Brief background, I am experiencing failures of knock sensors in my Vette, sometimes 3 a season, at first they flag up a 43 when I have been driving for a long time and then progressively it gets worse to the point that as soon as the engine reaches running temp the sensor goes down.

My Vette is a 1990, 6 speed, I am running a 383, full internal balance, 195 cc heads, Keith Black pistons, Eagle rods and lightweight crank, Fidansa flywheel, Scorpion 1.6rr, Comp Cams XFI 280 cam, Miniram, full length hedders, Rt cats, Corsa system.

I am just about to order yet another KS so any input as to the failures that I am having would be great.

TIA
Adrian
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Are you torquing them in at the correct specs? That could partially explain their lack of longevity. Also, check your sensor connections. Maybe a wire is burned or frayed somewhere.

 

Knock sensors are pretty bulletproof. Maybe the ecm is picking up excessive engine noise from your mods and setting a code.

 

Hi and thanks for the quick reply.

Yup 15ft lb, for the torque, wiring is all good ( the car has only done 30k miles )

 

Thanks for the quick reply.

Well I have a pile of broken ones here . When I fit a new one it will be fine for about 3 to 4 months and then the it will gradually get worse, and it doesn't show false knock counts when it starts to log a code either....

Adrian

 

I wrapped the threads on mine with Teflon tape in hopes of reducing their sensitivity some. I also have the LT4 module in the PCM. Don't know if a '90 has one there or not though.

 

Hi there, its not a problem with sensetivity that I have, when the sensor is good it is working 100% , the problem is that I am having 2 to 3 sensors fail a year for no apparent reason...

The 7727 ECM in the 90 / 91 uses a memcal which has the knock module built in...

Adrian

 

I see, maybe you could find a used one that would last a little longer. How do you know one has gone bad? Timing? Knock retard?

 

It will flag ESC code and pull timing in that order, while the sensor is failing there are no knock counts , it is failing as if the KS is open circuit, as soon as you change the sensor for another one all is well.

Adrian

 

Adrian,

You've certainly got a strange problem here. Most problems are either consistently there (if it's not a sensor problem), or it goes away permanently when the sensor is replaced.
It's been a while since I worked on something of that vintage, but a couple things come to mind. The harness connector to the knock sensor is reportedly only designed for about three insertions (after all, the knock sensor rarely needs to be removed). IIRC, it has a soft coating on it to better conform to the sensor lead, and offer some oxidation protection. The electrical integrity of this connection sometimes is compromised after a few removals and installations. It's anybody's guess on the "TTF" between when you plug it back in, and when the connection might go intermittent. Just something to keep in mind.
I don't remember, does this model year use a 100kOhm internal sensor or the later 4 kOhm sensor? If engine vibration or rough handling is damaging the resistor conection, the ECM could read this as an open sensor connection. What reading does an ohmeter show on a new sensor? If your system uses the 100 kOhm sensor, then a broken resistor isn't a big deal, as the 100 kOhm was only needed to bleed off voltage if the sensor was getting bounced around during shipping. However, if your model year uses the 4 kOhm sensor, then resistor integrity is mandatory. This resistor is one half of a pair that sets up the ESC input impedance, and is also used for detecting continuity via an A/D voltage reading.
I had a friend who raced (W2W) in SCCA several years ago, and he mentioned that he was frequently getting a sensor code in the middle of the race, and asked what he could do about it. I told him he could install one of the older 100 kOhm sensors (of the same 5200 Hz frequency), and then put a parallel 4 kOhm sensor in the wiring harness. The ECM wouldn't know the difference, and he wouldn't have the code issues occuring during the race anymore.
If you can refresh my memory on what sensor you're using, we can refine our plan of action a bit better.

 

Hi there, Many thanks for getting back to me.

Mine is the 4kohm sensor ( black top ) so the ECM needs to see the resistance to know that there is a sensor there, as it is the later 7727 ECM with the KM on the memcal, as for the connector I have changed it recently, IIRC I put a new plug on with the last sensor.

I did wonder if I could cheat by putting an earlier sensor in and add a 4kohm resistor into the loom

Many thanks
Adrian
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Do you still have any of the old (removed) sensors collected somewhere where you could put an ohmmeter on them, and see if the internal resistor connection opened up?

 

I did check one last time one went down and it was fine when it was cold, but I checked the one that is on the car when it was hot, (and throwing a code) and it was OC ....

Adrian

 

Well I have found a few of the old ones and one of them is OC but it still generates an AC signal if I tap the sensor with a screwdriver handle, so I am thinking that I might try that one in the car with a resistor in the loom ( as it is completely O/C )

 

Bump :d

 

Did you check the Knock Sensor Module? It's in the bottom of the PCM depending on the year.I have one for a 95' LT1 if you need it.

 

Hi there, mine is the 7727 so the module is on the memcal, so thanks for the offer .

Adrian