DTC 28

Fig. 22 Transmission wiring circuit. 1984:

Fig. 23 Transmission wiring circuit. 1985:

If this code is not displayed during the third pass of diagnostic codes, the problem is an intermittent failure. To help isolate the problem, manipulate related wiring while observing fourth gear status light. If failure is induced, status light will illuminate.
The ECM supplies a 12 volt signal on circuit 438 to transmission fourth gear switch. The fourth gear switch pulls this circuit to ground when transmission is in any mode except fourth gear. Fourth gear oil pressure causes ECM to sense operation when switch opens. Code 28 indicates that ECM is sensing 12 volts on circuit 438 when transmission is in reverse. Possible causes of this condition are:
a. Defective fourth gear switch.
b. Defective wiring or electrical connectors.
c. ECM not able to process signals properly.
1. Disconnect transmission electrical connector, then connect a jumper wire between harness connector pin B and ground.
2. Turn ignition on and enter diagnostics. Observe ``4th gear'' status light above ``Hi'' button on ECC control panel. If status light illuminates, proceed to step 3. If light remains off, proceed to step 4.
3. Connect jumper wire between ECM connector P-2 pin 22 and ground. If status light remains lit, check for faulty ECM or ECM connector and repair or replace as necessary. If light goes out, repair open in circuit 438.
4. Check pin B at transmission with a test lamp to 12 volts. If test lamp lights, proceed to step 6. If test lamp does not light, proceed to step 5.
5. Remove transmission oil pan, then disconnect 4th gear pressure switch electrical connector. Check pressure switch terminal with a test lamp to 12 volts. If test lamp now lights, repair open in wire between transmission connector and 4th gear pressure switch. If test lamp does not light, replace 4th gear pressure switch.
6. Start engine and shift transmission into reverse. If test lamp remains lit, proceed to step 7. If test lamp goes out, system is functioning properly and the fault is in the transmission.
7. Reconnect transmission electrical connector, then start engine and shift transmission into reverse. Enter diagnostics and observe ``4th gear'' status light. If status light illuminates, repair open in circuit 438 at the transmission connector. If light remains off, there is no malfunction in the system at this time and problem is intermittent.