that the 60 Amp Main Fuse is OK.
Inside the vehicle, the two fuses that need to be checked are the #7 10 Amp and #8 10 Amp fuse (see the illustration below).
FUSE CHECK RESULT: If in TEST 1 the battery voltage (with the engine running) was below 12.5 Volts, in TEST 2 the battery output circuit has continuity and all of the fuses are OK then the alternator is defective and replacing it should solve the no charge condition.