Alternator PCM Control Signal Tests –GEN-COM, GEN-MON (1999-2003 3.8L V6 Ford Windstar)

In this tutorial, I'm going to show you how to quickly and easily check the GEN-COM and GEN-MON signals of your 1999-2003 3.8L V6 Ford Windstar's PCM-controlled alternator.

You don't need any fancy/expensive diagnostic equipment —not even a scan tool. We'll be testing both signals using a regular multimeter set to Volts DC mode.

Although these are pulse-width modulated (PWM) duty cycle signals, I'll show you how to test them in Volts DC mode to verify that the alternator and PCM are communicating correctly.

NOTE: Before you begin these tests, it's critical that you've completed the alternator and voltage regulator checks found in this tutorial: How To Test The Alternator (1999-2003 3.8L V6 Ford Windstar).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L V6 Ford Windstar: 1999, 2000, 2001, 2002, 2003.

Before You Begin The GEN-COM And GEN-MON Signal Tests

The two tests in this tutorial —and their interpretations— only apply if you've already confirmed the following:

1. The alternator is not charging the battery.
2. Terminal 3 (BATT-SENSE) of the alternator's 3-wire connector has battery voltage present at all times.
3. Terminal 1 (GEN-MON) of the alternator's 3-wire connector has battery voltage present with the key ON and engine OFF (KOEO).
4. The BATT-SENSE and GEN-COM circuits (at the voltage regulator) show a resistance within the specified range of 950 to 1050 Ohms.

If these three conditions haven't been verified yet, please go and complete the alternator and voltage regulator resistance tests found in this tutorial before continuing: How To Test The Alternator (1999-2003 3.8L V6 Ford Windstar).

You can also find a detailed explanation of how the PCM controls the alternator's output through the GEN-COM, GEN-MON, and BATT-SENSE circuits in the following tutorial:

• PCM-Controlled Alternator Basics (1999-2003 3.8L V6 Ford Windstar).

TEST 1: Checking The Alternator's GEN-COM Signal

The GEN-COM (generator communicator) activation signal is the signal that the PCM sends to the alternator —via the red with pink stripe (RED/PNK) wire of the alternator's 3-wire connector— to command it to charge the battery.

This signal is a duty cycle (%) signal that can be measured with a multimeter capable of reading % duty cycle. However, for our test, we'll be using a standard multimeter set to Volts DC mode to verify that the PCM is commanding the alternator.

To correctly interpret our test results, it's important to understand what's happening on the GEN-COM circuit between the PCM and the alternator's voltage regulator:

• The voltage regulator sends 12 Volts to the PCM on the GEM-COM circuit (the RED/PNK wire).
• When the PCM is not commanding the alternator to charge, you'll see full battery voltage on the circuit.
• When the PCM commands the alternator to charge, it Grounds this circuit using pulse-width modulation (PWM). This Grounding action pulls the voltage down (it can go as low as 1 Volt).

In plain English, this means: the more the PCM pulls the voltage down (in the GEN-COM wire), the more it's asking the alternator to output current.

Since you've already confirmed that the alternator isn't charging the battery, the PCM will be actively commanding maximum output from the alternator. So, you should expect to see a voltage reading of around 1 to 6 Volts on the RED/PNK wire.

The key takeaway here is this: The lower the voltage you see on your multimeter, the longer the PCM is Grounding the circuit (more ON-time), which means it's commanding higher alternator output.

On a correctly functioning alternator (that is charging the battery), the voltage, you'll see on the RED/PNK wire will fluctuate —for example, you'll see 12 Volts and then it'll suddenly drop down to 7 or 3 Volts and back up again. This fluctuation is completely normal and simply reflects the PCM's constant adjustment of the PWM Ground ON-time to regulate alternator output.

One last thing: To perform this test, the alternator's 3-wire connector must remain connected to the alternator. To access the GEN-COM signal, we'll need to tap into the RED/PNK wire of the connector.

My preferred way to do this is with a wire-piercing probe. Since this is a weather-sealed connector, I avoid back-probing it to prevent damaging the rubber seals around the wires where they enter the connector. This is the tool I use and recommend: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

NOTE: This test requires the engine to run for about 5 to 10 minutes to complete. If the battery is weak or discharged, make sure to fully recharge it before starting the test.

GEN-COM SIGNAL: You can find a more detailed explanation of the GEN-COM signal and how it's used to activate the alternator in this diagnostic guide: PCM-Controlled Alternator Basics (1999-2003 3.8L V6 Ford Windstar).

Alright, let's get the ball rolling:

1. 1

   Reconnect the 3-wire connector to the alternator.

2. 2

   Connect the black multimeter test lead to the battery's negative (-) terminal.

3. 3

   Set your multimeter to Volts DC mode.

4. 4

   Connect the red multimeter test lead to the RED/PNK wire of the alternator's 3-wire connector.
   
   NOTE: The 3-wire connector must remain connected to the alternator.

5. 5

   Crank and start the engine and let it idle.

6. 6

   Check the voltage on your multimeter. It should read a voltage somewhere between 9 and 1 Volts DC.
   
   Since the alternator isn't charging the battery, you should see a voltage closer to 1 Volt.
   
   NOTE: There isn't a specific hard-coded voltage value for this test —instead, what matters is the range and behavior of the signal. Any voltage reading below 12V and closer to 1V (on the RED/PNK wire) indicates that the PCM is actively commanding the alternator to charge the battery.
   
   So even if your multimeter doesn't show exactly 1 Volt —say it shows 3, 4, or 6 Volts— that's perfectly fine. These are all normal values and indicate the PCM is commanding the alternator to charge the battery.

7. 7

   When done, turn off the engine.

Alright, let's find out what it all means:

CASE 1: The GEN-COM voltage reading indicates the PCM is commanding the alternator to charge the battery. This is the test result you want to see.

This voltage drop shows that the PCM is actively pulse-modulating the circuit to Ground, commanding the alternator to increase output, and that this circuit is working exactly as it should.

Since you've confirmed that the alternator isn't charging the battery —and in this test section you've verified that the PCM is actively commanding the alternator to charge— you can conclude that the alternator is bad and needs to be replaced.

CASE 2: The GEN-COM voltage stays at battery voltage (12 V) and never changes with load. If your multimeter shows a steady reading close to 12 Volts, no matter how many loads you turn ON, it means the PCM isn't commanding the alternator to charge.

Possible causes include:

• An open or damaged GEN-COM wire between PCM and alternator.
• The PCM has an internal fault and isn't Grounding the GEN-COM circuit (rare).

TEST 2: Checking The Alternator's GEN-MON Signal

IMPORTANT: This test section assumes you've already checked and confirmed that female terminal 1 of the alternator's 3-wire connector has battery voltage present with the key ON, engine OFF (KOEO). If you haven't checked this yet, do so before starting this test.

The alternator sends your Ford Windstar's PCM a feedback signal to let it know how much load is being placed on the alternator.

By load, I mean how much current the alternator's outputting to charge the battery and supply the current of any accessory (like the headlights) you've turned ON.

This feedback is created when the voltage regulator pulls down the 12 Volts the PCM sends it on the GEN-MON wire —the violet (VIO) wire of the alternator's 3-wire connector.

Just like the GEN-COM signal, where the PCM pulls voltage to Ground using pulse-width modulation, the voltage regulator does the same with the 12 Volts of the GEN-MON circuit.

Here's what you need to know about the GEN-MON circuit:

• When the alternator isn't charging the battery, it doesn't provide a Ground to the circuit —so your multimeter should show 12 Volts on the VIO wire.
• As the alternator produces more current to charge the battery and power accessories, the voltage regulator provides a pulse-width-modulated Ground to the 12 Volts on this VIO wire. This pulls the voltage down, so your multimeter will show less than 12 Volts.
• The higher the alternator is working to charge the battery, the lower the voltage, you'll see on the GEN-MON wire. It can go as low as 1 Volt DC.
• The lower the output the alternator is producing to charge the battery, the higher the voltage, you'll see on the GEN-MON wire.

NOTE: This test requires the engine to run for about 5 to 10 minutes to complete. If the battery is weak or discharged, make sure to fully recharge it before starting the test.

GEN-MON SIGNAL: You can find a more detailed explanation of the GEN-MON signal and how it's used to activate the alternator in this diagnostic guide: PCM-Controlled Alternator Basics (1999-2003 3.8L V6 Ford Windstar).

Alright, let's get the ball rolling:

1. 1

   Set your multimeter to Volts DC mode.

2. 2

   Connect the black multimeter test lead to the battery negative (-) post.

3. 3

   Connect the red multimeter test lead to the VIO wire of the alternator's 3-wire connector.

4. 4

   Crank and start the engine.

5. 5

   Check the voltage on your multimeter.
   
   Since the alternator is not charging the battery, you should see a voltage closer to 1 Volt.
   
   NOTE: There isn't a specific hard-coded voltage value for this test —instead, what matters is the range and behavior of the signal. Any voltage reading below 12V and closer to 1V (on the VIO wire) indicates that the alternator is actively trying to charge the battery.
   
   So even if your multimeter doesn't show exactly 1 Volt —say it shows 3, 4, or 6 Volts— that's perfectly fine. These are all normal values and indicate the alternator is under load.

6. 6

   When done, turn off the engine.

Let's examine your test result:

CASE 1: The GEN-MON voltage reading indicates the alternator is under load. This is the correct test result and lets you know the alternator's correctly reporting that it's under load and trying to charge the battery and supply current to the electrical accessories you've turned ON.

Since you've already confirmed the alternator isn't charging the battery, this result further confirms the alternator's toast and needs to be replaced.

CASE 2: The GEN-MON voltage stays at battery voltage (12 V) and never changes with load. This usually means the voltage regulator's toast, since it's unable to pull the voltage down to indicate the alternator's under load. Replace the alternator to solve your Ford Windstar's no-charge condition.

More 3.8L V6 Ford Windstar Diagnostic Tutorials

I've written quite a few diagnostic tutorials for the 3.8L V6 Ford Windstar —and you can find them all in this index:

• 3.8L V6 Ford Windstar Index Of Articles.

Here's a sample of the tutorials you'll find in the 3.8L V6 Ford Windstar index:

• How To Test The MAF Sensor (1996-1998 3.8L V6 Ford Windstar).
• How To Test The TPS (1996-1999 3.8L V6 Ford Windstar).
• How To Test For A Blown Head Gasket (1995-2003 3.8L V6 Ford Windstar).
• How To Do And Interpret An Engine Compression Test (1995-2003 3.8L V6 Ford Windstar).