EHCU - Loss of Power or Ground
90chevy31Number: 90-314-5
Section: 5
Date: August 1990
Corp. Bulletin No.: 065014
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 resefting 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.