LOSS OF POWER OR GROUND 4WAL DIAGNOSIS INFORMATION

Subject: LOSS OF POWER OR GROUND

Model and Year: 1990 M AND L VAN

TO: ALL CHEVROLET DEALERS

An intermittent loss of power or ground to the the 4WAL Electro Hydraulic Control Unit (EHCU) may cause a variety of symptoms and can be very difficult to diagnose. Whenever diagnosing a 4WAL complaint technicians should first ensure that the system is well grounded and has a stable power connection. The following code sequences are common indicators of intermittent power or ground conditions.

LOSS OF TWO PIN GROUND:

4WAL CODES 73,86 or 73,86,66 or 47,66,68,48,43,44,53,54,86 or any combination of the above codes

LOSS OF POWER:

4WAL CODE 65

Other indications of possible intermittent losses of power or ground would include.

- Difficulty in resetting the codes

- Other vehicle electrical abnormalities, particularly the A/C and Blower Motor which shares the same power connection with the 4WAL system.

- Evidence or knowledge of prior electrical repairs which might affect 4WAL system power/ground.

When a technician suspects that a vehicle may be intermittently losing either power or ground to the 4WAL system, both ten pin and two pin connections to the unit should be inspected and cleaned with a wire brush of all corrosion. Following this, an appropriately sized star washer should be installed at each source connection to ensure a secure reliable connection. The 4WAL power connection is located just below the bulkhead connector of the vehicle while the ground is located on a stud at the radiator support.

Technicians should take note that power to the EHCU through the 10 pin connector is provided through the "brake" fuse.

Once the power/ground sources have been verified as good, technicians should clear the codes set and run three (3) function tests with Tech One to verify the repair.

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.