Hey guys, I have a 89 IROC-Z L98 MAF car. I got it on a OTC scanner the other day and the ECM is seeing a good hard knock every 7 seconds although using an oscilloscope(?) the motor was NOT knocking, just the ECM was seeing a knock. BLM's, Integrator, and 02 counts were showing the car to be running VERY VERY Rich. Thusly causing the very poor idle quality and overall crappy driveability.

I believe that the Wire harness that goes into the Knock sensor to have gotten burnt onto the SLP 1 3/4" header's Y-pipe since it was very close and it is shorting out. Not to mention that I saw a light smoke rising from there when I was trying to diagnose it a few months ago. Would the knock sensor if it were to be shorting cause all these symptoms??

Thanks,
Brian P
89 IROC-Z 355

 

ANYONE!!!????

 

Yes, an intermittently shorting KS wire will register as a severe knock to the ECM. Also yes there is a good chance that this could cause your symptoms as it would be constantly retarding timing.
However, if you had an oscilloscope on the KS wire and it wasn't showing anything when the ECM registered a knock then i would say the problem is in the ECM/ESC module. You can further rule this out by bypassing the KS w/ a 3.9k ohm resistor at the ECM. If you are still getting knock counts with the sensor and it's wiring bypassed you can be sure the problem is in the ECM.
Other than that it should be obvious what your problem is w/ scanned data to look at.
...ed

 

Ed,

I have had the motor listened in on and there was no knock coming from the motor however the ECM saw knock. I have switched in between 2 known to be very good ECMs and either one runs like this. I have also tried 2 different but known good stock PROMS for it, not to mention the Hypertech chip as well. I do believe that it is shorting on the Y-pipe due to how close the harness for it was and the light smoke I saw rising from the Pax side Cowl area that really smelt "wirey".

I don't know how to bypass the knock sensor but I was thinking about just unplugging the KS connector, disconnecting the Neg. Battery cable, then hooking the battery cable back up and start the car to see how it runs. I think that should answer many questions. But in the event that it doesn't the ESC should be Suspect you think???
Where can I get a new ESC and how much will it cost me to get it??

Thanks,
Brian P
89 IROC-Z 355

 

to bypass the knock sensor unplug your knock sensor
put a 3.3K resistor in line to ground and see if it still gives you the code
If it does you have a problem in the ECM or the wiring

------------------
1992 B4C
as you know
1 of 589 made!
Only mod - - K&N

 

I'm about 99.9% Sure that it is not the ECM or the PROM. I have been all over the ignition itself and completely dissected every possible lead with no solution.

Like I said, I really suspect that the Knock sensor or the ESC is to blame here and it's only a matter of getting the car back from the shop to diagnose and fix it. The only thing I am concerned about is the ESC, I know that it gets it's info from the KS but is the ESC controlled by the ECM??

Thanks,
Brian P
89 IROC-Z 355

 

Does anyone have the GM part number for the ESC???

Thanks,

Brian P
89 IROC-Z 355

 

The ignition module, under the distributor cap, is the part you are looking for (ESC). If you have ruled out the ECM, all that's left is the knock sensor itself or the ignition module. Make sure your wiring is good and then focus on those two components.

------------------
1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, MSD coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley module, ported plenum,
Ported Daytona Yellow stock base, Moroso valve covers, other stuff,
Streetdampr, Ruger P95DC, hot wife, new oil filter, !cats, !TBC, !AIR.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

Huh? yall are just getting this matter very confused. Put simply...

The failure is in one of 4 places. KS, the wiring, the ESC module or the ECM itself.

You can rule out the first 2 by bypassing the KS at the ECM. Pull out the wiring harness and figure out which wire is the KS wire. Now cut it and hook the ECM side of the wire to a 3.9k resistor, the other side of the resistor to ground.
If you are still getting knock then the problem is either the sensor or the wire. If you know the wire has a burnt through spot that can touch ground i don't even know why you're asking for help when an obvious solution is in front of you.

If you still get knock w/ it bypassed there, the problem is the ECM or ESC. The ESC is NOT the ignition module. The ESC is one of the things that lives in the memcal w/ the PROM. If you've already tried different ECMs and different PROMs and everything still happened then it would seem the problem is in the KS or wiring which brings me back to my other statement, why would you spend a bunch of time chansing a wiring problem when you know you have a burnt wire associated w/ that sensor.

if you just unplug the KS then it'll simply throw a code. You must bypass it w/ a 3.9k ohm resistor to make the ECM happy.

 

Oops ....Thanks, Ed.

 

Well, there's one addtional possibility, the OTC is at fault. I've read in two seperate diagnostic charts (one was from a Chiltons, the other I don't recall) that say "A scantool that has been known to register false knocks, should not be used" or similar.

Always led me to belive this may be an indication of a failing scantool, I dunno, but I'm assuming the OTC you have is an older model, may be best to try a different scantool before you start tossing parts into it.

 

On my brother's 87 GTA the ESC module was located on a bracket right next to the power brake booster. 89s may be different- never went looking. My 94 LT-1 has a knock (ESC) module inside the weatherproof case with the ECM.

The ESC module is NOT the same thing as the ignition module inside the distributor.

Definitely bypass the knock sensor itself with a 3900 Ohm resistor and see if the ECM is still registering knock. If so, you have a problem with the ESC module or the wiring, since you have already eliminated the ECM and the knock sensor itself from possibility.

Also, check your alternator's output- not the amps or volts, but if the diodes are going bye-bye. AC voltage leaking into a DC system tends to drive digital processors of all descriptions bonkers.