Bad Harness Causeing Phantom Knock
Jun 29, 2015, 02:46 PM
#1
Bad Harness Causeing Phantom / False Knock
Hello all,
I have spent the last month or so trying to take care of a "Knock" isue my car was having. Below are the steps i took and the eventual fix i found. Hopping this might help other people.
The problem.... Knock...
2500-4k RPM range, 75-110 load. Knock counts vary, some times it start like 1,5,21,0 other is it 2,3,3,4,5,4,4,3,2. It is only in that RPM range if i let it keep pasted 4k it is clean no problems.
The Testing
So all testing was done with a modified map. Timing on both the High and Low octane maps where set to 10 deg for all my knock testing. During the test car did not see over 115 load. It is a 9/1 compression motor on pump gas. This ECU and tune was also tested on in a buddy’s car with no problems. Knock modification tables worked in his car. Timing was dead on.
First:
We made a jumper for the knock sensor. Using a mistu connectors and some shielded wire to make a harness that would plug in the knock sensor and we could pin right to the ECU replacing the factor knock sensor shielded wire. The shielded was both grounded on the sensor side and the ECU side to make sure the shielding had a solid ground and we were not getting interference.
This made no change knock was still there is said load and RMP ranges. No luck on the next idea.
Next:
We decided to unplug the factor knock and plug in a known good one we had laying around. I tied it up out of the way making sure it could not bump anything then striped the end of the wire wrapping it around the threaded part of the knock sensor. Electrical taping it on there, I striped the other end of said wire and put an eye let on it bolting it to the chase. (Grounded knock sensor out of the block should be like a blind man in an empty room.)
Now I have knock 9 counts from 2500-4k. Knock voltage is the 0 volts for the most part but I see .0195-.034 sporadically throw the log. Also somehow in parts am getting down to -3 somehow even though all timing should be set to 10 deg.
Last:
I went in to the ECU under the Knock Control tables there is a table where I can set the ECU to ignore knock using RPM and Load. I set the ECU to ignore all knock under 130 load from 2k-5k. Knock sensor is still grounded as I described above.
Still I have knock 9 counts from 2500-4k. Knock voltage is the 0 volts for the most part but I see .0195-.034 sporadically throw the log. Also somehow in parts am getting down to -3 somehow even though all timing should be set to 10 deg.
Now at lest i was getting some where. After talking it over i desided to pick up another Engine harness. I was mainly thinking of the Last test i did. The knock sensor grounded and tie out of the way should of shown no knock what so ever. Also with the ECU being set to ignore knock and sill seeing this was beginning to look electrical.
First
Back probed the ECU using a safety pin and a test light. Checking to see if i have power leaking in the the grounds. I hooked one end up to the positive terminal of my battery. Placeing the test like to a ground it should light up nice and bright. A dim light or no light would mean i have Power leaking in to a ground. Key has to be on and everything powered up for this test to work.
No luck all
Next
Time to pull the harness. Disassembled the center console and half the lower dash. The harness traversal down the car passed the shifter. Pulled that all back to the fire way. Then disconnect the ECU. Now I disconnected all the sensors and pulled it on the engine side back to the fuse box. Then the hard part. You need to pull the strut brace. there are 3 nuts and 4 bolts that hold the harness on the fire wall. Then one bold on the engine side and one nut on the dash side the hold the boot in place where the harness gose in to the cab. The best way to fish this out is slow pull everything back to where is goes in to the cab. It travails throw some ABS lines it will all fit just have to be careful and second set of hand help out greatly. This all too about 3 hours.
So with both harnesses out i decided to start checking resistances. All but one ECU grounds, ground to the strut tower 4 in total. The 5th one grounds to the CAS. Agen back probing the ECU with a paperclip.
My old harness.
3-4 ohm of resistance with one ground being 10 ohms (assuming this was the single sensor ground).
New harness.
.8-1 ohm for all grounds.
Bingo we have found the problem. After installing the new harness did a log and the problem was gone. Also if i sent ECU to ignore knock it dose. My best guess is with such high resistance in the old harness that it was having trouble returning to ground causing electrical interference that i was just being seen as knock.
I am hoping this helps some one. This was a month of chasing this down. And this Knock i though i was having was the reason a year ago i decided to tear down the engine because i though it was sick. I don't regret building a motor but there was a lot of down time because of what i though was going wrong. IMO turning off the knock sensor and still seeing know was the biggest clue combined with when i grounded it out out side of the block it showing 9 counts (like it was disconnected).
I have spent the last month or so trying to take care of a "Knock" isue my car was having. Below are the steps i took and the eventual fix i found. Hopping this might help other people.
The problem.... Knock...
2500-4k RPM range, 75-110 load. Knock counts vary, some times it start like 1,5,21,0 other is it 2,3,3,4,5,4,4,3,2. It is only in that RPM range if i let it keep pasted 4k it is clean no problems.
The Testing
So all testing was done with a modified map. Timing on both the High and Low octane maps where set to 10 deg for all my knock testing. During the test car did not see over 115 load. It is a 9/1 compression motor on pump gas. This ECU and tune was also tested on in a buddy’s car with no problems. Knock modification tables worked in his car. Timing was dead on.
First:
We made a jumper for the knock sensor. Using a mistu connectors and some shielded wire to make a harness that would plug in the knock sensor and we could pin right to the ECU replacing the factor knock sensor shielded wire. The shielded was both grounded on the sensor side and the ECU side to make sure the shielding had a solid ground and we were not getting interference.
This made no change knock was still there is said load and RMP ranges. No luck on the next idea.
Next:
We decided to unplug the factor knock and plug in a known good one we had laying around. I tied it up out of the way making sure it could not bump anything then striped the end of the wire wrapping it around the threaded part of the knock sensor. Electrical taping it on there, I striped the other end of said wire and put an eye let on it bolting it to the chase. (Grounded knock sensor out of the block should be like a blind man in an empty room.)
Now I have knock 9 counts from 2500-4k. Knock voltage is the 0 volts for the most part but I see .0195-.034 sporadically throw the log. Also somehow in parts am getting down to -3 somehow even though all timing should be set to 10 deg.
Last:
I went in to the ECU under the Knock Control tables there is a table where I can set the ECU to ignore knock using RPM and Load. I set the ECU to ignore all knock under 130 load from 2k-5k. Knock sensor is still grounded as I described above.
Still I have knock 9 counts from 2500-4k. Knock voltage is the 0 volts for the most part but I see .0195-.034 sporadically throw the log. Also somehow in parts am getting down to -3 somehow even though all timing should be set to 10 deg.
Now at lest i was getting some where. After talking it over i desided to pick up another Engine harness. I was mainly thinking of the Last test i did. The knock sensor grounded and tie out of the way should of shown no knock what so ever. Also with the ECU being set to ignore knock and sill seeing this was beginning to look electrical.
First
Back probed the ECU using a safety pin and a test light. Checking to see if i have power leaking in the the grounds. I hooked one end up to the positive terminal of my battery. Placeing the test like to a ground it should light up nice and bright. A dim light or no light would mean i have Power leaking in to a ground. Key has to be on and everything powered up for this test to work.
No luck all
Next
Time to pull the harness. Disassembled the center console and half the lower dash. The harness traversal down the car passed the shifter. Pulled that all back to the fire way. Then disconnect the ECU. Now I disconnected all the sensors and pulled it on the engine side back to the fuse box. Then the hard part. You need to pull the strut brace. there are 3 nuts and 4 bolts that hold the harness on the fire wall. Then one bold on the engine side and one nut on the dash side the hold the boot in place where the harness gose in to the cab. The best way to fish this out is slow pull everything back to where is goes in to the cab. It travails throw some ABS lines it will all fit just have to be careful and second set of hand help out greatly. This all too about 3 hours.
So with both harnesses out i decided to start checking resistances. All but one ECU grounds, ground to the strut tower 4 in total. The 5th one grounds to the CAS. Agen back probing the ECU with a paperclip.
My old harness.
3-4 ohm of resistance with one ground being 10 ohms (assuming this was the single sensor ground).
New harness.
.8-1 ohm for all grounds.
Bingo we have found the problem. After installing the new harness did a log and the problem was gone. Also if i sent ECU to ignore knock it dose. My best guess is with such high resistance in the old harness that it was having trouble returning to ground causing electrical interference that i was just being seen as knock.
I am hoping this helps some one. This was a month of chasing this down. And this Knock i though i was having was the reason a year ago i decided to tear down the engine because i though it was sick. I don't regret building a motor but there was a lot of down time because of what i though was going wrong. IMO turning off the knock sensor and still seeing know was the biggest clue combined with when i grounded it out out side of the block it showing 9 counts (like it was disconnected).
Last edited by Seancj; Jun 30, 2015 at 03:48 PM.
The following users liked this post:
ViciousLSD (Apr 3, 2024)
Jul 1, 2015, 04:05 PM
#2
Nice fix and way to be persistent. I've seen situations where the low load knock is turned off and still shows knock in those areas. It's rare though.
Did you buy a new harness or a used one? I see them used on eBay for a couple hundred $$$. I can't imagine what a new one would cost.
Did you buy a new harness or a used one? I see them used on eBay for a couple hundred $$$. I can't imagine what a new one would cost.
Jul 2, 2015, 06:22 AM
#3
I picked up a used one, i look in to new or mil-spec ones and it was just to much to spend. Made sure it was out of an unmoded car with no splices.
Payed 190 shipped for it. Car has about 20 miles on it now with no isues. Besides a trans leak i have to trace down... :-(
Payed 190 shipped for it. Car has about 20 miles on it now with no isues. Besides a trans leak i have to trace down... :-(
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