SERVICE GUILD: INFO. ON REPLACEMENT OF ISOLATION DIODE

SUBJECT: INFORMATION CONCERNING REPLACEMENT OF ISOLATION DIODES

MODELS: 1983 - 1992 ALL TRUCKS

Many of the electrical systems on our vehicles use a diode to isolate certain circuits and protect them from voltage spikes. Some of the circuits that may use such a diode are listed below:

- A/C Compressor Clutch - 4WAL - Wiper - Charging System (hidden in wire harness) - Relays - Solenoids - Diesel Glow Plug Circuit

Obtaining replacement diodes can sometimes be a problem. A universal diode that meets the specifications in the attached chart can be used for the applications listed above.

SERVICE PROCEDURE:

When installing the new diode, use the following procedures to obtain a lasting repair:

1. If the diode is taped to the harness, remove all of the tape.

2. Paying attention to current flow direction, remove inoperative diode from the harness with a suitable soldering tool. If the diode is located next to a connector terminal, remove the terminal(s) from the connector to prevent damage from the soldering tool.

3. Carefully strip away a section of insulation next to the old soldered portion of the wire(s). Do not remove any more than is needed to attach the new diode.

4. Check current flow direction of the new diode, being sure to install the diode with correct bias. Reference the appropriate electrical diagrams and diagnosis manual wiring to obtain the correct diode installation position, Reference figure 1 for replacement diode symbols and current flow explanations.

Attach the new diode to the wire(s) using 60/40 rosin core solder. Use a heat sink (aluminum alligator clip) attached across the diode wire ends to protect the diode from excess heat. Follow the manufacturer's instructions for the soldering equipment you are using.

5. Install terminal(s) into the connector body, if previously removed in step number 2.

6. Tape the diode to the harness or connector using electrical tape. To prevent shorts to ground and water intrusion completely cover all exposed wire and diode attachment points.

- A universal diode with a 1 -amp, 400 PIV (Peak Inverse Voltage) rating can be used in all of the previously mentioned applications.

Diode Rating P/N Comments Brand Number ------ ------ -------- -------------------------------------- GMSPO N4001 8907496 1-amp, 50 PIV - All applications except Radio 276-1101 A/C compressor clutch. Will need to remove diode from connector assembly.

GMSPO N4004 16039386 1-amp, 400 PIV - A/C compressor clutch 276-1103 application Radio Shack

Parts are currently available from GMSPO.

Heat sink, part number 276-1567, can be obtained from Radio Shack.

We believe the diodes and heat sink listed in this article as well as their manufacturer to be reliable. There may be additional manufacturers or equiva- lent products. General Motors does not endorse, indicate any preference for, or assume any responsibility for the products from these firms or for any such items which may be available from other sources.

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.