Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1990 Geo Tracker L4-1590cc 1.6LFACTORY OEM SERVICE PROCEDURE
Idle/Throttle Speed Control Unit: Testing and Inspection
Chart C-2:
Wiring Diagram For Chart C-2:
CIRCUIT DESCRIPTION:
The ECM controls a solenoid to regulate an opening at the throttle body. The amount of the opening determines the idle speed. The ISC is regulated according to engine load. A stalling condition or rough idle could be caused by the ISC valve not functioning properly.
TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.
1. This will quickly tell if the ECM will increase the idle for added engine load.
2. If the solenoid has high resistance or no resistance, the valve will not operate properly.
3. This will simulate the ECM command for the ISC valve to open.
4. This test the circuit supplying B+ to the ISC valve.
5. This will check the ISC valve Driver circuit in the ECM.
6. Follow the ECM check in the Electrical and Electronic Specifications.
Factory Service Tools
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.