Name that SeNsOr

Its on the driver side, leaning over the driver fender its just to the right of the motor mount and threads into the block just above the oil pan... I would like to know everything possible about what it is and what it does. lol i managed to break it putting my new poly motor mounts in.

TIA
Tom

It would help immensely if you told us what year and engine. This data is based upon my own collection - 87XJ4.0, 88XJ4.0, and two 89XJ4.0...

On the driver's side of the engine, there are two sensors in the block - the lower one (almost right on the oil pan rail) is the knock sensor. It is readily identified by the fact that the connector looks like one of the fuel injector plugs and the two wires are wrapped in foil.

The next sensor up is the coolant temperature sensor - the one that reports to the ECU. Its signal has to do with fuel metering, and is something that should be checked if you are having a lot of fuel mix trouble. It can be checked against coolant temperature (with a thermometer) by using an ohmmeter - use the standard GM thermistor curve (and if you can't find that, I can supply it for you...)

If your XJ is 1991 and up, consult an HO owner - since I'm all RENIX.

5-90

hahaha sorry that probabaly would help, yeah its a 90 and it does kinda look like a fuel injector plug. well i guess I get to go buy a new knock sensor now... :-<

Tom

Yay it look like its only about a 65 dollar sensor.... i cant wait to go spend the last of my money. Ggrrr.....

If it's the knock sensor, don't be surprised if the new one doesn't look like the old one. I think they've updated it, you'll get a ring-shaped sensor and a stud that will thread into the boss. This will work - the sensor only needs a solid mechanical connection, since it reads sharp vibrations (caused by spark knock.)

I'd put some weak loctite on the stud (the blue, I think - I go by the numbers on that stuff. 222 or 242 would be fine, but 222 is best) so it won't go anywhere until and unless you find an "old-style" sensor.

Take a moment to make sure the foil wrap around the wiring is still solid (it's only on the lead for the knock sensor,) as it helps prevent false knock sensor signals (which would be caused by crosstalk from adjacent wiring.)

5-90

Yeah im thinking maybe i need to try a junk yard to get one. what will happen if i need to drive it without it connected. I drove it last night and it didnt do anything funny or make any funny noises.

live24wheel said:

Yeah im thinking maybe i need to try a junk yard to get one. what will happen if i need to drive it without it connected. I drove it last night and it didnt do anything funny or make any funny noises.

Click to expand...

Renix knock sensor doesn't seem to do much with engine timing. Maybe a few degrees retard under low-speed lugging situations. I haven't had a working KS for a long time but I don't let the engine lug either.

haha that could make sense, i was out driving it last night with the new motor mounts and form a couple stoplights i stomped it and it didnt want to go anywhere until it got to about 2500 rpm then it just opened up and roared.

The knock sensor is supposed to retaerd timing if it detects spark knock - which is not a big problem for us. Why? Low compression ratio.

However, if you any towing or similar high-sload low-speed, keeping the knock sensor is a good idea - and you should get a new one if possible.

How it works - the knock sensor is a piezoelectric device, meaning that it is centered around a crystal that generates a small electrical impulse when vibrated or "knocked." It is possible that the crystal can lose its piezoelectric properties in the presence of excess heat and/or vibration, so renewing it isn't a bad idea. How often? I'm not sure - but I see no reason it can't last well over 200kmiles. Piezoelectric devices usually use a quartz chip, and quartz is a pretty hardy crystal.

If there is no knock sensor, the ECU will not get a knock signal, and the timing will remain static within the limits of the advance curve. The knock sensor is only really noticed if it has anything to say.

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