Field Service Mode - Technically Speaking

James Berry Mar 12, 2014 0 Comment(s)
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No Flash in the Pan

Q: I’m having a problem with one of my ’85 Corvettes, which seems to be running rich.

As you know, if you jump terminals A and B (the upper-rightmost pinholes) on the Assembly Line Diagnostic Link when the key is in the On position, the Check Engine light will flash any existing trouble codes.

What I didn’t know is that there is another a mode that provides additional information. Some of the guys in my Corvette club told me that by tripping the same terminals, and also starting the car, I could tell if the engine was running rich or lean. If this is true, it would be a great diagnostic tool.

Unfortunately, I could not find any information on this method, and I don’t want to damage my car’s electrical system.

Does this mode really exist? If so, how do I get into it safely, and what information can I really retrieve?

Tom Harper
Via email

A: Your friends are correct: This mode, known as Field Service Mode, can be entered by jumping terminals A and B on the ALDL while the engine is running. It will not harm the electrical or other systems on an OBD-I–equipped vehicle such as your ’85.

When the vehicle is in Field Service Mode, you can observe the engine controller’s loop status—that is, whether the vehicle is in open- or closed-loop mode. You can also monitor the signal from the oxygen sensor to the engine controller, relaying a rich or lean condition. This information will be relayed through the Malfunction Indicator Lamp (MIL).

Interpreting a Flashing MIL Lamp

If the engine controller is in open-loop mode, the MIL will flash extremely rapidly, about two-and-a-half times per second. If the engine controller is in closed-loop mode, it will flash about once per second.

When the engine is first started, the system goes into open-loop operation. In open loop, the engine controller will ignore the signal from most of the engine sensors and calculate the air/fuel ratio based on inputs from the coolant and MAP sensors, along with a pre-programmed memory.

The system will stay in open loop until the following conditions are met:

  1. The O2 sensor has varying voltage output, showing that it is hot enough to operate properly (approximately 600 degrees Fahrenheit).
  2. The coolant sensor is above a specified temperature (approximately 104 degrees).
  3. A specific amount of time has elapsed after starting the engine.

When these conditions are met, the system will go into closed-loop operation. In closed loop, the ECM will calculate the air/fuel ratio based on a variety of sensors and keep it as close to 14.7:1 as possible.

When in closed loop, if the MIL is off more than it is on, the engine is running lean. If the opposite is true, this indicates that the engine is running rich. These displays are based on readings taken by the oxygen sensor.

In a lean condition, the engine controller will command the block learn/integrator to enrich the fuel mixture above the 128-count baseline. In a rich condition, the block learn/integrator will lean out the fuel mixture below the 128 baseline. (This number can only be monitored using a scanner.)

This 128 baseline number on the ’82 through the ’95 Corvettes comes from monitoring the Block Learn Multiplier (BLM). The BLM is the long-term fuel adjustment that the engine controller “learns” in order to keep the air/fuel ratio within acceptable parameters. The integrator works using the same idea, only for short-term adjustments.

The BLM value can range from 0 through 255, with a value of 128 (the midway point) being ideal. Changes below 118 or above 138 usually indicate a problem.

There is another mode that can be accessed through the ALDL that can be helpful in rudimentary diagnostics. It’s known as the Backup Fuel Mode, and it can be entered by inserting a 3.9K resistor between terminals A and B of the ALDL connector pinholes.

Backup Fuel Mode—more commonly referred to as “Limp Home” mode—forces the engine controller to rely on set fuel calculations in the engine-controller PROM, instead of the learned inputs in active RAM. Backup Fuel Mode is usually set at around 12:1 or richer. If your engine seems to run better in this mode, there is likely a sensor that is reporting incorrect data to the engine controller.

Tom, I would recommend that you purchase a scanner if you’re doing your own repairs. There is simply much more information that can be ascertained using one of these diagnostics tools.

Thanks for the question, and good luck.

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