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Old 10-28-2000, 08:17 AM
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In my last message I stated OVR, I meant OVP but what the heck. Where is the frequency valve located on this 380? How could this be checked? I know that this problem is some little simple thing!! Thanks for your help. If I was a little closer to Gainsville, I would bring it in for Steve to fix!!! I am about 300 miles North of Him.
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Old 10-28-2000, 09:27 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Well if you are 300 miles north of me, you should be in Donnie's back yard.

The frequency valve sits to the drivers side of the airflow meter and looks sort of like a electric injector. For your purposes all it needs to do is buzz while the engine is running properly. Its function is as a controlled leak to the lower fuel dist chamber pressure. It does this by varying the pulse width of the open close cycle. This is called the duty cycle.

For those into trivia here is an explanation of duty cycle (would be easier with pictures of a scope pattern). The valve is opened and closed an exact number of times a second (constant frequency). Say that the frequency is 10 (for demo purposes). That means that one cycle takes one tenth of one second. A 50% duty cycle would have the valve open one twentieth of a second (half) and closed one twentieth of a second. A twenty percent ON duty cycle would have the valve open 2 one hundredths of a second and be closed 8 one hundredths.

This difference in the amount the valve is opened varies the lower chamber pressure and alters mixture. This ability is used by the electronic controls to do feedback controlled closed loop mixture control. The O2 sensor in the exhaust measures the result of the original hydraulic mixture controls and minutely adjusts rich or lean (using variable duty-cycle)the mixture. It does this until the result as measured at the O2 sensor is changed in the opposite direction. In other words if the readings are first too rich the duty cycle is reduced the pressure drop is lessened the car goes lean until the O2 sensor sees this and opens up the duty cycle causing pressure to drop ..... back and forth ... this is called closed lop control.

For most power loss problems the only need is to determine that the valve is buzzing. If you have a VOM that measure duty cycle all of the testing and calibration of this system can be measured.

Because equal abilities to go rich or lean are important for system operation, a properly adjusted mixture will have the duty cycle at 50%. A car running at zero or 100% will be pegged too rich or lean for control to work.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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