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Troubleshoot a charging problem

Michael J. Thomas for Progressive Forage Published on 12 July 2018

Recently, I experienced an unusual situation with a ’98 Dodge Cummins farm and ranch pickup. My son and I were hauling materials to a fence project when a loud howling noise came from the engine compartment.

I quickly scanned my gauges. Sure enough, the volt meter on the dash had dropped to around 12 volts from its typical 14.5 volts.

I installed a new alternator – only to discover the new alternator would not charge. I called a couple local repair shops and the Dodge dealer to see about ordering it in and having one of them take a stab at it. The local guys were all behind on work and wanted to start by putting a new power control module (PCM) in the truck (to the tune of $1,300 before labor).

Given that I still needed to use the truck and could continue to do so by charging the batteries at night, I opted to troubleshoot and correct the situation myself. I knew I could install a PCM and, if nothing else, could save the shop labor expense.


If you have a similar issue and need to troubleshoot, start by checking all of the fuses related to the charging system. The integrated power module is located under the hood, usually on the driver’s side fender, and the interior fuse panel is located in the cab, either under the dash or inside the driver’s door.

It is not uncommon to find the large (140 amp) fuse between the battery and the integrated power module has blown. Beware; if you find a blown fuse, it likely indicates you have a short somewhere in the system and should locate and resolve that issue.

Fused link is inserted in the power supply vetween the internal fuse panel and the external voltage regulator

If the fuses are good, verify whether or not you have voltage on one of the field-exciter poles on the back of the alternator. To do this, you will need a multimeter or a test light. With the engine running, probe both field-exciter poles with a test light or multimeter.

If either one of the poles produces a bright light on the test light – or at least 12 volts on the multimeter – the alternator has failed, and replacement will solve the problem. To be safe, ask your parts store to bench test the new alternator before you leave the store.

If neither field-exciter pole generates a light or voltage, you have a problem in the wiring harness, a bad engine position sensor, or the circuitry that controls voltage regulation within the PCM has failed.

At this point you have two options: Take the vehicle to a dealer or mechanic you trust, or obtain a wiring schematic for the year and model and proceed to troubleshoot the system yourself.

Gregory Eppich, who owns and operates Eppich Farms near Handel, Saskatchewan, is a firm believer in the farm-tough value of older trucks and pickups. Eppich finds and rebuilds these older trucks for use on his farm.

Eppich said, “When you ask your dealer for a wiring diagram of the charging system, it is a good idea to have the VIN (vehicle identification number), as well as the year and model of your vehicle. You might be surprised at how many variations of wiring harness you may get for the VIN of your truck or pickup.”

If there are multiple diagrams for your vehicle, you will need to determine which one is correct. Locate the PCM. In most cases, the PCM is located on the firewall on the passenger side of the vehicle. To gain easy access to the PCM, you may have to remove some external parts.

The PCM is made accessible after removing the air intake box

Next, remove the wire harness plugs from the PCM. The wiring diagrams will have pin numbers associated with a wire color scheme. Begin checking the pin number of the diagram with the actual pin of your wiring harness. If you will have multiple schematics that apply to your vehicle, it will not take long to determine which schematic is appropriate.

Identify the wires that run between the PCM and the alternator field-exciters. If the probes of your multimeter are not small enough to penetrate the PCM plug without damaging the internal contacts, you can insert a small paper clip and touch the probe of your meter to the clip.

Using the appropriate ohm setting, check for resistance between the PCM and end of the wires where they attach to the back of the alternator. If you have no conductivity in one or both of these wires, you have a broken wire.

If you have good conductivity to the field-exciter poles, continue checking the grounds and 12-volt power into the PCM. To check for 12-volt power, you will need to have the ignition in the run position. Some systems will only send a brief pulse to the PCM until the engine is running, so you will need a helper to turn the ignition on while you are watching the test light or multimeter.

Eppich says, “If you don’t get a power pulse, you will need to check the auto shutdown relay. If that relay is good, the problem could be a failed battery temperature sensor or the engine position sensor. If your tachometer is still working, you can usually rule out the engine position sensor.”

If all grounds and the 12-volt power supply to the PCM check out, the voltage regulation system within the PCM has failed.

Installing an external voltage regulator

Cameron Bennett, co-owner and mechanic for G&T Enterprises, Salmon, Idaho, says, “If all of the systems in the vehicle are functioning correctly except for the voltage regulation system for the alternator, there is no problem bypassing the PCM with an external voltage regulator.

But you should know, one of the functions the PCM performs is to manage voltage regulation based on battery temperature. You will lose this, so if you do not already have a voltage gauge on the dash, you should install one. In this way, you can keep an eye on the voltage and, if the system is overcharging, you can remedy the problem before the batteries get damaged.”

External voltage regulators are available at your automotive parts store and are easy to install. Be sure to follow the directions carefully.

An external voltage regulator is mounted on the fender below the battery

Mount the regulator in a safe place on the fender or firewall and make sure it is well grounded. If you are concerned about the ground to the body of the vehicle, run an extra ground wire from one of the mounting screws for the regulator to a ground source on the engine.

Be sure to connect the 12-volt supply into the external voltage regulator to a 12-volt source that is only “hot” when the ignition is in the run position. Connecting to a supply that is “hot” all of the time will drain your batteries when the engine is not running.

Disconnect and tape off the wires from the PCM to the field-exciter poles on the back of the alternator, and connect the wires from the external voltage regulator to the field-exciter poles.

Make sure the batteries are fully charged before checking the alternator output. With the engine running, check to see how much voltage the alternator is producing. If the external voltage regulator is functioning correctly, the alternator should produce 14.5 volts.

If the voltage is more than a few tenths of a point higher or lower, adjust the regulator or check the grounding for the unit. If the unit is adjustable, be sure to keep the unit grounded while making adjustments. Failure to do so will damage the external voltage regulator.

Bennett says, “We’ve sent quite a few vehicles out the door with external voltage regulators over the past 20 years. A lot of them are still going today.”  end mark

PHOTO 1: An external voltage regulator is mounted on the fender below the battery.

PHOTO 2: The PCM is made accessible after removing the air intake box.

PHOTO 3: The fused link is inserted in the power supply between the internal fuse panel and external voltage regulator. Photos by Michael Thomas.

Michael Thomas is a freelance writer based in Salmon, Idaho. Email Michael Thomas.  

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