DRIVEABILITY Gearbox Fault Code 21 (19/06/97/NAS)

ENGINE STALLS & ILLUMINATES THE MIL ON RESTART

There have been cases of customer complaint of an engine stall, when the engine is cold. This typically occurs either when first selecting Drive or shortly afterward when the vehicle is stopped. These incidents occur before the vehicle has had a chance to warm up and can be accompanied by a MIL illumination and possibly a transmission defaulted to third gear. Test Book analysis can show a P1775 code (gearbox fault) and Transmission Code 21 (engine speed signal). This indicates that engine speed has fallen to zero with DRIVE selected.

REPLACE PROM IN GEMS ECM

In order to properly diagnose this fault the technician should retrieve P-Codes and Freeze Frame data and FAX a completed DTCR to LRNA. A new PROM has been developed which will prevent the stall at first stop. This is accomplished by providing a higher idle speed for the warm-up period.

TEST BOOK

1. Hook up Test Book and follow directions to retrieve fault codes.
2. Complete a DTCR along with freeze frame data.
3. Follow basic diagnostic directions as specified by Test Book.
4. If Test Book tests do not resolve the problem, follow the procedures below.

CHECK BASIC ENGINE SETTINGS

1. Check with Test Book to make sure that the Idle Air Control Valve (IACV) is within the specification of 20 to 25 steps with engine and transmission fully warmed. Adjust as necessary.
2. Check the Air/Fueling adaptations are within specifications (see Operation Pride article 1-06, TIB 12/03/96/US).
3. Make sure that the airflow into the engine at idle is 20-24 Kg./hr. for the 4.0 liter engine and 22-26 Kg./hr. for the 4.6 liter engine.
4. Make any repairs indicated by the above checks.
5. If no fault is found with the above checks, review freeze frame data. If this data shows an engine speed of 0 rpm and a coolant temperature of between 12-42° C (55-110°F), the problem occurred when the engine was cold, soon after start.
6. Contact the GEMS help line for assistance. The help line will release a PROM with a revised cold idle strategy that should eliminate the reoccurrence of the problem.
   NOTE: If the updated PROM fails to cure the symptoms (rough idle, driveaway stall, hesitation), it is possible that the problem is due to intake valve deposits:
   • The use of lower quality fuels may lead to intake valve deposits which can modify the fuel mixture and cause these symptoms.
   • The use of fuel system cleaners such as tank additives or fuel rail injector cleansers is NOT recommended as they have not been proven effective in eliminating these deposits.
   • The only proper way to resolve this problem is to either remove and clean the valves or to use walnut-shell blasting equipment with the valves installed.
   • In order to prevent deposit build up, the customer must be instructed to use high quality fuels with detergent additives.
7. Upon receipt of the revised PROM, refer to TIB 10/11/96/US and install the PROM using the procedures outlined.
8. Reset engine adaptations and set to IACV to 20-25 steps.
9. Perform Drive Cycle B.