The Hammer28

iTrader: (10)

I have a 97 Nissan Maxima that is throwing 3 codes. 1) After cat oxygen sensor, 2) knock sensor, and 3) speed sensor. If I have to replace all of these, it'll be very expensive and time consuming. I also heard that the speed sensor code is sometimes caused by a faulty guage cluster, and I definitely don't want to replace that. The car runs and drives perfectly, but I now have to pass emissions. Does anyone know if any of these codes can be simply tuned out like some LS1 codes? Someone please help me out. Thanks.

dan@masportspeedshop

iTrader: (6)

Best advice I can give is to never "tune out a code", even if it can be done. Masking problems doesn't get rid of them. Your problem may be a simple ground issue. None of the codes and sensors you mention share anything in common except for power and ground circuits. The first thing to do is to check that the ecm grounds are in good shape. Perform all of the following tests with the ignition switch ON. Many people perform a simple continuity test to ground, but the more effective approach is to perform a voltage-drop test. To do this, place the positive lead of your DVM on the ECM ground pin, and the negative lead of the DVM directly to battery negative terminal. Anything less than 50 millivolts (0.050 V) is ok, but less than 20 mV is considered ideal in the real world. Check ALL ecm ground pins for potential ground problems as they are not all necessarily connected internally. Pins 10 and 19 are important ground terminals. Test the ecm pins, then check the ground connection at the top front of the engine. This is where those ecm pins terminate to ground. If the grounds all test fine, move on to diagnosing each sensor. This requires a scope, so find, beg, borrow, or, well, don't steal one. Regarding your speed sensor, yes, it could be the instrument cluster. The speed sensor is a magnetic reluctor type sensor that sends an analog signal (ac sine wave) directly to the cluster. The instrument cluster then processes the signal and outputs a digital pulse (dc square wave) to the ecm (pin 29 of the ecm). If you (or someone you know) has access to a lab scope or shop scope, this can be checked easily by back-probing pin 29 of the ecm and verifying that a square wave is present when the vehicle is moving. If the signal is there, your ecm may be defective. If the signal is NOT present, it may be the cluster. Use the scope to check the sensor itself for an ac signal. If the sensor is outputting a signal but the cluster isn't, you know the cluster has gone bad. Always check signals right at the connector to ensure that the longest possible wire length is tested. Moving on to the rear O2, pin 4 is power (red/blk wire), pin 3 is ground (do voltage drop test), and pin 2 is the signal output. Use the scope or a DVM to verify a signal here of between 0 and 1 volt (also a sine wave). The rear O2 should give a "dead" signal. If the rear O2 produces a signal similar to the upstream O2 sensors, it tells your ecm (and the emission test tech) that your catalytic converter has gone south permanently and that you need a new one. Test the knock sensor by attaching the scope lead to the knock output line and tapping the block nearby with the handle of a screw driver. You should see a variance is the signal (between 0-5V) each time you tap the driver on the block. The knock sensor wire is white in color and is at pin 64 of the ecm. Performing these tests will uncover your problem. If you don't have access to a scope and your grounds all check out fine, you're up the creek. I do think that in all likelihood it is a simple ground problem, but I also don't know what else you have in the car (ie stereo eqmt, etc). Other high current electrical items can cause false codes to come up easily, so beware of those as well.
Good luck!

Billf6531

Will someone in Boca please go see this guy and buy him a beer. He's too good to lose.

dan@masportspeedshop

iTrader: (6)

Billf6531, I appreciate the kind words very, very much. After a tough day here at the shop dealing with a less-than-appreciative customer (understating to be polite), I surely appreciate seeing something this nice posted up about me. I'm hoisting one up to you, pal!! Cheers!!