# What controls the frequency valve ?



## veedublvr (Feb 2, 2001)

Ive got an 88 cabriolet with a 1.8L 5speed (CIS-E) ... 
What controls the frequency valve (electronically) ?
Mine stays relatively quiet, gets louder engine speeds up the the valve gets quiet...
What controls it ?
Dan


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## ABA Scirocco (May 30, 2001)

*Re: What controls the frequency valve ? (veedublvr)*

The CIS-E systems don't use a frequency valve, they use a differential pressure regulator to replace both the frequency valve and the control pressure regulator used in the early systems. But you don't have a CIS-E system, you have CIS system with Lambda and it does use a frequency valve.
The frequency valve is controlled by the fuel ECU. The sound shouldn't really change much, the frequency is steady at approximately 70Hz, what changes is the duty cycle i.e. the length of time per cycle that the valve is open per cycle. The ECU controls the frequency valve in response to the O2 sensor signal, if the O2 sensor voltage is too high, the ECU reduces the frequency valve duty cycle and increases it if the sensor voltage is too low. IOW, the ECU increases the duty cycle to richen the mixture.
The ECU also has a couple other inputs besides the O2 sensor, there's a coolant temperature thermo-switch ( Bentley calls it the oxy-thermo switch) it tells the ECU when the engine and O2 sensor are cold and this causes the ECU to ignore the O2 sensor reading and sets the frequency valve duty cycle at a pre-programmed level 60% to 80% depending on year, model etc. (off hand, I don't know what percentage yours would be).
There's also a full throttle enrichment switch (a.k.a. WOT switch). It's mounted on the throttle body and is actived only at wide open throttle, when activated the ECU sets the duty cycle to 60% regardless of the O2 sensor signal. Not all CIS model have a WOT switch. If yours has one, it's particularly important you make sure you system is in proper adjustment, make sure the warm idle duty cycle is at about 50%.
Check out the CIS links at http://www.badhabitrabbit.com for info on tuning CIS-Lambda systems.



_Modified by ABA Scirocco at 2:07 AM 6-21-2003_


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## veedublvr (Feb 2, 2001)

*Re: What controls the frequency valve ? (ABA Scirocco)*

What would cause the idle to surge then, bad coolant temp sensor ? Bad Ecu ? The thing is crackers, you can hear the frequency valve go from non existent to loud while the idle is bouncing...
Been a great help already , thanks
Dan


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## ABA Scirocco (May 30, 2001)

*Re: What controls the frequency valve ? (veedublvr)*

There are a few things that could cause that, check for vacuum leaks first, CIS engines are very sensistive to vacuum leaks, they can cause a variety of peculiar symptoms after that, put a dwell meter on the fuel system as described in the link I gave eariler, that'll tell you what exactly the frequency valve is actually doing and if it's not behaving erratically, use the dwell meter and a 3mm Allen wrench to set the mixture properly. If it is behaving erratically, look for the cause, most likely a flakey O2 sensor or a bad ground connection to the ECU. A bad ground has happened to me, it caused an intermittant failure of the ECU and in turn the frequency valve, my symptoms were somewhat similar to yours only more severe, the car was barely driveable it took a lot of head scratching before I figured that one out.
There is not coolant temperature sensor in the fuel system it's a temperature sensistive switch that turns on or off at a specific temperature, it one of the things that controls the warm-up behavior of the fuel system. There are also a few other things that help control the idle speed, cold start and hot start behavior as well. 
My advice to you is first get a Bentley manual if you don't already have one, it's very difficult to diagnose a problem without a good manual and for VW's, Bentley's the best. Then do a complete tune up. If your problems still persist, that's the time to dive into the Bentley methodically eliminating one thing at a time until you find the source of the problem.
Good Luck.


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## veedublvr (Feb 2, 2001)

*Re: What controls the frequency valve ? (veedublvr)*

Well, so far we have replaced the following
Exhaust manifold gasket
valve cover gasket
fuel injectors
injector seals
vacuum lines
vacuum canister
spark plugs
plug wires
cap
rotor
fuel pump (s) 
fuel pump relay
cold start pulse relay
alternator
air cleaner
fuel filter
fan switch
all of that was to find out that the original cold start pulse relay was installed incorrectly and that for the three years we owned the car the cold start valve was open and dumping fuel in... 
I think Ill try the O2 next
that leaves me with the ecu, cpr, freq valve, o2, 


_Modified by veedublvr at 1:04 AM 6-21-2003_


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## ABA Scirocco (May 30, 2001)

*Re: What controls the frequency valve ? (veedublvr)*

Just a hunch but, check the ground wires that attach to the intake mainfold at the cold start valve, remove then, clean the connections and re-attach them. One of these wires is the ground for the ECU, that was the cause of the problems I had. Also there are a few ground wires that connect to the engine at the corner of the valve cover, check these out too. Bad ground can cause a host of wierd, intermittant and difficult to diagnose problems. I've run a new ground wire directly from the battery to both of these locations just to be on the safe side.


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## ThatRuinedMk1 (Jan 15, 2017)

*getting a weird duty cycle reading*

doing this test on my '88 with CIS Lambda and I'm getting pretty much a constant reading of 67.5% at idle. According to the Bentley, this is around what the "ECU" defaults to when at WOT. The reading does not change when I adjust the mixture screw OR when I disconnect the O2 sensor, it does not default to 50% which based off of my understand is what it's supposed to be doing. So i think either my O2 sensor is bad, or something in the ECU is bad and is telling the frequency valve to stay in a WOT condition of 67.5% duty cycle. Im not sure if this is accurate or not and Im afraid of running my freshly rebuilt engine lean and cooking my valve seats again. Any input helps. Is there another way I could double check my AFR???


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## Butcher (Jan 31, 2001)

It's probably something to do with the temperature switch. Spend a little time with the wiring diagram and make certain all the wires are doing their thing. No reason to just throw parts at it until the problem is resolved.


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