INFO. ON THE TRUCK ELECT A/C SYS.(SELF DIAGNOSTIC FEATURES)

SUBJECT: INFORMATION ON THE TRUCK ELECTRONIC AIR CONDITIONING SYSTEM

VEHICLES AFFECTED: 1991-93 "C/K" AND TOPKICK/KODIAK TRUCKS

For the 1991 model year, self-diagnostic features were added to the Electronic Air Conditioning (EAC) system which are used in C/K and TopKick/Kodiak trucks. These features are also used in 1992-93 model year vehicles.

In these systems, the control head communicates its diagnostic message to the service technician by flashing the various symbols located on the display panel.

It is important for the technician to recognize that these flashing displays represent a DIAGNOSTIC MESSAGE, and ARE NOT A RESULT OF CONTROL HEAD FAILURE.

NOTE: When the technician is attempting to distinguish between normal diagnostic messages and a failed EAC control head, he should remember that all 1991-93 model year control heads utilize self-diagnostic features. However, 1988-89 model year product does not employ these features, even though these have an external appearance which is nearly identical to that of the later versions.

SELF-DIAGNOSTIC MESSAGES:

The control head will diagnose faults in the mode door motor, the temperature control door motor, and the A/C compressor system. When a fault is detected, the control head will flash the appropriate symbol on the display panel to notify the customer and/or technician of the problem. This flashing will last for two minutes following the failure occurrence. It will then stop to avoid distracting the driver. However, when the vehicle is restarted (ignition recycled), if the failure was present when the ignition was last ON, the display panel will again flash its diagnostic message for two minutes. This serves to remind the customer that the EAC system still has a fault and should be taken to the dealer for service. The diagnostic message will also reappear whenever the driver presses a function switch which would cause the faulty circuit to operate. In this case, as in the others, the display will flash for two minutes.

In summary, the control head will communicate a diagnostic message at following times:

1 WHEN THE FAULT IS FIRST DETECTED

2 WHEN AN AFFECTED FUNCTION SWITCH IS PRESSED.

o If a fault is present in the mode motor, pressing a mode function switch will cause the mode symbols to flash.

o If a fault is present in the temp motor, pressing a temp function switch will cause the temperature cursors to flash.

3 WHEN THE IGNITION KEY IS RECYCLED (after engine start).

o All the above messages will flash for two minutes.

REQUIREMENTS FOR SETTING/CLEARING FAULT CONDITIONS: -------------------------------------------------- The following chart outlines the three different diagnostic messages that the EAC control head may communicate. Letter codes are used in the FAULT, DEFAULT ACTION, and REQUIREMENT TO CLEAR columns. The meaning of each lefter code is described in the list appearing below the chart.

FLASHING SYMBOL FAULT DEFAULT ACTION REQUIREMENT TO CLEAR --------------- ----- -------------- -------------------- Mode Symbol A B C

Temp Cursors A B C

A/C Symbol D E F

A. Fault detection is based on the monitoring of the actuator motor FEEDBACK voltage.

o To determine if a malfunction has occurred, the control head looks for any of the three following conditions:

1. OPEN CIRCUIT - Indicated by a feedback voltage which is too high ()4.80 volts) to be considered normal.

2. SHORT CIRCUIT - Indicated by a feedback voltage which is too low ((0.20 volts) to be considered normal.

3. INOPERATIVE MOTOR - Indicated by a feedback voltage which doesn't change in response to motor drive commands.

B. While the fault is current, the control head will continue to send motor drive commands to the actuator motors.

o This will allow the motor to continue to move, if only the feedback circuit is faulty. However, when the motor moves without feedback information it will drive all the way to its mechanical limits, with no stops in between. Therefore: 1) The MODE MOTOR will stop at either VENT or DEFROST. 2) The TEMP MOTOR will stop at either VENT or FULL COLD.

NOTE: By noticing whether the motor will move in response to the EAC function switches, one can quickly determine if the fault is in the motor drive circuit or the feedback circuit. No motor movement indicates a motor drive problem. Uncontrolled motor movement indicates a feedback problem.

C. Pressing a function switch will always command motor movement. As the motor is moved, the control head looks for the correct feedback voltage. IT WILL CLEAR THE FAULT IF THE CORRECT FEEDBACK VOLTAGE IS PRESENT.

D. Fault detection is based on unusually rapid A/C compressor clutch cycling.

o If the A/C clutch stays on for less than 1.5 seconds, for 10 consecutive cycles, an A/C fault is indicated.

E. Once the A/C fault is detected, the A/C clutch will be disabled for all subsequent ignition cycles.

F. Removing BATTERY power from the control head is the only means of clearing the A/C fault and restoring A/C compressor operation.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.