ERRATIC IDLE OR CEL ON REVISED TPS VOLTAGE CHECK

VEHICLES AFFECTED: 1986 S/T With 2.8L TBI Engines

Some 1986 S/T trucks equipped with the 2.8L TBI engine may experience an erratic idle condition or a "Service Engine Soon" light that stays on. This condition may be caused by the Throttle Position Sensor (TPS) supplying incorrect voltages to the Electronic Control Module (ECM). A voltage test of the TPS switch must be made prior to replacement of the TPS using the Service Manual revision included in this bulletin.

If this condition is encountered and the testing indicates TPS replacement is necessary, a new TPS, P/N 17111725, has been released and should be used to correct this condition. The replacement procedures for this operation can be found in Section 6E2-C2-15 of the S15 Service Manual. When Performing the TPS voltage check, a revision must be made to the adjustment procedure on page 6E2-C2-15.

The revision should read as followa:

With ignition 'On', voltage between terminals "A" and "B" should be between 0.420 and 0.450 volts.

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.