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.

Fob-Training Program

Q: I understand it may be difficult to diagnose a problem via e-mail, but I thought I’d try. I just bought an ’02 Z06 with only 6,800 miles, and I’m having a problem programming the key fobs.

The car only came with one fob when I got it, and it didn’t work. I changed the battery, to no avail. My next step was to go to the parts department of my local dealership and order two new fobs. I then made an appointment to have the fobs programmed by my mechanic.

On the day of my appointment, I waited an hour and a half before being told by the mechanic that his programmer was “malfunctioning.” He recommended that I try the local Chevrolet dealer, but I’d like to avoid that expense.

I tried to program the fobs myself, but I can’t get them to work, either. I can get to the point on the DIC where it says FOB TRAINING, but after hitting the reset button, it won’t move on to the PRESS UNLK + LOK step.

Any input would be greatly appreciated.

Kent Tibbs
Via email

A: The procedure for programming your Corvette’s key fob should be located in the owner’s manual. That said, I’ve had GM vehicles give me problems when trying to program new fobs.

Let’s start by following the normal programming procedure; if that doesn’t work, we can try to “force” the vehicle into learn mode.

Method 1—Key-Fob Quick Learn

This procedure will only add key fobs, with new ones numbered in sequence based on the order in which they were programmed. For example, if two fobs are currently programmed to the Remote Control Door Lock Receiver (RCDLR), a fob programmed using this method will be numbered as Fob 3. This procedure requires that you have a known “good” (functioning) key fob, a replacement fob, and the vehicle key.

Note: Quick Learn is not available when the maximum number of fobs (four) has been programmed. In that case, the Driver Information Center (DIC) will display MAX FOBS LEARNED, and the programming procedure will terminate.

  1. Make sure the vehicle is off.
  2. Open the rear compartment.
  3. Place a known “good” key fob in the interior of the vehicle.
  4. At the rear of the vehicle, insert the vehicle key in the rear-compartment lock cylinder (located in the lower left) and cycle the key five times within five seconds.
  5. The DIC will display READY FOR FOB X (X = 2, 3, or 4).
  6. Place the replacement fob in the glovebox fob pocket with the buttons facing right.
  7. A beep will sound, and the DIC will display READY FOR FOB X (X = 3 or 4) or MAX FOBS LEARNED.
  8. If programming additional fobs, repeat Steps 5 and 6, or press the OFF/ACC portion of the ignition switch to exit programming mode.
  9. Operate the active portion of each fob to exit programming and verify correct system operation.

Method 2—Key-Fob Slot Programming

This procedure adds or replaces a key fob, and it allows you to choose the location (slot) to which the non-programmed fob will be “learned.” If you are replacing a fob, it will erase the fob previously programmed to that particular slot. This procedure requires that you have a known “good” (functioning) key fob, a replacement fob, the vehicle key, and a scan tool capable of programming key fobs.

  1. Make sure the vehicle is off.
  2. Open the rear compartment.
  3. Place a known “good” key fob in the interior of the vehicle.
  4. At the rear of the vehicle, insert the vehicle key in the rear-compartment lock cylinder and cycle the key five times within five seconds.
  5. Install a scan tool.
  6. Use the scan tool to access the following:
    • Vehicle Control Systems
    • Computer/Integrating Systems
    • Module Replacement/Setup
    • RCDLR
    • Program Key Fobs
  7. Select Fob X (X = 1, 2, 3, or 4) on the scan tool. This will be the slot to which you program.
  8. Place the replacement fob in the glovebox fob pocket with the buttons facing right. A beep will sound, indicating that programming is compete. The previously programmed fob is now erased.
  9. If programming additional key fobs, repeat Steps 4-8, or press the OFF/ACC portion of the ignition switch to exit programming.
  10. Remove the scan tool.
  11. Operate the active portion of each key fob to exit programming and verify correct system operation.

(Note that if you want to add or replace a key fob without using a scanner, it’s possible to do so using Method 1. Follow Steps 1 through 4, then stop and wait 30 minutes. The DIC will display “READY FOR FOB 1” after the time has elapsed. [It’s a good idea to install a battery charger to keep the vehicle battery from going dead during the wait.] I’ve had good and bad luck getting this procedure to work.)

Forced-Learn Programming

If the vehicle won’t go into programming mode, you can try to “force” it to do so using the following procedure:

  1. Make sure the vehicle is off.
  2. Open the rear compartment.
  3. At the rear of the vehicle, insert the vehicle key in the rear-compartment lock cylinder.
  4. Place a known “good” key fob in the interior of the vehicle.
  5. Turn vehicle “on” without starting it, then press the start button again to turn it off.
  6. After turning vehicle off, have a helper immediately turn the key five times. This may trigger the “FOB” message on the DIC. You can then proceed with the rest of the procedure as normal.

In closing, I’d like to note that this procedure has on occasion driven me to perform the following additional steps:

  1. Remove beer from refrigerator.
  2. Open can.
  3. Drink.
  4. Repeat as necessary.

Good luck, and keep us posted on your results.


Questions?

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at vette@sorc.com. Be sure to put “Technically Speaking” in the subject line.

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