Hard Trouble Code E28: Notes

NOTE: If this code is not displayed during second pass of diagnostic codes, it is an intermittent failure and cannot be diagnosed using this procedure.

The ECM supplies 12 volt signal on circuit Nos. 438 and 446 to transmission 3rd and 4th gear switches respectively. The 3rd gear switch pulls circuit No. 438 to ground when transmission is in any mode except 3rd or 4th gear. The 4th gear switch pulls circuit No. 446 to ground when transmission is in any mode except 4th gear. The ECM senses 3rd gear operation when switch opens due to 3rd gear oil pressure or 4th gear oil pressure, and senses 4th gear operation when switch opens due to 4th gear oil pressure.

Trouble code indicates that ECM is seeing 12 volts on circuit Nos. 438 or 446 when transmission is in Park or Neutral. Possible causes of this condition are: Defective 3rd gear switch, defective 4th gear switch, defective wiring, terminals, etc., or ECM not able to process signals properly.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

To begin diagnosis, observe 4th gear status light ("REAR DEFOG" light on CCP panel) to determine if failure resulted from 3rd gear circuit or 4th gear circuit. If light is off, go to step 6).
If light is on, determine if ECM can recognize grounding of circuit No. 446 at transmission connector. If "REAR DEFOG" light remains on, there is an open in circuit No. 446, or ECM. If light remains on with circuit No. 446 grounded at ECM, either ECM connector or ECM is faulty. If light goes off, circuit No. 446 is open.
If light is off with pin B grounded, then ECM is functioning properly and fault must be in transmission. If test light on pin B at transmission does not light, an open must exist in wiring or 4th gear switch. If test light fails to light on 4th gear switch terminal, it is open and should be replaced. If test light does light, open is between transmission connector and 4th gear switch connector.
If test light on pin B lights, 4th gear switch is working properly. If test light goes out when engine is started, switch is receiving oil pressure when it should not be. Service transmission.
If test light stays on, transmission connector should be reconnected and "REAR DEFOG" light should be checked for proper operation. If "REAR DEFOG" light is on, then fault must be improper mating of circuit No. 446 terminals at pin B of transmission connector. If light is off, malfunction is not present at this time.
If "REAR DEFOG" light was off without pin B grounded, determine if ECM can recognize grounding of circuit No. 438 at transmission connector. If "SERVICE SOON" light is on when out of diagnostic mode, there is an open in circuit No. 438 or ECM. If light remains on with circuit No. 438 grounded at ECM, either ECM connector or ECM is faulty. If light goes off, circuit No. 438 is open.
If "SERVICE SOON" light is off with pins B and C grounded, then ECM is functioning properly and fault must be in transmission. If test light on pin C at transmission connector does not light, then an open must exist in wiring or 3rd gear switch. If test light fails to light on 3rd gear switch terminal, then it is open and should be replaced. If test light does light, open is between transmission connector and 3rd gear switch connector.
If test light on pin C lights, then 3rd gear switch is working properly. If test light goes out when engine is started, then switch is receiving oil pressure when it should not be. Service transmission.
If test light stays on, then transmission connector should be reconnected and "SERVICE SOON" light should be checked for proper operation. If light is on, then fault must be improper mating of circuit No. 438 terminals at pin C of transmission connector. If light is off, then malfunction is not present at this time.