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Idle/Throttle Speed Control Unit: Adjustments


On engines equipped with an idle speed control motor, idle speed is controlled by an electrical signal from the ECM. Do not use the ISC plunger to adjust curb idle. The motor varies idle speed from a base (minimum authority) to a high (maximum authority) idle speed. Do not adjust unless diagnosis indicates a malfunction. Check for proper base and high idle speed as follows. See Underhood Emission Label.

Plunger - Maximum Adjustment:





NOTE: Prior to adjustment, check for an identification letter on the ISC plunger. If NO LETTER appears, using Tool J-29607, BT-8022 or equivalent, remove plunger from ISC Unit. Then, measure length of plunger from back side of plunger head to end of plunger screw - Dimension "A". Record this measurement for use at Step 17. Reinstall plunger in ISC unit, turning plunger into a preset position so that the distance measured from the back side of the plunger head to the ISC nosepiece is LESS THAN Dimension "B".


ISC PLUNGER ADJUSTMENT CHART


Identification Plunger Length Plunger Length

Letter Dimension "A" Dimension "B"

None 14.1 mm (9/16") 5.6 mm (7/32")

None 16.3 mm (41/64") 8.0 mm (5/16")

X 18.5 mm (47/64") 10.0 mm (25/6'4")

A 19.3 mm (49/64") 10.8 mm (27/64")

Y 20.5 mm (51/64") 12.0 mm (15/32")

S 21.2 mm (27/32") 12.7 mm (1/2")

Z 22.5 mm (7/8") 14.0 mm (35/64")

G 23.2 mm (29/32") 14.7 mm (37/64")

E 25.6 mm (1LL) 17.1 mm (43/64")

L 27.5 mm (1 -3/32") 19.0 mm (3/4")

J 30.0 mm (1-3/16") 21.5 mm (27/32")

N 32.0 mm (1-17/64") 23.5 mm (59/64")

T 34.0 mm (1-11/32") 25.5 mm (1")

Idle Speed Control Assembly - Typical:





CAUTION: DO NOT disconnect or connect ISC connector with ignition "ON" as damage to the ECM may occur.


1. Perform preliminary procedures - see emission label for latest instructions.

2. Before starting engine, place transmission in PARK (AUTO trans.) or NEUTRAL (MANUAL transmission), set parking brake, and block drive wheels.

3. Connect dwell meter to mixture control (M/C) solenoid dwell lead. Remember to set dwell meter on the six cylinder scale, regardless of the engine being tested.

4. Turn A/C "OFF".

5. Start engine and run until stabilized and in "closed loop" mode (dwell meter needle starts to vary).

6. Turn ignition "OFF".

CAUTION: Do not unplug ISC connector with ignition "ON" as damage to motor will occur.


7. Unplug connector from ISC motor.


8. Fully retract ISC plunger by applying 12 volts to terminal "C" and grounding lead to terminal "D" of the ISC motor connection.

CAUTION: DO NOT leave battery voltage applied to motor longer than necessary to retract ISC plunger. Prolonged contact will damage motor. Also, NEVER connect voltage source across terminal "A" and "B" as damage to the internal throttle contact switch will result.

9. Start engine and run until dwell meter needle starts to vary, indicating "closed loop" operation.

10. Place A.T. transmission in Drive.

11. With ISC plunger fully retracted, adjust carburetor base (slow) idle stop screw (minimum authority) to the RPM shown in the carburetor charts.

12. PLACE TRANSMISSION IN PARK (AUTO TRANS.) OR NEUTRAL (MANUAL TRANS.).


Idle Speed Control Assembly - Typical:





13. Fully extend ISC Plunger by applying 12 volts to terminal "D" and grounding lead to terminal "C" of the ISC Motor Connection. Leave power applied only long enough to extend the plunger.

14. Manual transmission: Using Tool J-29607 or BT-8022 or equivalent, turn ISC Plunger to obtain ISC adjustment RPM (Maximum Authority) shown in carburetor charts.

15. Automatic Transmission:

a. With transmission in PARK, using Tool J-29607, BT-8022 or equivalent, preset ISC plunger to obtain 1500 RPM.

b. With parking brake set and drive wheels blocked, place transmission in drive. Using Tool J-29607 or BT-8022 or equivalent, turn ISC Plunger to obtain ISC adjustment RPM (Maximum Authority) shown in carburetor charts.


Plunger - Maximum Adjustment:





16. Recheck ISC "Maximum Authority" RPM with voltage applied to Motor (Step 13). Motor will ratchet at full extension with power applied.

17. After adjustment of ISC plunger, measure distance from back side of plunger head to ISC nose piece, Dimension "B". Dimension must NOT exceed that shown by plunger type as either identified by plunger length or letter identification.

18. Fully retract ISC Plunger (Step 8). Place transmission in PARK (AUTO Trans.) or NEUTRAL (MANUAL Transmission) and turn ignition "Off". Disconnect 12 Volt Power source, Ground lead, Tachometer and Dwell Meter. With ignition "Off" reconnect four Terminal Harness connectors to ISC Motor.


19. "TRICKING" the ISC motor as described will cause the "CHECK ENGINE" light to come on and an ISC motor trouble code to be set. By restoring the system to normal operation, the light will go out, but the trouble code will continue to be stored as an "INTERMITTENT" problem. In this case, it will be necessary to clear the diagnostic trouble codes by removing the ECM fuse from the fuse block for about 10 seconds.

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