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BMW M3 E30 ICV Troubleshooting

ICV valve is a valve which controls the idle speed. It is used to increase the idle during warm-up and for example when AC is turned on. Also at the end of the article is a quick, easy and cheap way to avoid costly DME repairs.

ICV=Idle Control Valve, which controls the amount of extra air given to the engine when for example AC compressor is activated to raise the idle.

One of the M3 E30 typical faults is the Idle control system. System consists the valve which is controlled by Motronic ECU.

 

 


  

 

+12V supply voltage comes via relay to the 3-pole DC motor, which controls the valve. ECU contains following parts:

 

 

 

DC motor pins 33 and 34 are controlled by a pair of NPN Darlington TO-220 packaged transistors. CPU gives out a PWM (Pulse Width Modulated) pulses. When ADV11 is high, T560 will start conducting and gives ground to the motor which will turn. At the same time T561 will go into high impedance state. When ADV11 is low T561 is conducting and now motor turns to other direction. The pulse length is changed until target RPM is reached. If you notice, there is no current limiting for the current flowing thru the transistors!   

Typical ICV symptoms are following:

  • Low idle. Idle valve is closed, no air passes thru

  • Idle RPM is high, around 2000. Idle valve is open and air passes thru it.

  • Idle can be around 1200 RPM if your TPS switch is broken or not adjusted correctly.

Troubleshooting the system: 

  • Turn ignition key to position

  • Touch the ICV, which is located under intake plenum.

When valve is working, it should be vibrating and buzzing.  If valve is not vibrating, try with a known working ICV. It is safe to do this. If replacement valve vibrates, ICV valve is broken.

DO NOT TRY YOUR OLD ICV WITH ANOTHER ECU!

If ICV doesn't vibrate, most likely T560 and T561 are burnt and ICV valve is broken as well.  DO NOT REPLACE THE ECU IF YOUR ICV IS BROKEN. NEW ECU MIGHT BE DESTROYED AS WELL! 

But, why the transistors were burnt? Following picture shows why:

If the exterior body or the axle inside rotates a little bit, brushes will short circuit the commutator segments. Now instead of 20ohm load, current will flow directly via main relay battery 12V to transistors and to the ground. And since there is no current limiting for the transistors, smoke is the end result. When ignition is now turned on, T560/T561 will start to conduct and 12V goes directly to ground instead of DC motor coils, destroying the transistors or the ECU circuit board. 

How to test if ICV is OK? 

Remove the ICV. Measure the resistance between ICV pins. Resistances should be as follows:

  • 1-2: 20 ohms

  • 2-3: 20 ohms

  • 1-3: 40 ohms

It is commonly thought that if the resistances are as above, ICV is OK. This is not correct; you need to do one additional test:

Measure resistance between pins 1 and 3 while you are turning the valve slowly.

If at any point measured resistance is 0ohms, ICV is broken and brushes are positioned as shown in picture above.  

Transistors: Original ON588 transistors are not available anymore. Some say BDX53C is the perfect replacement but that didn’t work in my case. I used TIP31C transistors.    

How to protect your ECU from broken ICV? 

 

 

What I did is I added a small fuse into +12V wire which goes to ICV Valve. Now if ICV breaks, the fuse blows saving the transistors and my ECU. Fuse is a small 3A fuse, smallest small size automotive fuse Radio Shack had. 1A would be sufficient. Fuse is installed inside the ICV Valve connector boot as shown in this picture:

 

 

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