Is there a correct procedure for testing the output of an alternator on a 99 4.3 blazer.
I have done all the usual things, engine off, battery test gives 12volts, engine running 11.6volts and dropping. Load test, lights on, with engine off, 12volts and dropping. With engine running, 11.5 and dropping. Also tested at output terminal on back of alternator, same results.
Battery is GOOD, so i guess the alternator/regulator is shot, am i missing something, i dont see any any fuses or fusible links in the circuit, or are there.

 

Starting in 1999, the PCM controls the alternator. The red wire on the rear of the alternator should show battery voltage when the engine is off. With the engine running, it should show 14.2 volts. The smaller red wire goes to terminal "L" and voltage will vary depending on load and battery state of charge when the engine is running.

 

So if i dont get 14.2 volts from the big red terminal on the back of the alternator, i can assume its gone bad? The pcm cant affect the output from this terminal?

 

When the engine is running: If the PCM is not commanding the alternator to charge, voltage on the rear terminal will indicate somewhere around 12 volts, depending on the state of charge of the battery. This would indicate a problem with wiring or the PCM itself. On the other hand, if the PCM is commanding the alternator to charge, and the alternator is faulty, you'll get the same results. You need to probe the small wire on the "L" terminal when the engine is running to verify the PCM is commanding the alternator to charge.

 

I have checked voltage at terminal L, with the engine off its 0 volts, with the engine running its 0.11 volts, does this sound like a bad alternator?

 

The PCM applies a 5 volt signal to the "L" terminal of the voltage regulator to turn it on.
With a reading of 0.11 volts on the "L" terminal red wire:

1) The alternator may be faulty.
2) The red wire from the PCM to the "L" terminal may have an open circuit.
3) The PCM can't or isn't commanding the alternator to charge, (bad PCM).

To check the red wire: unplug connector #3 at the PCM and unplug the red wire from terminal "L" on the alternator. Check resistance from the PCM harness connector pin #5 to the harness connector at the alternator. Resistance of the red wire must be less than 5 ohms.

 

Captain, with your help i may be getting somewhere. As directed i pulled plug from back of pcm, looks like its never been off since it came out the factory, pulled plug from alternator and tested resistance, much lower than 5 ohms, so the wire checks out ok. Refit plug into back of pcm and start up engine. Check voltage at alternator plug, now reads 10.74 volts, could that have been a bad connection at pcm?
Does this mean that the pcm is working correctly and is telling the alternator to charge, if so then it must be a bad alternator, Right? As a side note, i noticed if the plug is not in the alternator when the engine is running, the battery light on the dash goes out, is that relevent in any way, thanks, Steve.

 

It's possible the connection at the PCM was poor. The PCM connector has a silicone rubber weather seal to keep moisture out; was that seal distorted, or missing? Did you check to make sure the pin and socket in the connector and PCM were tight and that they made a good tight connection to each other? Possibly a poor crimp on a terminal: If the insulation on the wire isn't stripped far enough, the insulation can be crimped and miss the actual wire. Another possibility is a loose crimp, the connection could loosen over time with road vibration etc. Just when I think I've seen it all, something new comes along so look closely.

10.74 volts at idle, without the alternator charging, is ballpark. BUT, the shop manual says the PCM applies 5 volts to turn the voltage regulator on, which may indicate a problem with the PCM.**

Here's something to try: Connect the red wire to the alternator while the engine is running, then: 1) back probe the red wire and check voltage. 2) Check voltage at the battery. Post the results.

** There is a "work around" that modifies where the "L" terminal gets its voltage. The modification changes it back to 1998 and prior, and this may be a viable option for you. The alternator part number is the same for 1998 and 1999, the only thing changed is the voltage source for the voltage regulator.

Another test you can try is to start the engine, connect a volt meter to the battery, disconnect the "L" terminal red wire, run a jumper wire from battery positive to the "L" terminal. Battery voltage must increase to around 14.2 volts, if not, alternator is faulty.

 

Update: i checked the PCM plug and seal is in place and looks good, rechecked continuity and resistance from pin 5 to alternator plug, again all good.
Next check, i bared some insulation off red wire plug and tested voltage from here to battery ground with engine running, 0.8 volts, battery itself showed 11.4 volts.
Finally the last check, which i now understand what we are trying to do, i cut the red wire where i bared off the insulation, made a jumper cable to the alternator plug and with it in the alternator started the engine. Connected jumper to battery positive, check battery voltage, still 11.4 volts!
Does this mean alternator is bad?
Steve.

 

Good job, yes, the alternator is bad.