Action Taken

WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.

ECM turns on SERVICE SOON light.
ECM assumes throttle switch is closed if brakes are applied, and TPS is less than 18 degrees.
ECM assumes throttle switch is open if brakes are not applied, or TPS is greater than 18 degrees.

NOTE: With Code E26 stored hard, the throttle switch status light (OFF) displays the "failsoft" throttle switch status. To display actual switch status, enter diagnostics and clear codes.

The throttle switch is part of ISC motor assembly. Terminal "B" of ISC motor 4-way weatherpack connector is throttle switch input to ECM. The ECM provides a 5-volt signal to terminal "B" of ISC. When throttle lever contacts ISC plunger, throttle switch closes, shorting 5-volt signal at terminal "B" to pin "A" which is a ground (closed throttle equals throttle switch, input voltage low). Code E26 is designed to detect a throttle switch input that is always grounded, and throttle switch never opens.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Test Number 1) To begin diagnosis, clear ECM codes and observe throttle switch status light (OFF on ECCP). When a code is present, status light will represent a "failsoft" value. Therefore, it is important that any codes present are cleared before proceeding. Diagnostics should not be exited since the ECM will not set a code while in diagnostic mode. The light should go off upon opening throttle. If light does not go off, refer to diagnostic decision chart for Code E.7.2 since throttle switch signal remains in closed mode.

Test Number 2) If switch test E.7.2 passes, check TPS for proper operation. If sensor is free and correctly adjusted, enter diagnostics and display engine data parameter P.0.1. Under condition of a Code E26, this throttle angle reading will be greater than 13 degrees at closed throttle. If opposite condition (low TPS signal) can be created, then self-diagnostics can be used to detect this failure by displaying engine data parameter P.0.1. With opposite condition created, a throttle angle reading of -10 to -7 degrees should be displayed.

Test Number 3) To create a low sensor signal, disconnect TPS connector. If throttle angle reading is -10 to -7 degrees, circuit No. 476 must be checked for an open. If voltage between circuits No. 474 and 476 is zero volts, circuit No. 476 is open. If voltage is 5 volts, ECM and wiring are okay. The fault must be improper mating of sensor terminals or a faulty TPS sensor. If throttle angle reading is -6 degrees or greater, there is a short to voltage in wiring harness or the ECM.

Test Number 4) Fault is most likely at ECM connector or ECM. Before ECM is replaced, perform CHART C-1: ECM REPLACEMENT CHECK .