No knock

'87 Turbo "T"

What a shitshow!
Woo-hoo right? WRONG!!

So I'm out testing with the Powerlogger today & I got 0* of knock. 16 pounds of boost 91 octane & ALKY. This car has had minimal knock before.

However it seemed odd....I just got a feeling ya know? So I test with the key on tapping block & sensor....nothing. Car in park idling knock on block & sensor.....nothing, Have wife rev the engine to about 1,500+ R.P.M knock block & sensor....nothing. No knock on SM or PL.

Have 9V DC on black & yellow wire at ECM & ground. No changes with tapping. Sensor bought maybe two years ago from Caspers & torqued to 14 lb. ft.

I know knock sensors don't just fail. Any ideas of where to look next? ESM? Or a way I can test with a DMM?

Thanks!
 

salvageV6

Daily Driver
I think you need an oscilloscope to see the knock counts the DMM's usually don't sample quick enough to show on the display.
16 pounds of boost with alky. should be easy I would think with 91 octane and never have knock.
 

'87 Turbo "T"

What a shitshow!
I'm not getting any at any time. Idle, part throttle, or WOT that is the issue. Zero.

Not even seeing any knock counts in the Powerlogger ever. Never registering on the Scanmaster. Seems like the system isn't working for some reason. I know it's worked with this sensor before. I had this sensor working with the SM before the PL upgrade.

Not sure if there's a way to check the knock circuit somehow with a meter.
 

Nigel

Active Member
You can wiretap into the signal from the ESC to the ECM (the 9V wire) and run it to one of the aux inputs on the PL. Configure the PL input for 0-5v. Then with just ignition on and PL hooked up tap the sensor with a something, I use a long socket extension. You should see a short down volts blip in the display. It won't drop to zero as the PL does not have a high enough sample rate, but you will see a drop in voltage. Don't smash the sensor, but make sure it gets a good tap so the ESC will pick it up. I don't recall the RPM thresh hold for the method you tried, maybe higher than 1500rpm to get the ecm to process the knock signal. Also, if the tap is not hard enough (this is hard to quantify over the net) it won't register.
 

'87 Turbo "T"

What a shitshow!
Thanks Nigel. I will give that a shot tomorrow. I have a wire I can use from my Caspers gauge.

Thought that it recorded knock with just plugging it into the ECM. So odd with the 2.1 chip it was fine, now nothing.
 

RUQWKNF

KEEPER OF SECRETS
Staff member
Check your ESC Module for signs of corrosion on the plug and pins on the module.
They do go bad and won't trip a code when they do bite the bullet.
Just a thought.
 

'87 Turbo "T"

What a shitshow!
Well guys I got nothin'. Cleaned pins on ESC module.

Have continuity on sensor wire to plug, from plug yellow/green wire to ECM, & ground.

KOEO 11ish volts at pink & black wire at plug. Swapped to old knock sensor & retested.....not registering anything.

Wonder if Eric takes out the knock on his chips until a certain RPM. Other than that I'm at a loss unless it's in the ECM itself.
 

Nigel

Active Member
Unplug the ESC and start the car, if it throws a code for the knock sensor, then I bet the ECM is good.
The knock sensor is grounded to the block by the threads, you want to make sure the threads are clean.
I did a bunch of stuff with the ESC just to understand what makes it work, but most of it is lost due to the crash. The little that is still saved is here
Knock system
My guess is if the 9v signal from the ESC to the ECM does not pull low when the knock sensor gets a tap, the problem is somewhere with the sensor circuit or the ESC. The time that the 9v signal goes low is very short, on the order of 10 milliseconds, depending on the noise intensity being picked up by the sensor.
The chip could have knock ignore parameters, you would have to check with Eric.
 

'87 Turbo "T"

What a shitshow!
Thanks for the help guys!

Talked to Eric. Needs to be over 1k+ RPM’s to register knock anything below ECM ignores it(makes sense).

Put in old knock sensor had wife take it up to around 1,700 RPM’s tapped on intake & got knock retard.

I’ll swap it one more time just to be sure, but it looks like I may have a bad knock sensor.
 
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