1985 LTD FI Idle Air Control System

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Well-known member
Dec 25, 2015
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Victoria, BC,
I have done some threads on the electrical aspect of my 1985 LTD FI model motorcycle. Have learned a lot from working on it and perusing the various forums and threads. I mentioned in one of the threads that I would be putting together a thread regarding the Idle Air Control (IAC) system on my '85 LTD. This information is also pertinent to the '86 SE-i FI motorcycles as well.

The IAC system is comprised of an IAC control valve, air hose, and two reed valves. These items are connected at various locations of the air chamber and the throttle bodies.

IAC 2.jpg

Air Chamber:
Air Chamber Underside 2.jpg

The IAC system provides additional combustion air on engine start, and during normal operation when the throttle plate(s) are closed during motorcycle deceleration.

The IAC system is used to provide additional air to the engine cylinders during a cold start because the ECU controls the idle of the engine. On start, the ECU causes the engine RPM to be some 300 to 500 RPM higher than idle and as the engine warms up the engine speed is reduced to the set idle RPM. To ensure there is sufficient combustion air, there is this IAC system.

The IAC valve air supply comes from the air chamber via an air hose connected to the underside of this air chamber. The next part(s) in this system are the reed valves, one for each cylinder bank. The air flows from the IAC valve via hoses to the reed valves.

The reed valve is only a one way valve activated with cylinder vacuum – for the electrical/electronic people in the crowd, something similar to a diode. The cylinder vacuum causes the reed valve to open admitting air into the combustion cylinder through the air intake pipe – when there is no vacuum the reed valve closes stopping the flow of air into the cylinder. This occurs continuously during engine operation. The amount of air is directly related to the number of vacuum pulses.

Have to revisit the IAC valve at this point. The IAC valve is a bimetallic controlled valve. Inside the IAC valve is an air restriction plate that is never fully closed. A bimetallic strip is used to open this restriction plate fully so that max air flow can be supplied to the engine cylinder through the reed valves. The bimetallic strip has an electrical supply of 12 VDC that causes the bimetallic strip to push up on the restriction plate opening it so that max air will flow into the system.

IAC VV internals:
IAC 1.jpg

IAC VV air opening when cold. You can see light through the opening:
IAC VV air opening when cold.jpg

IAC VV when warm. Notice the larger opening to admit more air to the IAC system:
IAC VV air opening when warm.jpg

This valve also has a engine water connections so that the engine coolant will flow through the valve to prevent this valve from “freezing” up during cold weather operations. This is not really relevant to us as most of our riding is during the better weather months and those of us that choose to ride during the winter months live in areas that do not really get that cold.

The IAC valve power is connected to the fuel pump 12 VDC power supply. When power is being supplied to the fuel pump, power also goes to the IAC valve. This power is controlled through the ECU. When the ignition is turned on, there is power to the fuel pump for 2-3 seconds to prime/pressurize the fuel system The IAC valve receives power, but it is not of sufficient duration to affect the bimetallic strip. The fuel pump immediately starts when the engine starts, and continues to run until the engine is shut down. During this time, power is supplied to the IAC valve and the bimetallic strip bends causing the air restriction plate in the IAC valve to open to the full position.

I had thought that this system would be more complicated; however, the simplistic design is quite effective in achieving the aim. I would surmise that this design is based on the automotive industry design and adapted for use in the '85 and '86 GL1200 FI models.

Here is a schematic of how the air flows in the IAC system:
IAC System_0001.jpg

This explanation of the IAC system in the '85 and '86 GL1200 FI model motorcycles is my understanding based on working on the system and seeing how it is connected into the engine. I have also read a few articles on this, but was never satisfied with what I had been reading. These articles in conjunction with working on the IAC system is what I have used to do this article.

As always, it is only my opinion, and I do invite comments regarding it.

Thanks for reading. Cheers


Well-known member
Feb 1, 2015
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Euless, TX
Could it be that the voltage is used to heat the strip and once the engine coolant is hot, the coolant takes 'over' from the voltage? I have no clue but thinking of some I've worked on my memory is pushing that way.