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Goodwrench9124
Goodwrench9124, ASE Certified Technician
Category: Saturn
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Experience:  Ase Master Certified, GM Master Certified Tech. Awarded GM Top performer,Hrybrid certified
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2002 Saturn SC1 SOHC - My car has a high idle in park and

Customer Question

2002 Saturn SC1 SOHC

- My car has a high idle in park and neutral. It varies between 2350 to 3500 RPM.
- The car accelerates beautifully, and I have no sputtering while it shifts up. However, on the down-shift the RPM jumps up to 2200 (All Gears).
- I have cleaned the throttle body, idle air control valve, and replaced the gasket for the throttle body. There are no vacuum leaks. I have not checked the intake manifold for vacuum leaks. I have not yet checked the electronic portion of the IAC or TPS. I bought the OHM meter today, but have not had a chance to check the voltage.

1. Check the TPS voltage, and verify if it defective or not.
2. Replace the ECTS

My main question is this: What other steps should I try to fix this problem? I'm pretty confident that if I replace the ECTS and TPS that it should fix it. However, I just want to see if I am missing anything. Thank you in advance for your expert advice.
Submitted: 5 years ago.
Category: Saturn
Expert:  Goodwrench9124 replied 5 years ago.

Hello, and welocme. you could be thinking right on the tps and ects. but before I would replace these parts, i would do two things first. even know you have cleaned the Iac, it could still be the problem. the iac has a electric motor that runs a pintle in and out. if there is a problem with this motor it will cause a high idle. also there are 4 wires to the iac, two of then are simple power and ground that runs the iac back and forth, there are two other wires that are feedback circuits to the pcm, so the pcm will know what position the iac motor is in. the pcm needs to know where the iac pintle psoition is, to know whether to command a lower or higher idle speed.

 

the two things I would check is the resistances at the iac motor, with the connector un plugged. here is the spec.

IAC terminal resistance:

A to B: 40-80 ohms
C to D: 40-80 ohms
All other combinations should be open.

 

if this checks out good, check the 4 wires between the iac motor and the pcm for being open or high resistance. if all checks ok, go ahead with your plan with the tps and ects.

 

 

graphic

 

 

The idle speed on DOHC and SOHC 1.9L engines is controlled by the PCM via an idle air control (IAC) valve. The IAC valve consists of a 2 coil stepper motor that moves a pintle valve in and out from the seat controlling the amount of air bypassing the throttle plate. The powertrain control module (PCM) controls the IAC valve by supplying ignition voltage or ground to any of the four circuits of coil A or coil B. When one circuit of a coil is supplied ignition voltage, the other side is supplied ground. Coil A and coil B are independent of each other, however both must be functional in order for the valve to move correctly. The IAC position is measured in counts, which can be read on the scan tool. When the engine is turned OFF, the IAC is retracted 80-100 counts allowing for maximum air during a restart. After the engine is started, the motor position command is based on ECT and throttle position (TP) sensor values. DTC P0507 sets when the actual engine speed is 200 RPM or more above the desired idle speed.

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