� GEARSApril2009

The title of this article says it all. Every year, more things outside the transmission affect

transmission operation. There’s no question that fixing customers’ vehicles is getting more complicated with every new model.

In this article we’re going to cover the relay control system used on late Dodge trucks and vans. These systems use a PCM-controlled relay to control voltage to the overdrive, lockup and governor solenoids. If the PCM detects a circuit fault for the relay or any of the solenoids, it only has to shut the relay off.

Affect On Us In 1997, the PCM started sup-

plying power to the electromagnetic rotor inside the alternator. At the same time, Dodge engineers tied a few other devices into the same power supply… including the transmission relay.

So the only way the relay can work is if the alternator and charging system are working properly, and there are no related codes. What it also did is give lots of people working on 42/47RE transmissions something else to worry about.

42/47RE Transmissions Won’t Shift Without the Alternator

by David Skora

GEARS April 2009 5

Alternator BasicsBefore we get into checking the

relay control circuits, we should take a moment to become familiar with alternators. After all, several of today’s hybrid cars and no doubt cars in the future won’t use transmissions as we know them. Instead, they’re going to have one or more alternators to move the car forward and generate electricity during braking (figure 1). Guess who’s going to fix them: You are, that’s who.

Inventors have been toying with alternating current (AC) generators for over 150 years. But it wasn’t until miniature diodes were available in the early 1960s that the modern alternator was born. That’s enough of a history lesson.

The reason alternators are the engi-neers’ choice for generating electric-ity is that they’re very efficient. Most alternators are in the range of 60-75% efficient. And with a little wiring modi-fication, they can also work as electric motors.

The two most common alterna-tor/motor designs are the Permanent Magnet Motor and the Induction Motor (figure 2). They can produce 3-phase alternating current or they can turn AC into rotating torque to drive the vehicle. When used for operation in a hybrid, these alternator/motors can be

as large and as heavy as an automatic transmission.

Automotive alternators include a rotor (electromag-net), a stator, and a set of diodes that form a rectifier. The rotor typically has six electromagnets which vary in strength depending on the field voltage applied to them. We’ll test this later in the article.

As the engine spins the rotor, the rotor induces alternating current into the stator. The stator is the coil of wire wound around an iron support and mounted to the alternator housing. The rectifier’s job is to convert AC into DC. At this point the alternator’s job is done.

Automotive alternators can produce more current than required, so a regula-tor controls the current level from the alternator to pro-vide just enough current to operate the vehicle’s elec-trical devices and keep the battery charged.

In case someone asks you, there are two stator designs. The most common

Figure 1

Figure 2

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6 GEARS April 2009

design is the Delta. These alternators typically produce less voltage but more current than the Star design, which can develop more voltage but less current. For automotive use, neither design has a big advantage over the other (figures

3 and 4).One difference is that the Star

design begins producing voltage at lower RPM. The alternators used on Dodge, GM and high-output Fords use Delta-wound stators. A few older 35-60

amp Ford alternators were Star wound. But most of these have been superseded by the Delta design.

Earlier, we mentioned that the rotor is a series of electromagnets. When the rotor spins inside the stator, the alterna-tor produces alternating current. The faster the rotor spins, the more current and voltage the alternator produces. Since electrical demands on the vehicle vary and the rotor spins at variable speeds, the easiest way to regulate volt-age and current is to control the voltage to the electromagnets.

TroubleshootingIf a late-model Dodge truck with

a 42-47RE unit comes in with a “trans relay stuck off” code or in limp mode, the first thing to do is troubleshoot the charging system. Don’t forget the basics like checking the battery condi-tion, state of charge, and making sure that the battery and alternator cables are clean and tight.

• Connect a battery charge tes-ter, voltmeter or current probe to the battery terminals.

• Start the engine.• Check the state of charge on

the battery and charging system with your test equipment.

Checking Source VoltageIf the alternator isn’t charging

properly, you’ll need to check the sys-tem, beginning with the source voltage to the alternator (figure 7). Check the white/dark blue or dark blue wire at the alternator connector (pin 2) for 12.2 volts (PCM source voltage).• If there’s no voltage present, refer

to the next heading to check for missing source voltage.

• If voltage is present, monitor the dark green wire at the alternator connector (pin 1) for voltage.

When the alternator is under a light load, a typical voltmeter reading on pin 1 should be 8-10 volts. If there’s no voltage, the rotor or brushes may be open.

When you turn the headlights or other electrical loads on, the voltage to pin 1 should drop to 4-5 volts (figure 5). This would indicate that the PCM is increasing alternator output.

If you need further verification that the alternator can produce voltage

Figure 3: Delta Design

Figure 4: Star Design

Figure 5

42/47RE Transmissions Won’t Shift Without the Alternator

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8 GEARS April 2009

and current, ground the dark green wire (full field the rotor) and see whether the alternator output increases.• If alternator output increases with

the dark green wire grounded, the alternator is good. Check the PCM for codes or the data stream for false battery temperature.

• If the alternator output doesn’t increase either with the PCM con-trol or grounding the dark green wire, replace the alternator.

IMPORTANT — Constant heavy use or additional load devices added to the vehicle’s electrical system can greatly reduce the life of the alterna-tor. Figure 6 is an example of a burned stator assembly from an overloaded electrical system.

Missing Source Voltage Perform these checks if the trans

relay is stuck off or you find no voltage at pin 86 of the relay socket panel with the engine running. Keep in mind that there could be several reasons for this. Decide which of these 4 situations is causing the source voltage to be miss-ing and perform the appropriate tests:

Issue 1: The PCM set an alterna-tor-related code.

Check the rotor field wiring (fig-ure 7) and alternator. After making the repairs, clear codes and check for cor-rect source voltage at the trans relay.

Issue 2: The circuit is open between the power source and pin 86 at the trans relay.

Test the circuits for opens or shorts to another circuit. You may have to bypass the faulty circuit with a new wire.

Issue 3: Some device may be con-suming more current than it should and drawing the voltage away from pin 86 at the trans relay.

Depending on the situation, the PCM may continue to supply the volt-age and not set a code. Using the appro-priate wiring diagram, locate all the devices sharing the PCM power source. Unplug them one by one, and see if the source voltage returns to trans relay panel pin 86. If there’s still no voltage at pin 86, see issue 4.

Issue 4: The PCM has an internal problem and can’t provide a voltage signal to pin 68 at the trans relay.

At this point, you have verified

that the circuits out-side the PCM are capable of working. This leaves the PCM as the remaining sus-pect. Use the next test to check the PCM.• Using a fused

jumper, connect 12 volts to the white/ dark blue or dark blue wire at the PCM.

• Verify that voltage is present at pin 86 of the trans relay panel. If so, the PCM is bad.

The wiring diagram (figure 7) pro-vides a general view of the circuits. Refer to the specific wiring diagrams for the year and model you’re work-ing on.

Each year, more and more of the vehicle’s control systems become inter-woven with the transmission and its operation. But by understanding the systems and how they work, diagnosis is no more difficult than checking a switch or solenoid.

Figure 6

Figure 7

42/47RE Transmissions Won’t Shift Without the Alternator

Normal Color

Dark Color = Overloaded

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