97 ZJ Cranks but won't start
 

Hi all,


I have read several posts on this topic, and most point to a bad crank sensor. I have the symptoms where the gauges do not show anything (fuel and battery), and it cranks and cranks but won't start. However, in the case of a bad crank sensor it sounds like if I unplug the sensor and turn the ignition to ON, I should then see something on the gauges, and I don't (they're still not registering anything).


Prior to encountering this issue I had been camping and managed to drain the battery leaving doors open etc. Got a boost from someone at the campground and managed to get home but when I put the jeep in park and got out to open up my gate to back the trailer in, by the time I got back in the jeep the engine had stopped. Subsequent attempts to start it (even after fully charging the battery with a bench charger) resulted in the cranks-but-doesn't-start behavior.


I haven't ruled out a bad crank sensor, and will try to check it out, but is there anything else I should be checking? I've read that some people have had a blown 20A fuse (heater circuit), which I guess is the same circuit that powers the PCM?


Thanks for any suggestions.

Update: Found one of the small 20 amp fuses was blown. Popped a new one in there and it started right up. Now just need to figure out why it blew in the first place. Sounds like the O2 sensor wiring is a common culprit, though I am wondering if the jump start prior to this happening could have fried it somehow?

I believe I found the problem. Further research pointed me to the rear O2 sensor wiring, which runs up over the drive shaft and is prone to having the insulation worn away. Sure enough, there was definitely some wear and some exposed wire (and this was obviously not the first time - looks like the previous owner had taped it up before). Didn't have time for any real testing but taped it up good and haven't had problems since (touch wood). I would like to figure out how to support those wires so they don't wear away again - I can probably zip tie them up away from the drive shaft if there's an appropriate place to fasten them to.

Easiest way I've found over the years is mount a nylon anchor to the floor with a sheet metal screw and zip-tie the wiring to it.

The main thing with 02 sensors is the wiring is important. If it's 4 wire, 2 of them are a 12V heater circuit. The other 2 are 5V and the signal sent back to the PCM is in MILLIAMPS. You can see where good connections are important. That's why I loath splicing 02 sensor wires. I'd rather spend the $$ on one with factory plugs on it. Bosch is a good example.

O2 sensors generate their own voltage because of a chemical reaction. There is no reference voltage for the sensor, but there is 12 volts to the heater circuit for the O2.