I've had so many folks come up to me and say: "I think the alternator is bad on my car because I disconnected the battery's negative terminal with the engine running and..." I cringe (internally) when I hear this. This is the worst thing anyone can do to test for a bad alternator. I'll show you a simple but effective way to diagnose a bad alternator with a multimeter.
If you're a professional tech with access to a professional Battery Load Tester machine, read on anyway. Whether you're an Auto-Tech or not this information will help you to successfully diagnose a bad alternator WITHOUT the use of expensive testing equipment. This technique will be another tool to add to your toolbox of know-how.
Common Symptoms Of A BAD Alternator
These are the basic symptoms of a bad alternator:
- The vehicle's (whether it's your car or a customer's car) instrument cluster has the battery light on.
- The vehicle has to be jump-started with another car or jump-start box (or whatever method).
- The vehicle will not stay running after the jumper cables/ jump-start box have/has been removed.
- The battery can be charged and the vehicle will run for as long as that charge isn't depleted.
- When the headlights are turned on, they are very dim and/or cause the car to stall, necessitating another jump-start.
The diagnosis (test method) described in the following pages has worked for me for as long as I've been working on cars and has given me a good success rate in correctly diagnosing a bad alternator. Let me just point out that this test is not intended to diagnose a short-circuit in the vehicle's wiring harness.
Before you turn the page, this article is one of two HOW TO TEST THE ALTERNATOR articles in this Site. You will find the link to the other article at page 3 of 3 of this article. This is the page that is tabbed: ‘How It Works’.
TEST 1: How To Test The Alternator With A Multimeter
This simple but effective test will save you some headaches. I recommend using a digital multimeter but you can use an analog multimeter also (don't have a digital multimeter? Need to buy one? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
To perform this test, it's important that the battery in the vehicle is fully charged. You can remove the discharged battery and replace it with a fully charged one or simply charge the vehicle's battery (on the vehicle or off) before test.
OK, the test is divided into three parts: PART 1- Testing the alternator voltage output with the multimeter in VOLTS DC mode. PART 2- Testing the continuity of the alternator's output wire (to the battery) with the multimeter in OHMS mode. PART 3- Verifying that the alternator fuse in the fuse-box is not blown (if equipped).
PART 1- We're gonna' start by testing the alternator's output at the battery itself. Whip out your multimeter and turn the selector knob to VOLTS DC. You can use an analog multimeter or a digital multimeter ( I prefer the digital ones).
Be careful, take all necessary safety precautions and use common sense when performing all of the tests in this article.
- Start the vehicle (it'll start and stay running if you have charged the battery.)
- Put the red multimeter test lead on the battery positive terminal.
- Put the BLACK lead on the battery negative terminal.
- Look at the reading. It should hover around 12 Volts.
- Turn on the headlights. Turn on the blower motor on high (A/C or Heater). Turn on the wipers. Turn on the radio. Turn on as many things as possible in your car or truck (the reading should start to go down hill to 9 Volts or less in about a minute's time. The vehicle may stall below 10 Volts).
- Turn off the vehicle
Interpreting The Results Of The Alternator Volts Test
If the test gave you the results stated above. The alternator is not charging the battery. Why? Because an alternator that IS charging the battery will produce 13 (up to 14.5) Volts with the headlights on or off and/or with everything on (so if the the test above gave you steady, and I emphasize steady,13 and up to 14.5 Volts with everything on, then your alternator is not bad).
Alright, the above test confirms that the alternator is not charging the battery. But, is it the alternator or something else like a blown mega fuse? Lets find out in Part 2...