Part 1

TEST CONDITIONS
1. Code EO24 not set;
2. Transmission in reverse or drive;
3. ECM is not commanding 3rd gear;
4. Vehicle speed is equal to or greater than 0 MPH.

FAILURE CONDITIONS
Third gear input to ECM not grounded (3rd gear present) for 6 seconds.

ACTION TAKEN
1. ECM turns on "Engine Control System Fault" telltale;
2. ECM commands BCM to display "Service Soon" message on Driver Information Center (DIC);
3. Cruise control is disabled;
4. Viscous Converter Clutch is disabled.

DESCRIPTION
The third gear switch is a normally closed (grounded) switch, located in the transmission, that opens when 3rd gear hydraulic oil is applied to it. The third gear switch has 12 volts supplied by the ECM on CKT 108. CKT 108 is grounded when the transmission is not in third gear (switch closed) and has 12 volts on it when the third gear switch opens (3rd gear oil applied).
Code EO28 will set when the ECM does not command 3rd gear and the switch stays open.

Possible causes of Code EO28 are;
1. CKT 108 open
2. CKT 697 grounded;
3. Faulty 3rd gear switch;
4. Faulty 3rd gear shift solenoid
or
5. Faulty ECM connector or ECM.

Notes on Fault Tree
1. Checking for transmission gear status at idle. °C will light if CKT 108 is open (3rd gear), °F will light if CKT 108 and 446 are open (3rd and 4th gear), and °E will light is CKT 446 is open and CKT 108 is grounded (4th without 3rd gear).
2. Checking for an open in CKT 108.
3. Checking for an open in 3rd gear switch at idle.

NOTE ON INTERMITTENTS
At key "ON", engine "OFF", manipulate transmission connector, transmission wiring, and ECM connector while observing the third gear status light (°C). if the third gear status indicator flashes on, repair intermittent open in CKT 108 (third gear).