99 Cherokee Turns Over But Doesn't Start - JeepForum.com
 
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post #1 of 4 Old 03-20-2017, 05:08 PM Thread Starter
sp1ffyj1ffy
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Unhappy 99 Cherokee Turns Over But Doesn't Start

Hello all,

I recently drained my battery the other day by leaving the lights on. I got the battery recharged at the auto shop and put it back in. Now it will turn over but won't start. We narrowed it down to a spark issue after testing for fuel problems with some starter fluid. We have since replaced the Crankshaft position sensor, camshaft position sensor, coil, and distributor cap all with no results. we have also recently replaced the throttle position sensor and an O2 sensor. My dad is coming up with some tools and a code reader tomorrow to try and tackle it and would love any suggestions.

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post #2 of 4 Old 03-21-2017, 05:52 AM
irishlt
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Hate to say it, but crank sensor would be the first culprit. Followed by coil.

There's always a chance you picked up a bum cps. Another easy thing to eliminate is to swap one of the other relays in for the asd relay to make sure that's not causing issues.

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post #3 of 4 Old 03-21-2017, 08:07 AM
CJ7-Tim
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Testing the CPS is a good idea.



For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.


A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at theignition coil can also cause similar symptoms.


The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will usually relearn the idle settings after a short period of driving and the engine will start and idle normally.


Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.


Ifthe symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage,the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

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post #4 of 4 Old 03-21-2017, 08:09 AM
tjwalker
It's the crank sensor!
 
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*Did you actually pull a plug, ground it, crank engine and observe? Knowing if you have NO spark or WEAK spark is a very valuable data point. You are looking for a strong, blue, snapping spark. Yellow/orange/white indicates a weak spark which may not be strong enough to start the engine!

*What brand crank sensor did you install? No lack of crap crank senors available. For cam and crank, I go with nothing but genuine Mopar.

*Your ignition coil can be tested for both primary and secondary resistances with a manual and a meter. Suspect the ignition coil for "weak spark" condition.

*Below is more on spark. You could try swapping the ASD relay with another one for starters. These relays are all of the same part number so swapping is not a problem.
----------------------------------------------------------------------------
Spark: 12 volts from ignition switch to the ASD relay; when the key is first turned on the PCM provides a ground to the ASD relay to energize it, then the ASD relay passes 12 volts to the coil primary side. If no crank sensor signal is generated after a couple of seconds the PCM cuts the ground to the ASD relay (and fuel pump relay too) and that cuts power to the coil (and fuel pump). Key to START and once again the PCM energizes the ASD relay. Now, a critical element is the crank sensor--no signal from the crank sensor and the PCM won't trigger the coil to provide spark.

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Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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