"Why won't my alternator charge?"
That is one of the more frequently asked questions in the help board. The following will help you troubleshoot and understand the charging circuit in Dodge trucks and RC's.
First a basic diagram for pre-EFI vehicles with an external regulator. This is a simplified diagram that does not show fusible links, nor passage of the alternator output wire through the bulkhead connector.
EFI vehicles are very similar, the major difference being in EFI vehicles, the EFI computer has an internal voltage regulator, and no external regulator.
NOTE: I have shown the main alternator output wire in the above diagram in red. In most (maybe all) factory wiring, the alternator output wire will be black
Dodge alternators ground themselves through their case (except 100 amp alts), through the bracketry, the engine block, then finally to the battery negative terminal through the negative cable. The 100 amp alternator has a ground wire running from the alternator case to the engine block/alternator bracketry. The first thing to check in any no charge condition is to make sure the battery negative cable, which should be bolted to the engine block or head, is in good condition, the ends are free of rust and corrosion, and that it is bolted tightly to the engine block/head.
If you have a 100 amp alternator, also check the ground wire from the alternator to the engine block. Make sure the wire is in good condition, and the connections are free of rust, corrosion, and paint, and tight. Replace if necessary.
On pre-'81 vehicles, the main power wire from the alternator runs from the alternator, through the bulkhead connector, through the ammeter, back out through the bulkhead connector, through a fusible link and to the starter relay or battery. Not a great design, as ALL electrical power on these vehicles runs through the relatively weak bulkhead connections (twice) and through the ammeter. See THIS ARTICLE
from Mad Electrical for more discussion on this design, and how to improve it.
'81 up to EFI vehicles have a similar alternator output circuit, but there is a shunt wire built into the wiring, allowing current to bypass the ammeter and bulkhead connector should the ammeter go bad. While this is still not the greatest design, it is better than the dash melting or even fire prone charging circuit used in the 70's. Additional information on ammeter operation and the '81 up shunt circuit can be read HERE
I mentioned the power from the alternator travels through a fusible link above. This is the second item to check for a no charge condition. On '81 up to EFI vehicles, this fusible link is located on the driver's inner fender under the hood. Look for a triangular rubber piece roughly the size of a half-dollar coin. One wire will be going into the rubber triangle's "point". That is the alternator power/charge wire. The 4 or 5 'wires' coming out of the flat end of the triangle are fusible links. They look like normal wires, but are normally labeled "fuse link" if you can read it through all the years of grime and dust. Some blown fusible links are very obvious, they look like they were thrown in a fire. Other times just the wire inside the insulation may burn out, leaving the insulation intact. The easiest way to check for a blown fusible link that looks okay to the eye, is simply to grab each end of the fuse link and pull. If it stretches, it is blown. In this case, you will need to track down the reason the fuse link blew (usually a direct short to ground somewhere in the wiring) and replace it with the same gauge fusible link.
If the alternator ground and output wire circuits check out okay, it is time to check the alternator field wires. The field wires are the remaining two small gauge wires connected to the alternator. These wires are normally green and dark blue, but I have seen green and red, and all variations of colors on vehicles with EFI. I have used blue and green for the field wires in the diagram above, as they are by far the most common.
The green field wire runs between one of the alternator fields, and the voltage regulator only. This wire is used by the voltage regulator to control alternator field grounding. Or more simply, this is the wire the VR uses to complete the alternator field circuit.
The blue field wire runs between the other alternator field terminal, to the VR, then on to a master splice in the engine compartment wiring that provides ignition switched power to everything under the hood. On '81-'87 trucks and RC's, this splice is located in the main engine wiring harness, just above and to the left (passenger side) of the bulkhead connector, almost right behind the power brake booster. This wire provides ignition switched 12 volts to the voltage regulator, and in turn to the alternator field. The voltage regulator senses the incoming voltage from the blue wire. If the voltage is lower than approximately 13.5 volts (that number varies with temperature, electrical resistance, and I am sure many other factors) it grounds the green field wire. This energizes the alternator field, and the alternator begins to produce a charge and sends it to the battery via the main output wire. When the voltage regulator senses voltage from the blue field wire over approximately 14 volts, it cuts the ground to the green field wire, de-energizing the alternator field, and the alternator stops producing a charge. The voltage regulator does this ground on/off switching hundreds of times per second, which is how it maintains the electrical system at or around 13.7-13.8 volts.
Now we are at the third item to check for a no-charge condition. The voltage regulator provides a ground to the green field wire through it's case. This means the bolts securing the voltage regulator to the firewall must be clean and tight. Rust, corrosion, paint, loose bolts, etc can all cause a no charge or poor charge condition.
Item four to check is the field wires themselves. Trace the field wires from the back of the alternator to the voltage regulator. Replace the wire(s) if you find any breaks in the insulation or wire.
For those of you with an overcharging
condition, check the green field wire carefully. If this wire is shorting to ground somewhere along its length (e.g insulation has rubbed through and the wire can come into contact with a valve cover, head, or intake) it will full-field the alternator, meaning the alternator will put out the maximum voltage it is capable of producing, hence, overcharging.
Item five to check. If everything up to this point has checked out, grab a voltmeter or a test light, and check for voltage on the blue field wire at the voltage regulator, with the ignition key turned to "on", but the engine off. You should read approximately battery voltage at the VR connector on the blue wire pin. (or the test light should light up). If you are getting no voltage, there is a break in the blue wire between the voltage regulator and the main splice. Unfortunately, this requires you to remove the wiring from the loom travelling across the firewall to find and repair the break. It is a short run of wire (maybe 3 feet) to check out, but pulling the wiring out of the factory tape & loom is difficult due to the wirings position on the firewall.
If all of the above checks out, and if you have not already, have the alternator tested at your local parts store. Replace as necessary. Note: it is recommended to replace your voltage regulator any time you replace the alternator. If your current VR is working fine, it is not necessary, but VR's are cheap, and I cannot count how many times I had a VR fail very soon after an alternator replacement. Now I just replace them both at the same time and don't worry about it.
If the alternator checks out okay, and all the wiring checks above check out okay, you have a bad voltage regulator (or a bad VR ground, see above). Pick up a new one at you local parts store, bolt it in, and go on your way.