Knock Sensors

[911/Q45]

Just wrapping up a Q project, replaced all the injectors, both knock sensors and associated harnesses. Before starting I checked everything electrically and got injector ohm readings all over the map, which corresponded with rough idle, intermittent CEL, etc. On the knock sensors I got an OL reading which I assumed to be a bad harness, so I ordered same without the sensors. Once I got it apart, I checked the sensors directly and got the same thing, even though they looked OK visually. So I overnighted fresh ones from Scottsdale and installed everything. Now that it's back together, I decided to check things electrically again to confirm the fix. The new injectors and harness are 12.5 ohms across the board, but the knock sensors and their new harness still show OL. I checked them about a year ago and got the proper 550K ohms and the car has run strong all along with no indication of KS failure. With all brand new stuff I'm stumped.

[maxnix]

Wish I could help, but Dennis always says to replace the harness with the sensors.

If both KS and harness combinations are checking the same, maybe it's your methodology?

[DAEDALUS]

Interesting. And there's no CEL code for them? They ground through the mounting bolt, not the harness. Any way they're somehow isolated?

[911/Q45]

No code. Bolted to the block, clicked on the new harness. I'd suspect my methodology if I hadn't done the test before with appropriate results. I'm especially puzzled because it quit reading out before I changed anything and still is that way after swapping all involved parts for new.

[maxnix]

Did you test to ground to ensure they are grounded? Dumb, I know, but it is mystifying.

[911/Q45]

This was making me nuts, so I went back and tried again. It turns out that my multimeter will read correctly for ohms and M ohms(thousand?million?I forget) but not for K ohms. So when I checked with M ohms selected it read 550 with a decimal in there somewhere, therefore it is an instrument malfunction. Whew, I did not want to pull that plenum back off! The bad news is I bought new knock sensors that I didn't need.

[ardvarkus]

The bad news is I bought new knock sensors that I didn't need.

Hopefully you can sell them- maybe t to someone who 'knows' you from this board...

Mine are (still) fine

Ard

[maxnix]

New knock sensors will save you another trip under the plenum if everything else is fresh. I say leave 'em and be happy.

[elwesso]

I agree with Brian.. The knock sensor replacement is SOO MUCH FUN that I certianly wouldnt want to do it again while hte plenum is off...

Im scarred for life by replacing those stupid black pucks!!

[maxnix]

Listen to the voice of experience. Wes doesn't want to go back there soon, and with good reason.

[elwesso]

Actually, I wouldnt mind taking off the plenum again (not hard at all with good hoses) but I CERTAINLY dont want to do the knock sensors again.....

Now that I know wehre everything is, plenum removal is a snap.....

[911/Q45]

The new sensors are under the reinstalled plenum and will definitely stay there! The old ones are in perfect condition(confirmed once I figured out what was wrong with my multimeter) and I'll send them to the first guy with a $20 bill.

[ardvarkus]

The new sensors are under the reinstalled plenum and will definitely stay there! The old ones are in perfect condition(confirmed once I figured out what was wrong with my multimeter) and I'll send them to the first guy with a $20 bill.

Man, just to have a spare if I need it... thinking... thinking...

Sold

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[911/Q45]

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[duck3986]

I haven't done knock sensors on a Q. What is it about replacing them, other than removing the plenum, that is difficult?

[911/Q45]

Once the plenum is off, they are tucked behind the coolant elbows at the bottom of the valley. Creative use of 1/4" drive sockets, extensions and ujoints plus the liberal application of expletives get it done. If you do them in conjunction with the coolant hoses, then the fittings come off and it's easy.

[elwesso]

Agreed with 911... Trying to get the coolant elbows back in you just have to keep banging them in there, yelling at them, cutting your hands, and throwing tools until you can finally torque them down...

Its unreal how something so easy (youd think) can be so hard.....

[maxnix]

Of course you realize Duck3986 has a VH41DE? Similar, but from the service manual it doesn't look like it entails the same degree of pain that the VH45DE does.

[ardvarkus]

Quote »

Its unreal how something so easy (youd think) can be so hard..... [/quote]

Man, ain't that the truth!

You look at those and think 'no sweat, just a few bolts'...Arrrggghhh