1982 - 1991 Corvette: Service Bulletin: Parasitic Drain on Battery Ammeter Connecting Procedure

Source: Chevrolet Dealer Service Bulletin
Number: 91-14-8A
Subject: Parasitic Drain on Battery Ammeter Connecting Procedure
Model Year: 1982-91 All Passenger Cars and Trucks

Vehicles that have one or more on-board solid state control module (such as an Engine Control Module, E.C.M.) may, at some time, exhibit a failure mode of a control module which may result in a high parasitic current drain on the vehicle’s battery.

The use of the conventional "Battery Electrical Drain Check" procedure may not find the cause of the electrical drain because when the battery cable is disconnected to install an ammeter, the solid state module that was causing an excessive current drain may not cause an excessive current drain once circuit continuity is restored. Cycling the ignition lock cylinder to the "RUN" and then to the "OFF" position may cause the excessive current drain by the module to recur, but the ignition lock cylinder should not be rotated to the "ACCESSORY," "RUN" or "START" position while an ammeter is installed between the battery terminal and the battery cable. Therefore, the following procedure should be followed when connecting an ammeter to perform a battery electrical drain check:

Assure that the ignition lock cylinder is in the "LOCK" position and all electrical accessories are turned off and all doors are closed.
Disconnect the negative battery cable from the negative battery terminal.
Install a side terminal battery adapter ST-1201 (or equivalent) to the negative battery terminal. Torque to 17 N.M. (13 lb.ft).
Connect a 3mm squared (12 gauge) jumper wire with an alligator clip on each end between the end of the negative battery cable and the side terminal battery adapter installed on the negative battery terminal.
Rotate the ignition lock cylinder to the "RUN" position.

CAUTION:
Do not rotate the ignition lock cylinder to the "START" position. Doing so will cause the 3mm squared (12 gauge) jumper wire to become extremely hot and may result in personal injury and vehicle fire.
Rotate the ignition lock cylinder to the "LOCK" position and remove the ignition key from the lock cylinder.

IMPORTANT: From this point, make sure that circuit continuity between the negative battery cable and the side terminal battery adapter is not interrupted.
Connect an ammeter (set on the highest scale), capable of measuring at least 20 amperes, between the negative battery cable and the side terminal battery adapter.

IMPORTANT: The ammeter will be connected in parallel to the 3mm 2 (12 gage) jumper wire.
Remove the 3mm squared (12 gauge) jumper wire installed in step 4.
Adjust ammeter scale to lowest scale possible without going out of range and note ammeter reading.
Continue with normal battery electrical drain check procedures.

WARRANTY INFORMATION: Use applicable labor operations and Times.

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.