6.5 L57 Engine - Cranks or Starts with Key in Run - kw charge crank drive electrical fuse gauge generator ground ignition inaccurate loss P3 short start starter unwanted voltage wire

When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

Engine cranks or starts with the key in the run position. The L57 6.5 non turbocharged mechanical diesel uses a diode in circuit 531/806. Circuit 531 powers the cold advance solenoid/engine coolant temperature switch and solenoids. The purpose of the diode is to allow the fast idle and cold advance solenoids to be energized when cranking during a hot restart. The intended action of this design is to slightly assist engine starting during a hot restart. The power supply to these two solenoids through this diode is redundant when the engine coolant temperature is below approximately 100 degrees F.

This is due to the fact that the cold advance solenoid/engine coolant temperature switch is closed when the coolant temperature falls below this point. The power supply to the cold advance solenoid/engine coolant switch is applied during run and start/cranking modes. Since the solenoids are permanently grounded, the switch controls the power feed to them. At coolant temperatures of approximately 100 degrees F or higher, the cold advance engine coolant temperature switch opens and interrupts power flow to the cold advance and fast idle solenoids. During engine starting when the coolant temperature is above approximately 100 degrees F, power will still be applied to these two solenoids when cranking through this alternative path provided by the diode. The diode itself receives power from circuit 531/806 which is the power feed to the starter relay. This relay is energized only during starting/cranking modes.

Cause: shorted diode in starter circuit. In addition, this diode is shown facing the incorrect direction on page 6-2173 of the 1998 P32/42 chassis manual book 4 of 4. The direction of the diode within this schematic should be reversed on this page. The prior model year service manuals appear to be correct. The respective pages numbers for specific model years are listed below.

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1995 PG/P3 chassis driveability manual, page 8A-A27-0

1996 P32/42 chassis book 2 of 2, page 8A-A22-1

1997 P32/42 chassis book 2 or 2, page 8A-A22-1

If this diode shorts, power is then allowed to be applied to the to the starter motor as soon as the key is moved to the run position. However, in order for this to occur, the cold advance engine coolant temperature switch must be closed. If this condition occurs, the starter can engage with the key in the run position. The diode is located behind the generator in the wiring loom (see page 8A-A201-14 in the P chassis 1995 driveability book, figure d100). The only wiring schematic that shows the diode is in the P chassis service manual. If the diode is found to be shorted the part numbers below can be used to correct the condition Parts information:

GMSPO 12112421

Radio Shack 276-1103

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.

(94 - 98 GMC P32 RV/Van Chassis) and (94 - 98 Chevrolet P32 RV/Van Chassis) and (94 - 98 GMC G3 Van / Savana) and (94 - 98 Chevrolet G3 Van / Express)

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