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Knock sensor being difficult...[200/83] posted by Jason Kneier on
Sunday, 9 January 2000, at 5:39 p.m.
I've been having trouble with uneven power delivery, especially at low to mid rpm. It causes jerking when crawling through parking lots and hesitation under acceleration. I believe its being caused by timing retardation by the knock sensor. This is on an '83 240 B23F with LH 2.0 (Bosch distributor, Chrysler ignition).
Checked timing - 12deg BTDC +/- 2deg or so at idle, seems pretty jumpy, but smooths out when throttle is applied.
Haynes says with key in position II/car not running, you should be able to measure 1.5V-5.0V on the knock sensor terminal. I get 0.9mV. Checking continuity on the brown wire, its good all the way to the control unit. Resistance to ground is about 47.5k ohm, so its not shorted. Next, Haynes says watch timing (with car running) and strike the manifold with a wrench - watch for the timing to get kicked down. Well, I wacked away, and got no response whatsoever.
My next diagnostic was cutting the wire near the control unit. I figured that the 1.5V to 5.0V I was supposed to be seeing was a bias voltage, and if there was some fault in the knock sensor, I should at least be able to see that voltage at the control unit with the sensor disconnected. I still got 0.9mV with key in position II. So next, I clipped the wire back together and clipped into it to watch the voltage while the car was running. I got a fluctuating value of around 44mV that dropped down into the 25mV range slowly as the car began to warm. Striking the manifold didn't seem to affect sensor output at all.
So, can any of you Bosch experts shed some light on my problem? Do I have a bad knock sensor, or is the problem in the control unit? I'm an EE, and have no qualms about opening the control unit and diving deeper if need be. Thanks...
--
Speed Racer '83 240 'R', '74 164 E, '93 940 OL1 (Smithfield, NC)
Re: Knock sensor being difficult...[200/83] posted by abe crombie on
Sunday, 9 January 2000, at 7:20 p.m.
On that system the knock sensor is not a piezo sensor but more a crude switch sensitized to the resonant frequency of the spark knock. I think you might have a problem with the wire terminals at ign ECU causing your voltage readings to be out of spec.
The kicker is that I don't think the knock sensor could cause the problem you are experiencing.
Disconnect the wire at the point where you cut it and drive it to see if it is cured. The knock sensor sugnal being absent on that system doesn't cause any terrible defaults.
You could have a cracked solder joint on circuit board where the connector pins are attached to the ign ECU PC board. You can remove the gel coating and inspect. Re-soldering is an acceptable fix.
The erratic timing marks are normal. The input from the hall sensor in distributor gives crank position inputs in 60 gegree intervals and the engine speed varies enough between signal intervals to give unstable speed calculations and the result is the mis-timing you are seeing. It stablizes if you rev engine to 1200 RPM or above. This phenomenon is why the switch was made to flywheel signalled systems with only 6 degrees between crank position references.
The knock sensor test by whacking on block (better than intake) next to knock sensor is only effective with throttle opened as the closed throttle signal is supplied to Chrysler ign ECU and it cancels knock sensor retard activity.
BTW, if you have Bosch ignition systems you can't do the whack it and watch it test for knock sensor operation. The Bosch systems are cylinder specicific and you would have to knock within a few crank degrees of a given cylinder firing and be monitoring the timing light attached to that cylinder's plug wire. I don't think any of us are that good.........