# Frequency valve, what does it do?



## vdubsir (Dec 8, 2009)

83 GTI 8v 2L ABA/1.8 hybrid, 272 cam
Can anyone tell me exactly what the frequency valve does as it pertains to fuel flow or pressure? 
I know that it changes duty cycle dependent on the 02, WOT, etc. but is it just an emission thing? 
My freq valve is not making the common buzzing noise and I'm wondering if it's actually effecting my motor enuf to cause problems? 
Also looking at the wiring dia I have, it shows a Oxy sensor control unit, where the F is it located? I finally found my O2 relay and things seem like they're doing what they're supposed to. 
Any help would be awesome!


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## ABA Scirocco (May 30, 2001)

*Re: Frequency valve, what does it do? (vdubsir)*

The frequency is used to modify the mixture, you can think of it as fine tuning the control pressure.
If your frequency valve isn't buzzing, your mixture with a warm engine with be WAY out of whack, plenty enough to affect drivaability. A while back I had problems with my Oxy sensor control unit, it turned out to be a bad ground for the ecu, in my case, it was one of the ground wires connected to the engine at the cold start valve.
I'm not sure where they put the Oxy sensor control unit on the 83 GTi but, on Sciroccos, it's under the rain tray on the left side, marked in red on this pic.


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## vdubsir (Dec 8, 2009)

what am I looking for for a control module? the only thing i have up near there is my ign control module. 
So what your saying is that this will change the control pressure depending on the O2 reading/WOT/temp sensor?


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## ABA Scirocco (May 30, 2001)

*Re: (vdubsir)*


_Quote, originally posted by *vdubsir* »_what am I looking for for a control module? the only thing i have up near there is my ign control module. 

I thought maybe the Sciroccos were different, that seems to confirm it. Maybe somebody with a Rabbit, can help out with that.

_Quote, originally posted by *vdubsir* »_So what your saying is that this will change the control pressure depending on the O2 reading/WOT/temp sensor? 

That's not strictly 100% accurate but yes, you think in those terms.


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## 84_GLI_coupe (Apr 3, 2001)

*Re: (ABA Scirocco)*

It's under your dash between the glove box and the passenger-side wall.


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## vdubsir (Dec 8, 2009)

thanks, I'll check it out. anything in particular that makes the freq valve stop doing its thing? or just the relay? Anyone know what pins to jumper to override the relay?


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## ABA Scirocco (May 30, 2001)

*Re: (vdubsir)*

Pull out the relay, look at the bottom, they'll typically have the pins labeled as in this picture. 








Identify the slots in the relay socket that correspond to these pins, now, here's a wiring diagram for this type relay. This numbering scheme is used on all Bosch style relays. So from this you can see that in order to bypass the relay, you need to make a connection from the slot corresponding to pin 30 to slot corresponding to pin 87.


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## teutoned (Jul 29, 2008)

*Re: (ABA Scirocco)*

in


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## 84_GLI_coupe (Apr 3, 2001)

*Re: (teutoned)*

Check for power at the frequency valve, the control unit triggers ground. Use jumper wires on the valve to check to make sure it's responsive to 12V and ground, polarity doesn't matter if I remember.
edit: Also, what are you doing for an exhaust manifold or header? At first I was using a stock single-wire O2 sensor located at the collector of a 4-1 header, and anywhere but full power it just wasn't getting hot enough to read properly. I replaced it with a 4-wire (heated and grounded) and my problem went away instantly.


_Modified by 84_GLI_coupe at 3:19 PM 2-12-2010_


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## vdubsir (Dec 8, 2009)

So if its run via ground signal and it doesn't have 12v and the wiring is all clean does that mean my control unit is junk? can I just supply 12v to the freq valve?
I have a aba mani to a tt downpipe. the bung is halfway down the pipe, haven't checked the O2 yet, I would imagine it could use a new one (looks to be original one from 83)


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## 84_GLI_coupe (Apr 3, 2001)

*Re: (vdubsir)*

The frequency valve gets its power from pin 87b on the relay that feeds the control unit. The relay isn't your typical Bosch 4/5 pin relay, since it does not have the '87a' inverted output. Pin 87 feeds the control unit, 87b feeds the frequency valve, and the relay is triggered by the output of the fuel pump relay (before the fuel pump fuse).


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## vdubsir (Dec 8, 2009)

according to my wiring dia both wires to the freq valve come from the control unit??? you're saying the power to the freq valve actually comes from the relay? if this is true that might be why its not working (i have the relay pins jumpered.) also my relay is a non bosch one and the pin numbers don't correspond with the bosch ie I dont have a 87b, which position is 87b?


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## 84_GLI_coupe (Apr 3, 2001)

*Re: (vdubsir)*

American-built Rabbits have crap wiring/electrical that doesn't match anything German made. I'll have to dig for my MK1 Bentley manual, I was looking at the Cabriolet manual which is more accurate for stock wiring for MK1 GLIs (MK1 Bentley has nothing). 87b is the same as the normal output of the relay, not inverted. The control unit controls the frequency valve by triggering the ground, while the valve gets full ignition power.
If I find my manual I'll check what it says for the GTI. If it's anything like the other models that use that fuel system, the frequency valve gets power from a different pin on the relay, so you might not be powering it with your jumper.


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## Racer57 (Feb 18, 2010)

*Re: (teutoned)*

Is that the actual relay out of your car? Was it recently changed? I just went through this on my car. The other poster is correct that you should have a relay with 87 and 87B. The correct relay provides power to both the OXS part of the ECM and the frequency valve. That relay gets power at the #30 terminal from the fuel pump relay. If you jumper the fuel pump relay, you should hear the frequency valve buzzing in the airbox. If you suspect that relay is in fact the wrong one, jump from #30 to 87 and 87a plug into the fuse box and listen for buzzing. I noticed mine went bad when I went to check the dwell at the lambda test plug, it was dead and no buzzing heard.


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## vdubsir (Dec 8, 2009)

that wasn't my pic or anything, and doesn't look like my relay, mine has a aftermarket relay in it. I jumped power to 87a and no buzz, 30 gets power directly from the fuse box (not from fp relay(all looks factory)) do I have the wrong relay maybe? It has like 3 red wires coming into it(rd, rd/blk, rd/wht), a large red supplying power, and a brown wire(gnd). 
If this isn't the correct relay (looking at the wiring dia it looks correct) which one is it then!?!
also since I put power to the freq valve and no buzzing does that mean its bad?


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## mk1 coupe (Feb 28, 2010)

*Re: (vdubsir)*

i have an 83 jetta any my frequency valve is going whacko. it all started when i put a new o2 sensor in. i have a 4 into one header . and the thing buzzes and when it cuts out it makes the car idle whako. did a few tests the bentley told me to do. and i think its a defective valve. anyway to double check the valve


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## ODIE1234 (Jun 22, 2006)

*Re: (84_GLI_coupe)*

where would you hook up the 2nd o2 wire if you put in a heated one?


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## BMAN (Jun 9, 1999)

I need a little help on my 84 GTI. I'm not getting any power to the frequency connector and the engine won't rev. Here's a picture of my fuse panel could someone tell me what relay controls the frequency valve? Thanks I know its an old thread


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## BMAN (Jun 9, 1999)

Thanks it turned out to be a bad relay (white one) it had a date stamp of 5/1983 so its time for a new one. It seems the stock relay 431 951 253 C has been superseded to 7M0951253. Napa Auto Parts seems to have them along with a new relay connector. Mine seems a little over heated with age.


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