my '96 cherokee was failing to start unless i had a fresh charge on the battery and threw P0340... i replaced the crankshaft position sensor with a mopar part and it started right up, then went back to same troublesome behavior.
i refreshed the grounds, upgraded the cables with the big-7 2 gauge kit from jeepcables, it started right up, then later in the afternoon didn't start. put a fresh charge on the battery and it started (once), then didn't. put a charger
on it in Start mode and logged OBD data and it started fine with the voltage starting at 14.4V and only dipping to about 14V. without the charger on it, the battery voltage pre-crank was 12.3-12.4, but dipped to 11.9 during cranking
(which was fast) and it didn't start (tach stayed at 0 rpm) and threw P0340...

i'm thinking it's time for a new battery, but i'm curious: what is the minimum voltage required as input to the PCM voltage regulator to ensure it has enough power to work correctly?

and: i've seen several posts about dual battery setups to power accessories separate from the starting battery, but is a dual battery configuration ever recommended to make sure the
PCM gets what it needs during cranking?

thanks!

 

update--i didn't change anything after yesterday's failed attempts to start, but this morning it started 3 times in a row with cranking voltage dipping down to the low 10's...

 

Do a load test on the battery. Free at AZ, Advance, O'Reilly, etc.

Dual battery set-ups usually have a "start" battery and a "house" battery (deep cycle to run peripherals). Or you could get dual purpose batteries. I have a dual battery set-up. On a boat.

About the only time you should use both batteries to start would be if both batteries are low and you're stuck and need to get going.

 

P0340 indicates an issue with the Camshaft Position Sensor, not the CrPS...

I repeatedly run the regular ol' Napa lead acid battery in my 94 down to the 10.5v range according to my stereo and although the starter is noticeably slow, it has started every time without issue.

 

a few things. some of your questions are a bit hard to answer, so I will boil it down

how old is the battery?...a date code is stamped on the top of terminal

If it was more than 5 years old, I would probably just replace it, as its about die, the main suspect, and you can keep hold of it for the minute. Best if you can find on on special!

In the meantime, just do a parasitic current draw test, as there is either a current draw, your battery is dying, or your starter is on its way out

Is the starter original ? (they begin to crank slow)

You should be able to charge your battery to minimum 12.5V. It should be able to hold steady over 12.3V for at least 24hrs, if nothing is used, just test with multimeter

If it fails this test, its 99% battery failure (unless it has parasitic current draw over 30mA)

examine your cables, terminals, grounds

 

Starter possibly oil soaked? Mine was because of leaky oil filter adapter.

 

some updates:
thanks for all the suggestions!

so the battery is at least 5.5 years old, i've used a TOPDON BT100 load tester and it tested "good" but while i've read that many batteries last a very long time, i won't be surprised if this one is nearing the end of its life.
my starter is a new Bosch, and while the voltage is dipping a bit during cranking, it still cranks fast and the engine has lately been starting reliably...
but the engine is still dying with with what seems to be an intermittent problem, which is why i was asking if anyone knew what the minimum voltage requirement is for the PCM to function properly.

to help diagnose the problem, i kept a fuel pressure gauge on the fuel rail (it reads around 48-51 psi during these tests) and gathered OBD data for several PIDs, and i think i'm seeing a pattern that leads to the engine dying...

here's a chart of all the data i'm capturing:


one thing that caught my eye is that the Ignition timing advance is generally 17 degrees, the O2 sensor is averaging about 0.48 , but just before the engine dies, the O2 voltage goes to zero (note the red arrow in the chart),
the ignition advance goes to 24 and the engine stops, and then after about a minute the O2 voltage starts to build back up:




and when i tried to start it again, it started up just fine and ran for many more minutes before i shut it off. i saw the same pattern of O2=0, advance =24 just before the engine died. out of curiousity,
i also gathered data for a run when i pulled the ASD relay out, of course the engine died, but the advance didn't jump up, and when i plugged the relay back in, the O2 voltage started building again.

no codes are being thrown.

in having read many accounts of various crank-no start, random stalling posts, i'm now wondering if:

1. the ground signal from my PCM to the ASD relay is intermittently failing
2. the ASD relay is intermittently failing
3. the upstream O2 sensor is intermittently shorting enough to trip up the ASD relay but not shorting badly enough to blow a fuse

so i'm going to have a look at the wires leading to the O2 sensor and the wires in the PDC and the wires going to the PCM...

any other hypotheses/suggestions welcome!

 

Your diagnostic skills appear excellent

A BIG suspect on any XJ fault is failed wiring

I would start with the No1 O2 sensor. Either unplug it, or make a piggyback harness

a piggyback harness is the way to go with the other suspect sensors

The plastic insulation fails, causing short or open circuits

I just came up with a brilliant solution for my failed auto trans harness, will make a topic later

 

so I unplugged the upstream O2 sensor... started fine and gathered an hour's worth of data and the engine didn't die, O2 volts = 1.0 constant, as expected

plugged it back in and gathered another 25 minutes of data and the engine didn't die, O2 voltage varied as before when engine didn't die

undid the wire bundle that runs along the top of the manifold carrying the coolant temp, o2, and injector wires... the tape was hard and brittle
and many wires had apparently gotten hot enough that their insulation had blended (dunno if it got so hot that the copper made contact--will be inspecting the insulation)... i separated all the wires, re-started and gathered 20 more minutes of data and everything looked like it does when the engine doesn't die...

will try again in the morning. the anecdotal experience has been that if it's going to exhibit the problem, it does so when everything starts out cold (we've had some temps near freezing lately), then after 5-10 minutes, the o2 voltage goes to zero, the ignition advance goes to 24 (is this too much?), the engine stops, and no codes are thrown.

i think that still leaves me with intermittent ASD relay failure (which i have read can lead to engine stalls shortly after stalling, but it seems like that behavior would be repeatable on subsequent starts after the first stall) or something causing the o2 voltage to go to 0.0, sending the ignition advance to 24 which might just be too far advanced for the engine to keep running? or (please not this) an intermittent PCM failure?

someone on another site had suggested these steps to test the ASD relay:
1. You should always get 12 volts at pin 16 (red wire) because it is connected to the battery. You can test that by removing the ASD relay and measuring the voltage at socket 16. If you don't get 12 volts then the fuse is bad or the wiring from the battery is bad.
2. When the relay is closed, you will get 12 volts at pin 17 (green/orange wire)
3. But in order to close the relay, you must put the relay back in the socket and turn the key on. In order to measure the output voltage while the ASD relay is in the socket, you need to put a jumper wire into the socket at pin 17, insert the relay into the socket, turn on the ignition key and measure the voltage at the jumper wire. If you don't get 12 volts, either the relay is bad, or the circuit that energizes the relay is bad. Either way, you won't get a spark.

i'll add a jumper wire to socket 17 to see what the voltage is next time it dies (hopefully it'll die again in the morning)...

 

just checked terminal 87 (socket 17) on the ASD... i see continuity to ground, 14+ volts when the engine is running, and a few millivolts when the key is on, but the engine is not running.

am i incorrect in thinking that this is not a good test of an intermittent failure in the ASD relay because whether due to the O2 sensor or the ignition advance, the PCM will kill the engine just the same as if the ASD relay had an intermittent failure causing its contacts to separate?

 

2 immediate and easy suggestions..(I think you are on the right track)

1) swap the ASD relay with an adjacent one...notice several are the same ? (on my 96ers)...i think the A/C relay is the same, see what happens.

2) Unbolt the PDC and carefully lift it up to examine the underside especially, wiring etc, female slots for the relay legs etc etc

verify a good earth from ASD relay (unless earth is supplied by PCM)..I cant remember. but i think it gives 12v so earth must be wires/connectors

I have a publicly available link to download the 96 FSM from my Google Drive Public, and i posted it on this site, and I recently came across it, and it still works (Google Drive is a pita)

There is also the 96 parts and operating manuals, and the drivetrain diagnostic manual. If you own a 96, please free download them, you may have to search my username

 

swapped the ASD relay with the radiator fan relay... same stalling observed for multiple starts... it starts and runs fine for 5-10 minutes before dying.
O2 volts = 0 is coincident with the stall, at which time there is a delay which averages about 1 minute (64 seconds) before O2 volts starts building back up.

So......

• it seems unlikely to me that both relays would fail in the same way after swapping, so i don't believe the ASD relay is the problem (unless they had the same duty cycle history--not impossible, but it doesn't feel right)
  • i pulled the ASD relay and found that it activated with as low as 4.8 volts, and stayed activated down to about 3.6 volts...
• the stalling seems most likely to happen in the mornings after a cold night
• this morning, it stalled after 7 minutes, trying to immediately re-start failed
  • but turning the key OFF and then re-starting was successful, then it stalled after running about 3 minutes

These behaviors have me thinking that the ground from the PCM to activate the ASD relay is failing intermittently, in a time-dependent manner that may be related to temperature (say, a component in the PCM heating up?) and i find the delay before the O2 sensor starts to recharge suspiciously consistent (time for a component in the PCM to cool down?)... i know the harness wiring/grounds could be playing a part, but having refreshed all the grounds i consider them unlikely, and considering the apparent correlation between stalling, temperature, and O2 voltage recovery time, i have a hard time imagining how a wire could lead to that behavior (not saying impossible, just saying it's hard for me to imagine (then again, as some have said "it's a jeep"))

seems like the next thing to test is stalling behavior with the ASD relay jumped to ground so that it doesn't rely on the PCM? seems it would also be prudent to gather temporal data directly from the PCM ground rather than trying to infer it from the OBD data...

i'm curious: i know the O2 sensor voltage should vary between 0-1, but as values vanishingly small indicate a mixture approaching atmospheric mixtures, it seems to me that O2 going to 0.0 is indicative of something else... if the heater suddenly failed, would the value immediately go to zero? or does zero indicate a complete loss of signal to the PCM just as if i'd cut the return signal wire?

 

i pulled the ASD relay and added a jumper between 30 and 87...

engine started fine, ran for about 12 minutes, then O2 voltage went to zero, the engine stalled, but this time the O2 voltage stayed at 0.0 and didn't start building up again as it's done when the ASD relay was in place...

while this doesn't rule out a possible fault in the ground from the PCM to the ASD relay, i believe it shows that there is at least one problem elsewhere.

is there another way the PCM can shut the engine down besides the ASD relay? or is it a case that if the O2 voltage goes to zero, the PCM tries to make the
fuel/air mixture so lean that it ends up shutting off the fuel by driving the injector pulse duration to zero? and i wonder why the O2 voltage would stay zero instead
of building back up? is it only the O2 heater that is energized by the ASD relay?

Happy Thanksgiving Everyone!

 

Have you replaced have you replaced the front O2 sensor with a quality item (after first CATEGORICALLY ruled out its wiring) ?

That would be my first "suspect" (after bad harness or relay socket on PDC)

As to the way the engine operates with ASD etc, their is a long section in the FSM that goes through this in elaborate detail

The "Drivetrain Diagnostic Manual" is written by Chrysler to assist techs back in the day to actually fix them. It has flowcharts. Even though it is for OBD1 and talks of the DRB scanner, its all useable for OBD2 and use a multimeter, that is on my Google Drive free download

 

I haven't replaced the O2 sensor yet, because:

• i have yet to categorically rule out its wiring
• and i was so enthralled with the OBD data that i neglected to consider the case where the O2 sensor might've been correctly indicating an unrealistically lean mixture for a running motor
  • kept my eye on the fuel pressure gauge during one of my tests... noticed it was on the low side (38 psi) and the idle was getting a little rough, but didn't die
  • so i did another test, and after letting the O2 sensor settle into its normal pattern, i pulled the Fuel Pump Relay
    • which caused the engine to die after a couple of seconds
    • caused the O2 sensor voltage to go to zero, then take it's customary 60 or so seconds to start ramping up again (see below).
    • so i'm thinking i should focus my efforts on the fuel pump and its circuitry--from the low pressure i saw when i kept my eye on it, it seems clear that the high pressure i noted the other day might not've been maintained when i wasn't looking...