Hello,
I like many other have replaced the knock sensors in my '94 850 Turbo wagon. And shortly thereafter the check engine light illuminated and the fault code showed the rear knock sensor. A local Volvo dealer checked my work and tested the sensors and said I did everything correctly, and the sensors function properly.

OK. So let's assume the knock sensor is operating correctly, and the ECU is not faulty, and there are no bad grounding wires. Here's my question:- Can a component , when failing intermittently, cause the engine to knock and thus the knock sensor to detect knocking? If so, what would those components be?

I read a while back that the knock sensors have approximately 4meg ohm of resistance. I also read they may have 30 meg ohm. What do they have for resistance?

What I thought I might do is disconnect the knock sensors at the connector, and then put the proper resistance across the connectors on the wire harness. What effect might this have? I don't know!! I'm hoping the ECU is NOT faulty and never gets trip by a knock code again.

Input welcomed.

Thanks,
RickM

I've seen this occur many times now so I've been there/done that. You'll get the knock sensor code(s) and the first thing that the old SB recommends is to replace the two knock sensors with the new and improved type......BUT the car later returns with the same code. The problem is NOT the knock sensors, it's the ECU. Lately when I see this I'll get a used '94 ECU and replace it first, before doing anything with a knock sensor and guess what? The problem's solved and the car doesn't return with that problem.
ECUs aren't cheap new but these days you should be able to locate a good used one for a fraction of the new part price. Takes seconds to replace too.

You know, I'll have to agree with the ECU. This same car exhibits other oddities, for example: surging - slight, but noticable say about 50 to 200 hundred rpm on the tach, and the downshift indicater light will flash and the car (while in 'D' and NOT winter mode) starts in third gear, and the christmas tree light effect - all the various indicator lights on the dash twinkle for about two seconds! (That I would guess is a faulty ground.) Hmmm, but with all that it averaged about 26.6mpg over fifteen hundred miles! Not bad!

RickM

My understanding of a knock sensor working is that it detect the shock wave coming from ignited fuel. Thus a "knock sensor fault" may not be knock sensor at all but a lack of detection due to misfire.

First, the ohms is the resistance. I don't know how those sensors work, but I would guess they might be variable resistors relative to heat, but not sure. So...if you replace them with the appropriate resistance, they won't act as sensors, or they will act as if the sensors don't sense anything... I don't think a fixed resistor would be a reasonable test, repair.