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Barney Eaton

knock sensor

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Has anyone had a problem with their "knock sensor"?

My 1991 has fluctuiating gas mileage and rough idle, I ran diagnostics this week and the spec on the knock signal (ED17) is 0-255, mine reads 1200+. I suspect the sensor is telling the ECM that there is a knock and the ECM is adjusting timing, causing the mileage problem.

Because of the heat, all I have done is locate the sensor. I will start by checking the connection. To my knowledge, I don't remember anyone having a known problem with this sensor. I also would think it is a common part and the salvage yard would have a huge supply.

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Does the knock signal keep increasing while the engine is running or is that the number that appears when you enter diagnostics? It is normal for the knock counts to increase just from starting the engine and sometimes when the engine is under heavy load such as at an upshift. I have found the best mileage when the engine is running close to the knock threshold. I would watch ED16, knock retard, to see if you are actually getting a reduction in spark timing. It should not trigger retard for a few random knocks in normal operation.

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The reading was at 55-60 mph on the road. I have not gone back and checked it at idle. Sorry for the lack of info but it is hot here and will be until the middle of Sept. Just looking to see if anyone had experience/suggestions.

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I had to replace my knock sensor just a couple of months ago.

I kaet getting readings of 999. The when the engine was shut off it would reset to 0.

The readings made no sense as when I was really standing on it the bumbers would not go up.

And when I was driving normal they would not go up. I never did figure out when they increased.

I just changed it and the numbers went back to normal.

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There are always a bunch on startup and may be a few on initial acceleration such as from a stop but should not increase at idle or cruise.

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I have not had a knock sensor problem on my Reatta, however I did on a Mazda Millenia I used to own. On the non-supercharged Millenia V-6 the sensor is buried and requires the removal of mos of the intake system. I knew I was getting rid of the car soon and could feel a loss of power when check engine light was on due to timing at full retard. My "repair" is not recommended for a good car you want to keep but for me I went to the U-Pick and found a Millenia with the engine already dissassembled and grabbed the sensor for $12.00 and wrapped it in expanded foam, hid it under some wiring and plugged it in. Reset the OBD-2 and on regular it would not knock or ping no matter how I tried to lug the engine. Power and mileage came back and no more check engine staring at me. I did show the buyer of the car, he did not care as he was putting in the supercharged atkinson cycle engine in from his totaled Millenia-S

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It is relatively easy to get to on Reatta. Is a mushroom shaped sensor that screws in the block. It is pointing at the firewall, above and behind the oil filter.

Number of counts does not really matter, it is whether it is increasing. Can also see the retard in the timing advance value, ED08 and knock retard ED16.

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Has anyone had a problem with their "knock sensor"?

My 1991 has fluctuiating gas mileage and rough idle, I ran diagnostics this week and the spec on the knock signal (ED17) is 0-255, mine reads 1200+. I suspect the sensor is telling the ECM that there is a knock and the ECM is adjusting timing, causing the mileage problem.

Because of the heat, all I have done is locate the sensor. I will start by checking the connection. To my knowledge, I don't remember anyone having a known problem with this sensor. I also would think it is a common part and the salvage yard would have a huge supply.

I recently had the same problem with my "88 Reatta. With a rough idle I looked at the knock sensor that usually gave me readings over the range limits. The advice I was given was to wrap on the power steering support and see if the knock number jumps. I could get mine up to 1200, with the sensor always holding the highest reading. This would reset on a restart.

This showed the sensor was doing its job....but I replaced it anyway for $35 using the Reatta Store site....sensors. The end result...... it was not the problem. In my case the fuel pump was in early failure, the TPS was jumping too much and needed replacement and the ICM was just starting to overheat and cook the green. Sometimes you need to open up your ICM to see what is going on.

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This is my most-recent issue. ED 17 reads 9999. I took her for a spin, and it drops to 5400, no help. I check ED 16 and it reads a constant "17". So, it appears I'm running 17 degrees retarded timing. I can't wait to solve this, and see how nicely the car runs.

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well, I'm in trouble now ! Installed new knock sensor, and ED 17 still reads 9999 at idle. It intermitently drops into 3000 and 5000 range, and for a half a second might read 100. No code 043, so presumably CKT 496 is at its proper voltage btw. 1.5v and 3.5v. ugh, maybe its an ecm snafu

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Well, here's the fun part..... I disconnect the lead to the knock sensor, fire up the motor, and ED 17 is unaffected, it still reads 9999. So, some open wire somewhere ? but why no code 043 ?? Befuddled.....

Edited by ChrisWhewell (see edit history)

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Just checked the vert: key on engine off ED17=0

Start and idle with some revving ED17=376 and constant.

Probably some piston slap when started cold (well 90F).

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checked FSM, it is CKT 496. First test in flow chart is "ign. on" and then measure for voltage in ckt 496 which is basically disconnect connector from knock sensor and measure from there to ground. My reading was zero volts. Chart next indicates this means either 1) ckt 496 is grounded, 2) ckt 496 is open or 3) ECM is bad. Something also mentioned in FSM about a "pull up resistor" which should be present at the knock sensor terminal, YD9. I can't remember seeing a resistor there when I swapped the ecm last year. its all on page 8D1-54, 55 of my book. but my book is not the full FSM, it is the "New product information" version, but I adjudge it to be accurate.

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Just to add to the knock sensor data...

 

18 hours ago, padgett said:

Just checked the vert: key on engine off ED17=0

Start and idle with some revving ED17=376 and constant.

Probably some piston slap when started cold (well 90F).

 

I checked mine and ED17 reads 0 with the engine off and a constant 251 with engine running. I'm guessing it's not so much the number that is important but how much change the ECM detects.

 

2 hours ago, ChrisWhewell said:

Something also mentioned in FSM about a "pull up resistor" which should be present at the knock sensor terminal, YD9.

 

I read what the FSM is saying a little differently. I think it means 5 volts should be present at ECM terminal YD9 if the knock sensor is disconnected. I believe the pull-up resistor is inside the ECM. The knock sensor appears to keep the voltage the ECM sees between 3.5 and 1.5 volts during normal operation.

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Correction. With Ign On, I now read 4.9v at the knock sensor connector. Problem was, one of my voltmeter wires has an open in it, discovered that when replacing a rechargeable battery in another device, couldn't get voltmeter to read 1.5 v on a battery that was brand new. Next in the sequence is to check the connector itself, make sure it is contacting the knock sensor contact. If so, next step is to remove ECM and check memcal to make sure it is properly seated in ECM. If so, then replacement of memcal is the remedy, according to the FSM. Maybe the problem is just the connector if I am "lucky."

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not exactly the smartest connector, in my opinion. Has a male rod that goes into a hole in the knock sensor, but looks like mr. male rod of the connector might not have the girth fo make good contact with internal walls of female hole on knock sensor. Ugh. maybe I can get a piece of copper foil in there

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Installed a new knock sensor today to hopefully get better fuel mileage.  I get an average of 15mpg which is mostly city driving.  I also do not seem to be able to climb the mountains around here very easily.  My test mountain goes from 600 feet to 3,400 in just a few miles.

I will do a road test tomorrow to see how it behaves.

Just a few tips:  The sensor is located above the passenger side CV joint (see the copper colored thing in the first pic).  Use a 22mm socket with a medium extension and stubby ratchet.  If you have a "Santa" belly like mine then lift the front of the car up with a jack and use jack stands - I drove the right front tire up on a Rhino Ramp and could barely fit!  Wear safety glasses as plastic pieces of the connector kept breaking off and hitting my right eye (I wear glasses) when I tried to remove it - when I got the old sensor fully removed I got some drops of oil on my right eye, lips and into my right nostril!

 

DSCF2284.JPG

DSCF2278.JPG

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Yeah, I hate going under jacked-up cars, I put mine on a jack stand for this job on passenger side only, but also kept the floor jack snug on the body, in case for some reason the chinese steel in the jack stand decides to take a permanent vacation. Once you know where the sensor is at, you can later feel your hand down there when the vehicle is on the ground. My next step will be to locate the YD9 terminal on the ECM, and check the continuity in the wire between YD9 and the connector of the knock sensor. With luck, there's an open or short in that wire. Otherwise, I need to find a replacement ECM. Hey AZVet, did you have good luck after you put in the new Knock Sensor ?

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Test drive is a constant ED16 0 and ED17 158.  The car accelerates nicely and idles smooth as it did before the change.  I will have to do a longer drive to test the fuel mileage.  At least I didn't screw anything up.

One plus was that I pitched the crappy crescent wrench I bought at the Ace Hardware bargain bin and sprung for a new Craftsman.  The cheap one would change width when I bumped it against anything.  Grrrrrrrr.  When the cheap one would not hold its spacing I was forced to crawl out from under the car and search for the correct socket instead.  It's not easy at my age.🧔

 

 

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