Knock Sensor

[boatwrench]

1990 Suburban R2500 7.4L/THM400/4.10:1

The Suburban lost power and the engine light started flashing rapidly. It set a code 43. Went to all-data.com service manual and it indicates a knock sensor. Local parts store has one for $40. Problem is I didn't think the 454 had a knock sensor. Searched here and confirmed with this post.
http://www.67-72chevytrucks.com/vboa...ighlight=knock

So the question is. Is there a knock sensor on a 454 and where is it located?

Thanks.

[78 Chevyrado]

If it's fuel injected, it has a knock sensor. Look underneath the truck and look right above where the oil pan attatches to the block. Where the coolant drain plugs usually are they replaced one of them with the sensor

Here's a pic of one, but it doesn't go in a big hole like the picture implies:


Should have one wire going to it. When you remove it, have the new one all ready to go in because when you pull it out, coolant will start running out fast.

[boatwrench]

Still can't find it. Been underneath the burb 3x's with no luck. Is it on the drivers side or the passengers side?

[BLE 'BURBAN]

Most likely on the drivers side obstructed from view behind the motor mount

[BLE 'BURBAN]

Just consulted my O.E.M. wiring diagram and it shows it on the PASSENGERS SIDE no the D/S as I previously stated. Just above the starter in the lower center of the block.

[screwballl]

looking at the OBD codes for 1990 GMs, it shows:

43 Electronic Spark Control (ESC) Circuit

Granted the ESC and knock sensor is very similar in function, with these trucks they are different sensors (unless I read the explanation wrong). It should be right in front or right above the starter on the passenger side. Also remember there should be an ESC module as well.

[boatwrench]

Just back from a few days at sea and had a chance to crawl under there. Right in front of and above starter. How I could miss it do not know. I'll change in the morning, clear codes and report back. Thanks for the assist.

The ESC module is on the firewall above the HVAC evap/blower motor. The knock sensor is an input for this module. I decided to "guess" it was the sensor based on my friendly counter person at the local parts house telling me he sells more knock sensors and has never sold an ESC module.

[ChevyTech]

Knock sensors are more fragile than they look. I like to only tighten them by the hex on the base on the sensor, not by the hex on the plug eng of the sensor.

The Knock sensor contains a piezoelectric crystal.

[boatwrench]

OK.

I replaced the knock sensor. The passage was not machined through to a coolant passage. I cleared the codes and set the timing to spec 4* BTDC. Warmed it up in the driveway and took it out for a roadtest, still a dog - no power, but no code set.

The ESC module is $125 from the dealer. My local parts counterman told me I am only the 2nd person that has ever asked for one.

I am switching modes as my daughter's car won't start so focus has shifted for the time being.

[ChevyTech]

What many manuals will not tell you is that a code 43 can be set by something loose rattling on or in the engine. Lifters, cam, rod, crank and piston noises all have the potential for causing false knock signals to be produced.

Loose torque converter bolts can cause a false knock signal to be detected. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging.

If you are getting false knock signals this could be why your truck feels like a dog.

It is much more likely that the problem with your vehicle is caused by a rattling part rather then a bad ESC module.

[boatwrench]

Chevy Tech,


Thanks.


OK I'll be looking for loose items and pull the inspection cover down and check the torque convertor bolts in the morning. Can I "test eliminate" a false knock by disconnecting the switch creating an open circuit?

[BLE 'BURBAN]

Since you're in California, chances are slim you've got a block heater installed on your engine.
But if you do,what kind is it?
Freeze plug style or circulating tank type?
The tank type have little check balls in them that will rattle around while the engine's running and cause the knock sensor to give a false reading as well.
Just another probable cause to look at.

[ChevyTech]

Originally Posted by boatwrench

Chevy Tech,

..... Can I "test eliminate" a false knock by disconnecting the switch creating an open circuit?

It you unplug the knock sensor it will set a trouble code when it runs its self test (see criteria below). However if you unplug the knock sensor after the self test has run, you can test drive it and see if it has more power, which will help determine the timing is in fact what is causing it to feel sluggish.
NOTE: you don’t want to drive it for an extended period this way because the system will not detect any pinging and the engine could get damaged.

Each time the Vehicle is started and certain criteria is met (coolant temp, MAP reading, engine RPM), a self test of the knock senor circuit is run. The ECM adds spark advance while anticipating a knock signal. If no knock signal is detected during the test a code 43 will be set. Some GM manuals state that two consecutive tests must fail for a code to set.

Anything that prevents the knock/ping or prevents the system from detecting it will cause a code 43 to be set. A bad knock sensor, ECM failure, wiring, Timing way retarded or a very rich mixture can cause the code.

The self test is not the only condition that will set a code 43.

Here is an interesting store that pertains to this topic:

I helped a friend of mine from an import dealer fix a code 43 a few years ago. His code started setting after installing a crate engine. After six months at two Chevrolet dealers and an import dealer, nobody wanted to work on it anymore. The circuits had been tested by several people and my thought was a mechanical knock was causing the problem. The decision was made to tear down the engine. Removing the oil pan revealed the problem. The metal baffle in the oil pan was broken loose on one side. Under certain conditions the loose baffle would rattle and the knock sensor would detect it as pinging.

Low circuit voltage on the circuit from the ESC module to the ECM for longer than 5 seconds is the other criteria (for 1989 C/K1500 350 auto) to set a code 43. When the baffle would rattle for an extended period of time, the timing would back down, the knock sensor still detected at knock the module would lower the voltage on the circuit signaling the ECM it detects a knock.

My point is, if something is loose or rattling it can cause this code, and every test in the manual won’t find the problem.

[boatwrench]

I had a long patrol this time...anyway...

Pulled the inspection cover down and the flex plate/convertor bolts are tight. I decided to throw in the towel and take to the dealer. Unfortunately the economy has hit here hard and the two dealers I normally use are gone. So it's at the GMC truck center, I expect a call tomorrow.

12 hrs later...
Got a call, they told me plugged exhaust. The Cat is 3 years old, the muffler (flow master) less than one year and only installed as the original rotted out. The OEM parts are no longer available.

The flow master was to loud for my taste. I'll search the forum for recommendations. This is a single exhaust.

[BLE 'BURBAN]

Couldn't find an online illustration of the exhaust for a 454 equipped 1990 R2500 but here is a pic of one with a 350 engine. With the exception of the Y-pipe every thing else should be the same. This is from NAPA's website.

[ChevyTech]

Let us know if the code problem is gone after the exhaust work.

Often if the exhaust is restricted the sound level is unusually quiet.

There is a possibility that the y-pipe’s internal layer is collapsed, if it is a factory dual wall pipe.

[boatwrench]

This is what the tech wrote on the repair order:

"While driving, noticed a lack of power when wide open. No misses or backfires. Checked for fuel and air restrictions, none found. Removed O2 sensor and installed adapter for pressure guage. Found 8 psi of back pressure when RPM's are raised. Muffler and Cat look new, possible problem with engine causing repeated exhaust restriction".

I had the cat replaced three years ago to pass CA smog and the muffler replaced this past spring as the pipe rusted out from the muffler rear. Got an estimate today from a shop that does cut-n-burn replacements for $270 from Y-pipe back to exhaust tip. They did my Jeep's exhaust two years ago and it's holding up. The only question was they stated 2-1/2" exhaust, I thought the Suburbans were 3"? The Y-pipe is original.

I'll keep you all informed and am appreciative for the assist.

[BLE 'BURBAN]

Originally Posted by boatwrench

The only question was they stated 2-1/2" exhaust, I thought the Suburbans were 3"? The Y-pipe is original.

I'll keep you all informed and am appreciative for the assist.

If you look at the illustration I posted you'll see the part number "7335794" this is repeated 4 times across the pic and right next to it is an illustration of an exhaust clamp as well as the #3 indicating a 3 inch diameter exhaust clamp for the 3" exhaust system from the factory. I had to replace the intermediate pipe, muffler & tail pipe on mine back in June with these parts as mine was all rusted to hell as well. Still running the O.E.M. CAT/ Convertor though but will be replacing it next summer with a smaller sized high flow 3" in/out model.