Ok, recent thread on here drove me past the point of looking at wiring diagrams and giving direction. Somethings are just easier explained with pictures and tested against a known good working system.
So in an effort to help others, I tore my own truck apart in the dark outside in Alaskan November temperatures with a few simple test tools, a head lamp and an IPhone camera so bare with me if the pictures aren't the greatest.
I do prefer that testing be done with test light when possible. A meter is useful tool but if you don't 100% completely know how to use/test with it you can easily be lead in the wrong direction. Just throwing that out there from 11 years now of turning wrenches for a living. See it happen all the time. No current flow no voltage drop. Testing circuits when they are loaded is best. Also when touching pins,
TOUCH them only, do not force whatever probe you are using it in. This will spread the terminal and cause poor connections.
First of all you are going to have to look at a diagram, going to make you.
On this diagram we can see
- Powertrain Control Module or PCM - Controls Alternator Fields (Turns the alternator on/off)
- Power Distribution Center or PDC - Fuse box under the hood
- The Generator or Alternator - If you need this explained....
1. There is a Black/Grey Wire between the Gen/Alt and the PDC which contains the 140amp fuse and then a red wire to the battery. This is the large charge wire that electricity flows from the Gen/Alt to the 140amp fuse in the PDC and onto the battery. This is the large wire on the large stud on your Gen/Alt and you should have battery voltage or alternator output voltage on this stud. Check the fuse if you don't, or wires associated with that circuit.
2. Now into the meat of things, there are two wires from the PCM to the Alternator.
- Here is the two pin plug that plugs into the back of the Alternator, notice the Green and Dark Blue Wires.
- The dark blue wire is the voltage supply to the Generator fields. Meaning this is a positive wire and should have 12V on a meter or light a test light like this when the test light is hooked to battery negative as in the picture. (A meter would be the same with the negative lead) Engine
MUST be running, key on engine off does not work for this test.
For demonstration purposes pictures are taken with engine off so the test light is not lit in these photo's.
- The green wire is the control wire, meaning the PCM grounds this wire to turn on or off the alternator fields
(DO NOT THINK YOU CAN JUST JUMP THIS TO GROUND FOREVER TO BYPASS THE PCM IT WILL FRY LOTS OF STUFF BECAUSE THE ALTERNATOR WILL BE FULL FIELDED TO 16+ VOLTS) This is a ground circuit so with the truck running a test light or meter hooked to battery positive should show 12V or light like this.
If you have power on the blue wire while the engine is running and can hook 12v to ground on the green wire while the engine is running and voltage on the large stud your alternator is bad, but if you don't have one of the other.........
This is where the PCM lives behind your air filter box, notice I removed mine for better access,
TURN THE KEY OFF before disconnecting.
The connector closest to the passenger side is the Grey C3 connector. The Connector in the middle is the White C2 connector. Reference the wiring diagram at the PCM. We are concerned with pin 25 in the Grey C3 connector as it is the opposite end of the dark blue wire at the alternator. We are also concerned with pin 10 of the White C3 connector as it is the opposite end of the green wire at the alternator.
NOTE - The connectors have a locking tab on the top and bottom, do not force, use shop air if need to blow out dirt so the locks release and you can unhook the connectors.
Noticed the pins are numbered, double and triple check your are on the correct pin when testing.
Grey C3 shown here
White C2 shown here
Now using your meter set it to continuity or resistance test and check between pin 25 in the Grey C3 connector and the BLUE wire pin at the alternator plug. There should be a connection between these two pins.
Next test the Pin 10 of the White C2 connector to the GREEN wire pin at the alternator plug. There should be a connection between these two pins.
Meters vary from make to make as to what they will read (some beep) when showing continuity vs an open circuit. Simple test is to simply look at what the meter says when the leads are not touching each other..... like this.
VS
With the leads touching each other. (no resistance, good connection)
This is what you want to see when testing both of your wires between the PCM and Alt plug
Now if these wires both test good, more than likely your PCM is bad. To be 100% positive it is the PCM, I would pull the plastic cover off of the C2 and C3 connectors and back probe the blue and green wires and see if you have 12Vs that way, this would eliminate the entire harness and test only the PCM. I personally would back probe the pins at the PCM and test just like I showed at the alternator plug with the test light.
How you go about fixing the issue is up to you. With the manual transmission trucks that is all these two wires do. BUT with the Automatic trucks notice in the diagram that the BLUE wire also powers the transmission relay through a splice.
I'll try to update this with information as to PCM repair or external voltage regulator solutions. Hope it answers some questions.
Engine running for testing at the plug. Key off before unhooking PCM.
