Reset computer after changing knock sensor?

I picked up a knock sensor earlier today from the wrecking yard, threw it in but I only disconnected and reconnected the battery, I didn't switch the ignition a million times to reset it (I don't recall what the number of times was), and I didn't take it for much of a "relearning drive" to let the computer relearn. I vaguely remember reading somewhere that you should do one of the two..or maybe neither at all after changing a sensor, but I can't remember. Can anyone verify this for me? 1990 Cherokee 4.0l
I also spotted a nice front skid sitting in the back of a jeep..forgot to ask how much they wanted for it since they seem to be a pretty pricy wrecker..$30 for a "used-new" sensor..but I guess it still beats the $110 that chrysler quoted me.

Don't need to bother - if you've got a knock sensor, you've got RENIX. If you've got RENIX, you don't have the "Keep Alive Memory" (KAM) that OBD has, so there's nothing to relearn.

And, there are knock sensors available in the aftermarket for about half of what the dealer wants. Besides, the knock sensor only gives with a signal when something is going wrong (the engine is knocking,) so if you run quality fuel and don't have deposits, it's just along for the ride anyhow.

If you have a 1991-up XJ or MJ, the knock sensor was deleted by Chrysler...

The Renix does have Keep alive memory. It's in the fuel injection manual. It doesn't store codes in a typical fashion, but it lists them in a numercial number of failures 0-15 0 means not within last 15 key cycles, 15 meaning last fifteen plus this start.

It doesn't come out and say it directly, but it does hint around that it does adjust to different driving conditions.

You can read KAM and the failures+live sensor data if you have a Chrysler DRB II and jeep/eagle adaptor.

You can also, as I assume this person did, check the resistance of the knock sensor to check if its defective.

Check the knock sensor by watching timing and tapping the block near the sensor - it should retard. There isn't any real "resistance" to the sensor - it's a piezo sensor, meaning it's got a crystal that generates a voltage when it's tapped or vibrated.

Looks like I'll have to go re-read that manual after all - I've been spending too much time reading up on emphysema and trying to avoid my mother-in-law (which, coincidentally, is why I'm also reading up on emphysema...)

mine was broken in half, the plastic casing was broken clean off around the "head" of it. I didn't bother testing it, since it was clearly pretty mangled..just dangling there like that. I plugged it back together but I'm not sure if that was sending a signal to really retard the timing, since it was running pretty sluggish in the past few weeks since it started getting cold. In the summer, when it was hot I was getting some spark knock when I ran 87 octane fuel, sometimes a little even with 89 octane. It hasn't had any pinging that I can detect since it cooled of, but it's felt like the thing wasn't firing quite right or something, figured it wouldn't hurt.
None of the Canadian auto parts retailer online (that I know of anyways) had replacement knock sensors, and the local parts stores all quoted me quite a high price...after I spent 5 minutes explaining what a knock sensor is and where to find it.

89CherokeePioneer said:

The Renix does have Keep alive memory. It's in the fuel injection manual. It doesn't store codes in a typical fashion, but it lists them in a numercial number of failures 0-15 0 means not within last 15 key cycles, 15 meaning last fifteen plus this start.

It doesn't come out and say it directly, but it does hint around that it does adjust to different driving conditions.

You can read KAM and the failures+live sensor data if you have a Chrysler DRB II and jeep/eagle adaptor.

You can also, as I assume this person did, check the resistance of the knock sensor to check if its defective.

Click to expand...

I don't have a copy of that manual, can you tell me how I could reset it? Just on/off on the ignition 15 times?

The AMC242 engine has fairly low compression and does fairly well from a 'quench' point of view, so 87 octane should be no trouble (I've run worse...)

Sounds like you might have a good deal of carbon buildup in your cylinders. Here's how to handle that little nuisance...

1) Disconnect the catalytic converter at the flange. You'll need a new gasket for this, but they're fairly cheap (in a pinch, use some high-temperature gasket material and coat with a thin layer of RTV copper.) Tie the converter inlet out of the way (you won't be driving for this.)

2) Get a trigger spray bottle full of clean water - preferably distilled or reverse-osmosis (RO) filtered. No need to add anything.

3) Remove the snorkel tube from the airbox (make sure you're not anywhere dusty!)

4) Start the engine, and have a helper hold the engine speed up around 900-1000rpm.

5) Using the finest spray possible, shoot about one full spray of water into the open snorkel tube about every 20-30 seconds. Watch for black clouds coming from the exhaust (this is why you disconnected the cat and tied it out of the way. This is the carbon that is in your cylinders...) Repeat as necessary until you don't get any clouds anymore, then about three or four times after that (just to make sure.)

6) Shut the engine off and put it all back together.

WHY THIS WORKS

What is happening is that the liquid water you're spraying into the intake gets sucked into the cylinders, where it tends to be absorbed by any deposits in them (on the pistons, on the head, on the valves, ...) Combustion temperature 'flashes' up around 1800*F - this is enough to "flash boil" the water in the deposits. The sudden expansion of the water into steam literally blasts the deposits loose from whatever surface they're on - you're actually "steam cleaning" the inside of your engine. This is a harmless process.

The reason you disconnect the air filter is so that it won't stop the water from going in and doing its work.

The reason you disconnect the cat is so you don't coat the matrix with carbon and wreck it.

This problem is slightly worse on RENIX XJ's, due to the EGR valve. Since it cycles exhaust gasses back into the chamber, it also serves to recirculate carbon into the intake stream, which gets deposited on damn near everything (I know, stupid idea.)

It should take you about a half-hour, tops. Cheap, easy, and fairly quick -and it works better than all the expensive chemical cleaners out there (my criterion for "environmentally friendly" is this - if I can use the same amount or less of a "friendly" solvent, it's friendly. If I have to use significantly more, it's not. This is why I don't like non-chlorinated brake cleaner - for the job, trichloroethane is almost an order of magnitude better than acetone/MEK/toluene in the non-chlorinated stuff. Carburettor cleaner doesn't work worth a damn on brake dust...)

I actually came across another post here:
http://www.naxja.org/forum/showthread.php?t=922442&highlight=carbon+steam+cleaning
where you talked about doing this. I did it this past weekend and noticed it ran quite a bit better and was more responsive on the throttle. I think I could probably benefit from doing it again, and will probably do so in a month or so..I'm just in the middle of moving right now which is a hassle.

clunk said:

I actually came across another post here:
http://www.naxja.org/forum/showthread.php?t=922442&highlight=carbon+steam+cleaning
where you talked about doing this. I did it this past weekend and noticed it ran quite a bit better and was more responsive on the throttle. I think I could probably benefit from doing it again, and will probably do so in a month or so..I'm just in the middle of moving right now which is a hassle.

Click to expand...

Yeah - the "trick" was old when I learned it about 25 years ago. Doesn't matter - still works well!

The nice thing about it is you can do it as often as you think you need to, or as a preventive (I usually do it before I go get a smog check, for instance. They're not going to fail me easily...)

There is no key on key off to reset anything. Disconnect the negative battery cable and reconnect it. This will, if there is any drivability data stored in any sense of the word, reset it. Don't get hung up on that though. The manual eludes to it, doesn't come out and say it.

About the steam cleaning, I've heard that before. Oddly enough, engines that I've torn into with blown head gaskets seem to be oddly clean.

89CherokeePioneer said:

There is no key on key off to reset anything. Disconnect the negative battery cable and reconnect it. This will, if there is any drivability data stored in any sense of the word, reset it. Don't get hung up on that though. The manual eludes to it, doesn't come out and say it.

About the steam cleaning, I've heard that before. Oddly enough, engines that I've torn into with blown head gaskets seem to be oddly clean.

Click to expand...

That's because the same thing happens with the blown head gasket (when it's a coolant leak.)

If you look carefully, you should notice a slight green tinge, in addition to the overall cleanliness of the cylinder surfaces. That's from the coolant...