Has anyone tested a knock sensor? I have an intermittent "bucking" problem with my brick which occurs when fully warm. The tach shows revs but the engine hesitates badly. I was wondering what symptons are present with a bad knock sensor? Can I

Work already done includes; new hall sensor, clean and regap plugs, rotor and wires fairy new, fuel filter fairly new, both relays soldered within last 3 months, cleaned throttle body and IAC, air filter good, no vacuum leaks

Questionable items; wiring harness (spring project), fuel pressure (no pressure gauge in my tool box).

Input always appreciated.



1987 745Ti, 298,000 km, M46

Well, first, check your OBD to see if it's giving you a trouble code. If it gives you 1,4,3 it means your knock sensor is broken. If not, you can remove the knock sensor and test across the two metal connectors that the plug goes into using a voltmeter. You should get around 5 megaohms of resistance. If you get something other than around 5, then your KS is dead.

I just replaced mine recently, and it can get kind of cramped in there. I didn't even bother taking it out to test for the 5 MOhms. I tested it, and I got nothing. No conduction at all. So it was definitely broken. The new one had about 5. FCPGroton carries them for cheap, around $38.

This should be covered in the FAQ as well in the Engine Tune and Performance section. Look for the word 'knock sensor' on that page and it should tell you something like what I just told you.

Good luck!
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

I do not think the 1987 models have OBD because I have looked for the box near the driver's side strut tower, and have not found it.

Your car perhaps has a pig tail around the left fender where you can connect an LED and read codes. Don't bother. I did on my car and it showed KS was OK, but when I put the multimeter across KS connectors I read 300 K Ohms resistance (too low). A new KS from FCPGroton took care of that. Like Invar says, look for around 5 Mega Ohms resistance.

to around 10 ft. lbs +/- 3 I believe.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Volvo specifies that the torque for the 1987-on sensors with the more "rounded" profile should be 14.5 ft lbs.

hmm i wonder if i should tighten it more then. i brought it up to 10 roughly. i'm not sure if i have the rounded ones. i think i do...
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Well, the November 1989 edition of the 700 series Specifications manual shows a tightening torque of 8 ft lbs for the earlier, flatter, sensor, and 14.4 ft lbs for the newer. A note in the Ignition Systems manual states that "Type (C) and (D) [the newer version] sensors are less sensitive to tightening torque than the earlier types. The design permits the torque to vary from 11 - 18 ft lbs without any adverse effect on the performance of the device". The older sensors (pre 1987) are pretty much flat on top. The newer version isn't rounded actually, but has a raised section in the center with sloped sides (I'm beginning to realize why technical writers are well paid). If there's still a question which one you have, email me and I'll send a diagram. I just barely managed to squeeze in a 3/8 in drive torque wrench and had to muscle to top radiator hose out of the way.

yeah, if you could send me some sort of diagram or picture of the KS, that would be great. thanks!

tamk2@rpi.edu is my email
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'