# Knock Sensor Control Unit and Rev Limit



## saporter (Sep 10, 2003)

O.K.,  Here we go again.
Anyone think they can figure out how to modify the 8V or 16V knock sensor control unit to eliminate the rev limit? I'm talking about opening the box and bridging a solder joint or something, so the ground to the fuel pump relay is always grounded when the control unit has power, i.e. ignition on.
Someone needs to try it.
Sean


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## Racer_X (Jul 14, 2002)

*Re: Knock Sensor Control Unit and Rev Limit (saporter)*

I thought the rev limit was strictly a function of the fuel pump relay. And I thought if you used the older fuel pump relay, that removed the rev limit.
I could be wrong on this, but that's the way I thought it worked.


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## diablerouge (Mar 30, 2004)

*Re: Knock Sensor Control Unit and Rev Limit (Racer_X)*

rev limit on cis-e is definitly NOT a function of the fuel pump relay. i have a 4 lead relay on my fuel pump so i can switch it when i want, and i still hit the rev limiter pretty bad at the track. i have a 16v ecu here i might try hooking up just for the hell of it after i get some injectors that actually work.


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## 87 VW GTI (Aug 15, 2003)

I'm pretty sure it cuts ignition. By the time the pressure went down the in the fuel system, you'd be WAY past rev limiter.


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## diablerouge (Mar 30, 2004)

*Re: (87 VW GTI)*

i am almost certain that it cuts ignition. the only way it would be able to limit fuel would be to change the dpr current. it would be much simpler to cut spark or back out the timing, which is what it seems to do (retard ignition). cutting fuel is also not in the interest of keeping cars from blowing motors, which is exactly what the rev limiter is there for.


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## saporter (Sep 10, 2003)

*Re: (diablerouge)*

Well, here is the way I'm looking at it. 
The knock sensor control unit has an input for RPM from the Hall sendor. There must be a way to trick the knock sensor control unit to ignore the RPM input above 6K.
What about removing the RPM input to the knock box all together?
Sean


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## Racer_X (Jul 14, 2002)

*Re: (saporter)*

The signal from the hall sender to the knock sensor controller is what ultimately fires the coil and makes the spark plug spark. If you cut it off, you will have no spark at all. Also, the knock sensor controller provides all ignition advance functions. There is no mechanical advance or vacuum advance on the dizzy in the knock sensor system.
I'm in my A2 Bentley now, and it does appear that the knock sensor controller has a rev limiter in it. HT engines are limited to 6200-6400 RPM's. RD engines are limited to 6570-6630 RPM's. I'm guessing that if you have the HT engine and cut the wire in the harness to use the RD advance curves, you'll probably get the RD limit as well. 
The PL (16V) engine has a limit of 7200-7300 RPM's, but the entire knock sensor circuit looks identical to the RD. I wonder if there's a jumper or switch inside the KS box that is set differently on the PL engine, or if the entire controller is different. It looks like the PL engine KS box would be a direct swap into the RD harness.


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## saporter (Sep 10, 2003)

*Re: (Racer_X)*

Right, that's what I'm getting at. Maybe we could open a box, find the internal rev limiter and cut it. Just un-solder the joint. I don't know if it is a chip or a resistor, or what though?
Sean


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## Racer_X (Jul 14, 2002)

*Re: (saporter)*

The knock sensor controller is a rather primitive embedded microprocessor with some analog-digital and digital-analog circuits on it. It's not really easy to figure out how it works or to change the way it operates. 
Back about 10-12 years ago, when I was racing an A2 GTI in SCCA Improved Touring, we took a look at modifying / tuning the knock sensor system. We never could figure out how the thing worked and I eventually just installed a complete MSD ignition system. It's probably not worth the time and effort to figure out the stock system when a highly tuneable aftermarket system is available at a fairly reasonable price.


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## ABA Scirocco (May 30, 2001)

*Re: (Racer_X)*


_Quote, originally posted by *87 VW GTI* »_I'm pretty sure it cuts ignition. By the time the pressure went down the in the fuel system, you'd be WAY past rev limiter. 

My car is equiped with a fuel cut-out limiter only. Take my word for it, cutting the fuel pump has an immediate and dramatic effect. It's a very harsh and crude method but it's definitely a very effective rev limiter.
There is a discussion about the rev limiter in this thread http://forums.vwvortex.com/zerothread?id=1217606 some parts of that discussion apply here too. 
I have a knock sensor control unit from an 86 Jetta Carat in my 86 Scirocco. My Scirocco still uses it's original CIS-Lambda fuel system and the original type of fuel pump relay. In my case, there is a rev limiter built into the fuel pump relay but this was NOT the case in the 86 Jetta Carat, GLi or Golf GTi, in those cases, the fuel pump relay is grounded through the knock sensor control unit, i.e. the rev limiter in knock box is, in part or in whole, a fuel cut-out. 
I have heard other people say that there is also a ignition cut-out limiter built into the knock box as well. My experience with the knock box leads me to believe this is NOT the case because, when I install the 059F FP relay mentioned in that other thread, my rev limiter disappears ENTIRELY, I can rev past 8000 rpm without a single miss. 
So in conclusion, *IF* I'm right and the only rev limiter is the fuel cut-out then, the solution put forward by der_panzer in that other thread should eliminate your rev limiter.



_Modified by ABA Scirocco at 9:06 PM 7-2-2004_


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## saporter (Sep 10, 2003)

*Re: (ABA Scirocco)*

I'm going to open-up an extra CIS-E knock box I have laying around. I'll try to figure out a way to keep the ground to the fuel pump relay active whenever the knock box has power from the ignition.
Pin #10 on the box is the ground to the fuel pump relay. Pin #3 is the ground for the knock box. I'll try to figure out how the box cuts the ground to the fuel pump. It must be conditional, based on the RPM signal from the oxygen control unit, pin #2 on the knock box.
Sean


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## Racer_X (Jul 14, 2002)

*Re: (saporter)*

Just be very careful that you keep the zero RPM cutoff function of the fuel pump relay. If you don't, and you get in an accident, you could have a very bad fire if you break a fuel line. I've seen several really bad car-B-Q's start that way. 
And I can verify that you can use the KS box with the older fuel pump relay that gets the signal from the coil (1983-1984 Rabbit GTI fuel pump relay for example). Just leave pin 10 unconnected and connect th earlier fuel pump relay as it would normally work. That relay will do the zero RPM cutout and the earlier ones have no rev limiter. I've run KS ignition on an older A1 car with the A1 fuel pump relay and there was no rev limit in that setup.


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (saporter)*


_Quote, originally posted by *saporter* »_O.K., Here we go again.
Anyone think they can figure out how to modify the 8V or 16V knock sensor control unit to eliminate the rev limit? I'm talking about opening the box and bridging a solder joint or something, so the ground to the fuel pump relay is always grounded when the control unit has power, i.e. ignition on.
Someone needs to try it.
Sean


I have had the same thought. Reverse engineering the KS box might be difficult so I took the simpler path of just grounding the low side of the FP relay coil while I come up with something to shut the pump off at the right time to prevent feeding fuel in the event of a wreck. The way I envision this circuit is it needs to differentiate between an engine being started (leave the pump on) and one that just stopped (shut the pump off) or starting could be problematic - or slow - like it can be with the stock CIS-E system. Having a user settable high RPM cutoff would be nice too. From what I have read about the 5 pin FP relay, it might be a ready made solution just needing some tweeking to set the cutoff.
By the way the CIS-E in my '86 GTI uses the 4 pin FP relay, and the KS controller does shut it down but not the ignition. 
One nice thing I have observed about grounding the FP relay coil is the car starts much more quickly. 


_Modified by wclark at 3:12 PM 7-4-2004_


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## ABA Scirocco (May 30, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (wclark)*

Here's a description of an old style CIS fuel pump relay circuitry http://www.dolphinsci.com/relay.html . This circuit could very easily be adapted to ground out the fuel pump relay and still provide the zero RPM cut-out.


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (ABA Scirocco)*


_Quote, originally posted by *ABA Scirocco* »_ Here's a description of an old style CIS fuel pump relay circuitry http://www.dolphinsci.com/relay.html . This circuit could very easily be adapted to ground out the fuel pump relay and still provide the zero RPM cut-out.

Nice find. Thanks. I ordered the F and H variants of the relay this afternoon. I plan to open them up and see which will do the job best (0 RPM and 7000 RPM fuel cutoff). The wiring to the 4 wire FP relay socket on the '86 is a little different than earlier cars that use these but the socket retains the added slots so once I decide which to use and how best to alter it, getting the right stuff on the right pins should be easy.


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (saporter)*

A friend passed me a used fuel pump relay that originally came from a Rabbit. Its a different part no than the 321 906 059 C listed in the article above, but a little reverse engineering showed me it has the same circuitry - a zero RPM fuel shutoff, no rev limiter. I wanted to give it a try.
As the physical socket at location 6 on my relay panel, for the fuel pump relay, is different than this one, which looks a little like the seatbelt and wiper relays, I decided to install it in whichever empty slot it would fit and to wire it so it controlled the ground to the regular fuel pump relay in place of the KS unit. As it happens in my case relay location 5 is empty and has a physical layout compatible with this old relay. I moved a couple spades around in the socket. Since I am controlling ground in this case and not powering the fuel pump directly I moved the ground spade to the added relay sockets pin 87 position. I then reused 3 of the other spades in that location by clipping off their wires about 6" back up the harness. I tapped into the coil (pin 1) and ignition wire (pin 15) going to the upshift relay socket, and tied the regular fuel pump relay control wire to the added relays pin 30 location. Internally to the added relay I tied the ground from pin 87 to the ground spade pin 31 to pick up ground for the relay itself. On this added relay pin 31 is one of those smaller spades and this saved me from moving another spade on the panel.
Works like a charm. The time constant of the internal timer of the added relay shuts off the fuel pump relay in a little under a second from when the engine is stalled. The TC is .54 seconds, the same as the one in the article - uses the same RC combo. 
I intend to mess a bit with the F and H variants of the 321 906 059 I ordered when they arrive to see what they offer.


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## ABA Scirocco (May 30, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (wclark)*

I'll be interested to see what you come up with when you reverse engineer one of the rev limited relays like the 059H. I can't see them being based on the 555 like the 059C, I suspect you find it's based on a small microcontroller maybe something like Motorola's 8 pin MC68HC908Q.


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (ABA Scirocco)*

The 321 906 059 F and H I ordered came from busdepot.com. They are manufactured by Kracker. Both use the OTP 8 bit processor from National - the COP8SAB720N. Circuit boards and components are identical except for the location of one SMT jumper. 
These must be a newer, cost reduced version of the 24.1400.40 Kracker unit shown in thread http://forums.vwvortex.com/zerothread?id=1217606. My H variant has the same Kracker part number markings but uses digital processing rather than analog timing to control things.
It looks like these newer pieces either do what you want or they dont. Not much room for tweeking here. The good news is the relay pin layout is the same as the fuel pump relay in my A2 GTI relay panel with the addition of the 5th pin for the tach input. So assuming one of these doesnt have the restrictive rev limiter (hopefully the F variant), it will be a cleaner solution to the zero RPM cutoff than the 2 relays I am using right now... To those reading this for the first time remember with the CIS-E system the FP relay socket needs 2 changes, the coil ground side needs to be grounded and not controlled by the KS unit and a small spade carrying the tach signal from the upshift relay socket needs to be added here for this to work.
Today I test them on the bench.


_Modified by wclark at 4:55 AM 7-13-2004_


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (wclark)*

I tested the 321 906 059 F and H.
Both have 0 RPM and rev limit fuel cutoff functions. The H tested on the bench at 210Hz (6300RPM) and the F tested at 273Hz (8200 RPM).


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## ABA Scirocco (May 30, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (wclark)*

8200 for the 059F huh, that would explain why I've never hit the limit, I've pushed 8000 before but just barely 8100 maybe but no higher. 
BTW I'm just curious, what does your bench test set-up consist of?


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## wclark (Mar 28, 2001)

*Re: Knock Sensor Control Unit and Rev Limit (ABA Scirocco)*

For this I used a variable DC supply and an AF squarewave generator, plus a freq counter.


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## 85vdub (Apr 9, 2003)

looks like i need the 059f then. right now my stock relay cuts out between 6400 and 6700... cuts out inbetween those r's and never in the same spot the next time. I've really like to be able to his 7k at least with this 286 cam.


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## wclark (Mar 28, 2001)

*Re: (Pred)*

From other articles and my own junk box relays it appears Kracker used to make the relay cutoffs based on analog timing circuits. At some point, probably recently they have obviously moved to a digital solution. The analog solution can have some variation based on temp, humidity, and even noise on the coil wire. A digital solution is going to be immune to most of these so I expect the processor based relays from Kracker will be pretty consistent relay to relay and car to car. 
By the way, I was pleased at the prices and delivery from busdepot.com for the ones I bought for this.


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