Testing A Bad Alternator: Symptoms And Diagnosis

TEST 2: Testing The Battery Circuit Of The Alternator

Part 2: Before we condemn the alternator, we have to dig just a little deeper. We need to check that the alternator's output is actually reaching the battery. We'll do this by checking the continuity of the output circuit of the alternator. Why?

Because the alternator's charge arrives at the battery thru' this circuit. As you can see in the figure (the arrow), this circuit is the main wire that comes out from the alternator.

This wire goes to the battery (or to the main under-hood fuse box and then from there to the battery). All manufacturers install some sort of mega fuse (not to be confused with a regular fuse) or a fusible link in this circuit. The Continuity Test that you're about to perform will check to see if this fuse or fusible link is blown. I'll explain it step by step. Here it goes:

1. Disconnect the NEGATIVE terminal from the battery .This is important. Do not proceed without first disconnecting only the NEGATIVE battery terminal from the battery!
2. Put the multimeter in OHMS (Ω) mode (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
3. With one of the multimeter's test leads (either of the two because the polarity of the leads is not important), probe the center of the stud to which the main wire's terminal is bolted to on the alternator.
4. With the remaining multimeter test lead, touch the center of the battery's positive post.
5. The multimeter should show continuity. This will be an Ohms reading around 0.5 Ohms or so.
6. Or the multimeter will show an open. This will be an infinite reading that is usually rendered by the letters OL on the multimeter's display.

Results Of The Alternator Continuity Test

If the multimeter showed continuity in the circuit, the mega fuse or in-line fusible link is OK. Around 95% of time stopping here and calling the alternator bad and replacing it, is hitting the nail on the head. Why?

Well, because up until this point you have verified that the alternator is not producing a charge with the multimeter voltage test. And now with this continuity test you now know that the reason for this is not an open in the battery output circuit of the alternator.

The other 5% of the time we have to check one more thing. You can skip the last three paragraphs and go to the next page to find out about the last item to check TEST 3.

If the multimeter showed and open in the circuit, the mega fuse (or in-line fusible link) is blown and is not letting the alternator's charge reach the battery. Replace the mega fuse (or in-line fusible link) and check to see what may have caused the mega fuse to have blown (after all, a fuse doesn't burn for no reason).

It has been my personal experience that when the mega fuse or in-line fusible link is blown, it's usually due to the wire shorting against a hard edge on the engine or frame. How does it happen? Normally it's thru' human error. Someone removed the wire from its original routing and fasteners. In its current routing the wire has been rubbing up against a hot and sharp surface for the past several thousand miles. This surface starts to saw thru' the wire's insulation finally resulting in the short-circuit.

After repairing the short (if applicable) and replacing the mega fuse (or in-line fusible link), retest the alternator to see if it's now charging the battery.

TEST 3: Checking The Alternator Fuse

Part 3: This last test may apply or not to the vehicle that you're working on. This will be to check for an alternator fuse. This is a regular blade type fuse NOT like the mega or in-line fuse that protects the output circuit of the alternator. This fuse will be either in the engine compartment fuse-box or in the instrument panel fuse box inside the car.

Now, why would the vehicle not come equipped with this fuse? Because most of the newer vehicles are rolling out with computer controlled alternators and will not have this particular fuse.

How do you find it's exact location (if equipped)? -If you're a tech in a shop, this info will be in a charging system wiring diagram for that specific make and model. If you're a non-Tech (not in a shop), check your owner's manual or your car's (or truck's) repair manual.

OK, there are three possible outcomes in this Part 3 and they are:

1. If equipped with this fuse and it's NOT blown, then the alternator is bad.
2. If equipped with this fuse and the fuse IS blown, replace the fuse and retest the alternator to see if the charging voltage has gone up to 13 or more Volts.
3. If NOT equipped with this fuse, you have a computer controlled alternator. There's a good chance it could be a fried computer (it does happen) and/or an open-circuit problem somewhere between the alternator and computer. There is a way to further test this, but it's beyond the scope of this article. But more than likely, based on my experience, it's a bad alternator.

I hope this article was helpful and in the least, informative. The last section of this article will provide you with a couple more insights into the matter.