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CPS Crank Fault

Black_Black

NAXJA Forum User
Location
Colorado
I have now tried 5 sensors, I'm getting .2-.3v AC from them. My CPS's in the past have always been around .2v AC and the Jeep started fine.

I tested the resistance to the ECU and it's .1 ohm both sides.

I got another ECU today, and same result - CPS crank fault on Renix Engine Monitor.

What now any ideas?
 
Cam sensor is reading good on Renix Engine Monitor. Though I didn't have one for years maybe I should try unplugging it.

I'll check on the bellhousing bolts, also getting an O-scope today so I'll see what the pulses look like before and after the harness.
 
If you don't follow "DEXj's" or DexJeeps... wtv he goes by.. mentioned how these things get heat soaked very easily. I always have issues with that on my 89 MJ. Even have an access panel in the transmission tunnel for ease of access. Take a bottle of water and poke a hole in the top. Spray it down and try to let things settle for a moment. IMO the CPS on these things is just a very bad design. Besides the heat, it gets SOAKED in oil if you haven't noticed. (Magnetic sensor vs oil... not a good combo) Don't forget it might be something as stupid as the pigtail connector on the harness. Everyone has their opinions there. You decide if OEM connection will be kept. Wish you luck.
 
Here's a picture of when I scoped my 89. A/C spikes are roughly 5 volts peak-to-peak. Renix should look like this. HO and later should be nice square waves.

Renix-CPS-Signal-2.jpg
 
Nice Oscilloscope! Learned something new today. Didn't think even for a moment that Renix produces a saw wave. Not just that, but that it could be measured in A/C. (But the input on your oscope is set to DC readings..?)

not speaking on experience using those things.. but I have owned a couple Moog synthesizers, bla bla bla, and understand the purpose.
 
The signal was slow enough and low voltage, so I didn't need a/c coupling which tends to distort lower frequency signals. A/c coupling is mostly used when looking for noise on higher voltage lines.

This was the CPS at idle. The frequency and amplitude both go up with rpm, whereas the HO has the signal conditioning in the sensor and it continues to put out the same voltage square wave. As the Renix sensor gets weak, the voltage seems to drop which explains why it often show up as a hard to start condition.

The speeds sensors on the 98+ AW4 also work the same way as the Renix cps. I scoped that when I was figuring out the speed sensor mismatch between an 89 and 98. The later 98+ AW4 speed sensors spikes started with virtually zero volts and got up to 30 volts at higher speeds (tested by running in driveway with the xfer case in neutral).
 
Lawson you have a lot of great insight. We should keep this thread alive regarding this specific diagnosis you have been running.

The original poster of this thread has not checked back in a couple weeks.. but maybe he should check for wiring faults as well.
Renix 4.0s have a sloppy harness.
 
Agree, in addition to measuring resistance from the sensor to the ECM (which looked good per OPs post), I would check for shorts to ground or battery voltage.

Indeed, wiring harness and connector issues were the reason for the 86-88 TSB that rewired the CPS directly to the ECM. The added resistance of the C100(?) connector and the unconditioned crank signal was causing issues.
 
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