When it comes to GM flow charts for diagnosing idle speed control problems or a Code 35, factory charts vary according to model based on the diagnostic programs built into the ECM. For instance,some ECMs contain software to work in conjunction with a bi-directional scan tool. This allows the technician to use a scan tool for extending or retracting the IAC, as opposed to using a hand-held IAC driver tool.

The problem that I have with these charts is the idea of using a node light as a piece of test equipment. These node lights contain bi-color LEDs. Typically the threshold between red and green on a bi-color LED is above or below 1.5 volts. With the IAC circuits being a 0-12v toggle, there is a lot of room for error that an LED would not show. Also, the circuit is pulsed from 1-40hz. Can the human eye see missing pulses at 40hz? I don't think so.

The GM chart is a good chart, but I recommend using a DSO or GMM in place of a node light to perform these tests. With a DSO/GMM, you could see missing pulses or amplitude and frequency problems.

Quick test procedures

Field Service Mode (KOEO)
With the vehicle in this mode, the ECM is commanding the IAC motor to seat for a minimum idle speed check. This is done by pulsing each IAC circuit at a rate of 40 hz/ 50 percent dc.

After initiating this mode, set DSO to dual trace and back probe each pair of IAC driver circuits at the IAC. Look for a good square wave with a consistent duty cycle and frequency. Note: There are a few models that may not pulse the IAC circuits in Field Service Mode.

If this is encountered, move on to engine running tests below. Snap Throttle (KOER)
Set up DSO for dual trace. Connect to one pair of the IAC coils (high/low). With the engine running, snapping the throttle will cause the ECM to rapidly pulse the IAC to pull it open.

Capture this waveform and review. Perform this on both pairs of drivers.

Hint: Setting the DSO's trigger to single shot makes easier work of capturing the signals.

After duplicating the problem, I checked the basic systems such as fuel, ignition output, vacuum and DTCs. All preliminary test results were good. No DTCs!

With the problem only occurring on closed throttle, I decided to perform an IAC test in Field Service Mode. That's when I found the problem - defective ECM! The test revealed that the ECM had lost control of coil A on the IAC.

While recognizing the complaint as an idle circuit problem, I decided to check the health of the AIS driver circuits. Using Actuator Test Mode in my scan tool, I initiated the AIS test (key on-engine off). What this does is pulse each IAC coil 3 seconds on and 3 seconds off to allow the technician to check the health of the circuit. By backprobing each coil with a DSO, the pulses are very similar to GM in Field Service.

The problem is identified to be a defective Idle Air Control. After replacement and rechecking the car, the problem was cured. I did expect that the computer would be damaged from this, but to much surprise, it was not! This was a definite bonus.

I hope that these procedures shed a little more light on idle control problems. There are many more cars that I'm doing research on and I'll keep you updated. Until then, practice these examples and I think that you'll find them very useful.

Case Study I
1990 Oldsmobile Delta 88 - 3800 V6 Automatic Transmission
Complaint: Car intermittently dies on decel. Sometimes will not idle.

I always recommend a new IAC with the ECM on these problems. An intermittently shorted IAC could take out the new ECM.

Case Study II
1988 Plymouth Voyager - 2.5 4 Cylinder Standard Transmission
Complaint: Car will not idle cold. Intermittent high idle at stops.

Mike Sauer is employed by Linder Technical Services and works with the company's diagnostic hotline. Sauer also runs a dyno-equipped test lane to tackle driveability problems for area shops, which allows him to stay up to date on today's complex vehicles.

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