Testing of alternators off the car is the least desirable way to do it. All bench testers can do is test for proper charging voltage, (13.75 to 14.75 volts), but that is a product of the tester. On your vehicle that is a product of the voltage regulator which is in the Engine Computer. You also must test for full-load output current which requires about five horsepower to run the alternator wide open. Most bench testers only have a one horsepower drive motor. You also must test for a failed diode which will reduce the alternator's output to exactly one third of its maximum rating. 30 amps from the common 90 amp unit is not sufficient to meet the demands of the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
To start the troubleshooting, measure the two voltages on the two small terminals on the back of the alternator. This has to be done while the engine is running. We can skip one step in the description since we can assume the brushes are okay on a new alternator. On one terminal you must find full battery voltage. The key is what's on the second terminal. It must be less than battery voltage, but not 0.0 volts. Typically you'll find between 4 and 11 volts.
On that second terminal you're going to find 0.0 volts, the same as on the first terminal, or something in the 4 - 11 volt range. That finding will tell us where to go next. If both have exactly the same voltage, there is a break in the circuit going to the voltage regulator, or the regulator is defective. That would be very uncommon but could be why someone replaced the engine computer.
If you find 0.0 volts on both terminals, there is a break in the feed wire. That one won't be very long because it's spliced in with the wires that feed the ignition coil(s) and injectors.
If you find between 4 - 11 volts on the second terminal, the entire input circuit / voltage regulator is working. The problem then is in the output circuit. Measure the voltage on the large output terminal on the back of the alternator. That must always be exactly the same as battery voltage. If you find it's higher with the engine running and 0.0 with the engine stopped, there's a large bolted-in fuse that's blown. That is the first thing that should have been checked.
Tuesday, February 23rd, 2016 AT 2:50 PM