# Frequency Valve?



## joedapper (Oct 6, 2009)

Ok, I am new to german engineering. Coming from toyota/nissan/subaru I am used to things making sense and being very bare minimum. Ive been helping a buddy of mine work on his mercedes diesel and now this 81 cabriolet and i can say with a great amount of certainty that the germans really love their engineers given the amount of craziness on what should other wise be a simple engine. With that said, what is the frequency valve? What does it do and why does it cost like $300. I've found a few things like how its used in systems with an O2 sensor but thats about it. The reason im asking is because on all 3 nissans, a toyota and a subaru, i dont think they use one. Why do the germans have to be different.


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## GeekWagens (Oct 7, 2009)

*Re: Frequency Valve? (joedapper)*

It controls fuel air ratio via the o2 sensor input.


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## joedapper (Oct 6, 2009)

*Re: Frequency Valve? (GeekWagens)*


_Quote, originally posted by *GeekWagens* »_It controls fuel air ratio via the o2 sensor input.

But isnt that what the deal in my air box that controls the CIS is for?








edit: Is it redundancy?


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## WaterWheels (Aug 14, 2005)

*Re: Frequency Valve? (joedapper)*


_Quote, originally posted by *joedapper* »_ Ok, I am new to german engineering. Coming from toyota/nissan/subaru I am used to things making sense . . . i can say with a great amount of certainty that the germans really love their engineers given the amount of craziness on what should other wise be a simple engine. . . what is the frequency valve? What does it do and why does it cost like $300. Why do the germans have to be different.

The frequency valve switches on and off XX times a second based on the output from the O2 sensor control box. It varies the control pressure that presses on the top of the fuel distributor to fine tune the mixture, an emissions gadget you might say. 
Sorry if this offends you, but you seem to be a little critical of a system and engineering that you don't even understand, no? I really doubt the Toyota's, Nissan's or Subaru's you dealt with used any form of CIS fuel injection, could be wrong but I think they were all some form of L or LH-Jetronic (or a copy of them). To be honest, CIS is a much more simple design if simple is what you like. You can fix it with spit, bubblegum and Band-Aids if something goes wrong, which is seldom. Electronic injection on the other hand can not be patched up with what's in your trunk to get you home though. Unless of course you carry around repair parts and test equipment.


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## vr2jetta (Dec 16, 2007)

*Re: Frequency Valve? (WaterWheels)*


_Quote, originally posted by *WaterWheels* »_
Sorry if this offends you, but you seem to be a little critical of a system and engineering that you don't even understand, no? 

I agree. I have owned quite a few Japanese cars like Toyota, Isuzu and Nissan and find that my VW is the easiest to repair of them all. Whether it be the fuel injection or anything else. Everybody has their preferences I guess.


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## joedapper (Oct 6, 2009)

*Re: Frequency Valve? (WaterWheels)*


_Quote, originally posted by *WaterWheels* »_
The frequency valve switches on and off XX times a second based on the output from the O2 sensor control box. It varies the control pressure that presses on the top of the fuel distributor to fine tune the mixture, an emissions gadget you might say. 
Sorry if this offends you, but you seem to be a little critical of a system and engineering that you don't even understand, no? I really doubt the Toyota's, Nissan's or Subaru's you dealt with used any form of CIS fuel injection, could be wrong but I think they were all some form of L or LH-Jetronic (or a copy of them). To be honest, CIS is a much more simple design if simple is what you like. You can fix it with spit, bubblegum and Band-Aids if something goes wrong, which is seldom. Electronic injection on the other hand can not be patched up with what's in your trunk to get you home though. Unless of course you carry around repair parts and test equipment.

Not offended in the least bit. I'm actually excited. I used to "not believe" in german engineering, when i was a strictly JDM guy. Its just learning new things and how they work. I believe you on all that repair stuff and limping home. And i may be jumping the gun a bit on the "over engineering" bit. But seriously, I pop the hood on the 4runner and can point out every part and what it does. I do this on the Cabbi and i get past the engine and transmission and a few more parts and im done, its like a mystery under there. Thank you for the detailed explanation though, i really do appreciate people taking time to explain the theory behind the parts. Thanks.


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## ABA Scirocco (May 30, 2001)

*Re: Frequency Valve? (WaterWheels)*


_Quote, originally posted by *WaterWheels* »_
The frequency valve switches on and off XX times a second based on the output from the O2 sensor control box. It varies the control pressure that presses on the top of the fuel distributor to fine tune the mixture, an emissions gadget you might say. 


That's almost right on the money,except, the frequency valve operates at a fixed frequency of approximately 70hz, it's the ratio of on time vs off time (i.e duty cycle) that varies. 
The control pressure is what determines mixture, the lower the control pressure the richer the mixture. The primary means of controlling that pressure is the control pressure regulator, the frequency valve, oxygen sensor and ecu work together to fine tune the control pressure. This isn't 100% accurate but you can thing of the frequency valve like a fuel injector but instead of supplying fuel to the engine, it bleeds away control pressure so the more time the frequency valve is open, the more pressure to bled away thus richening the mixture. In this way, the ecu, using the signal it gets from the O2 sensor can manipulate the control pressure somewhat by varying the duty cycle of the frequency valve.


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## WaterWheels (Aug 14, 2005)

*Re: Frequency Valve? (ABA Scirocco)*

Ah, come on now







If you just wanted to make a posting to go deeper into how something works or just to boost "Post" numbers that's fine with me. True I didn't go into very much detail as I didn't really see a need for it. But just how does one get this:

_Quote, originally posted by *ABA Scirocco* »_
That's almost right on the money,except, the frequency valve operates at a fixed frequency of approximately 70hz, it's the ratio of on time vs off time (i.e duty cycle) that varies. 

*from this:*

_Quote, originally posted by *WaterWheels* »_The frequency valve switches on and off XX times a second based on the output from the O2 sensor control box. 

Maybe it was not as detailed or long winded as some might like, but it was correct (on the money if you like).


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## joedapper (Oct 6, 2009)

I understood both ways, I used to work in satellites and Freqs are what they are all about.


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## DasTeknoViking (May 25, 2008)

*Re: (joedapper)*

sorry to bump this thread.... but I just found it via Google while searching for Frequency valve info.
Just picked up a 87 Golf GL with 1.8L CIS 8V under the hood.
The Frequency Valve is disconnected. When I hook it back up the RPMs sky rocket to 4K and just stay there. 
This said, it runs but misfires a little @ idle. It has no power at all too.....
What should I do ? I remember working on a few old Audis with CIS, but its been over 10+ years since I have messed with CIS. My memory is pretty bad.


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## ABA Scirocco (May 30, 2001)

*Re: (DasTeknoViking)*

What I think might be happening there is someone was having some sort of an issue with the ecu and they decided to disconnect the frequency valve then to compensate for the lack of a functioning frequency valve, they adjusted the mixture screw until the engine idled okay, now when you reconnect the frequency valve, the mixture becomes a lot richer than it needs to be. Get a long 3mm Allen wrench and with the frequency valve reconnected, try backing off the mixture screw and see what happens, keep track of how much you've turned the screw, just in case you need to undo what you've done. 
For a good description of the CIS system and instructions for adjusting it go to http://www.mikegabriel.net/vw/badhabitrabbit/


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## chickenfriend (Jan 31, 2005)

*Re: Frequency Valve? (WaterWheels)*

* But just how does one get this:
Quote, originally posted by ABA Scirocco » 
That's almost right on the money,except, the frequency valve operates at a fixed frequency of approximately 70hz, it's the ratio of on time vs off time (i.e duty cycle) that varies. 
from this:
Quote, originally posted by WaterWheels » 
The frequency valve switches on and off XX times a second based on the output from the O2 sensor control box. 
Maybe it was not as detailed or long winded as some might like, but it was correct (on the money if you like
*
---------------------------------------------------

I can see how ABA Scirocco got that, and I disagree that your statement is correct. 
If you intended to mean that "XX" is a fixed number (the frequency), then it is incorrect to suggest it is a variable, as you did in the second part of that statement.
However, if you intended to mean that "XX" was the duty cycle, then it would also be incorrect to describe the switching on and off as "times per second", since "duty cycle" is typically defined in this situation as a percentage of time on for any time span.


_Modified by chickenfriend at 9:35 PM 11-24-2009_


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## DasTeknoViking (May 25, 2008)

*Re: (ABA Scirocco)*

Thanks for the link, I've read it.... man what a simple system. 
What are the chances of the ECU being fried ? I wouldn't be too surprised...


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## ABA Scirocco (May 30, 2001)

*Re: (DasTeknoViking)*

I'd say the chances are slim, the CIS ecu is pretty robust, I'm sure people have fried them but off hand, I can't think of anybody has. When I last had an ecu problem, it turned out to be bad ground.


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