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No fire, Not the CPS, i think

cushdaddy

NAXJA Forum User
Location
Xenia
New to this site, but not new to Jeeps, Have a 69 Commando, had a 89 XJ waggy, and a grand waggy. New Jeep project is not going smoothly. 1998 Cherokee Sport. Just bought it from a guy at work. Sorry, this is going to be sort of long.

History.

Previous owner had new radiator installed at a shop. 3 days later it overheated and the radiator blew the sides out. He parked it because he thought the engine was blown. 3 years later I bought it. It was stored in a garage and there is no evidence of mice. He has a lot of cats.

11-1-13 Got it home Friday night and parked it.

11-2-13 Saturday, Pulled the battery cables and cleaned the corrosion and reconnected. Started charging the battery. Flushed the old coolant out and cleaned everything. Then, new radiator, plugs, wires, cap, rotor, t-stat. Double checked everything and tried to crank it over. Cranks over, no start. Voltage gage bounced around when I tried to start it. Checked and no spark. Read about the problematic CPS and disconnected it to see if the gages would come back. No fuel gage or voltage gage.

11-4-13 Reading on here and the Jeep forum about how to get spark. Tried a few things, installed Autozone CPS because they were open, but had to extend the wires. Checked the O2 sensor wires for shorts, I don't have the manual yet so I cant trace the ignition path. I have checked the fuses and the relays. The fuses check good and there is battery voltage at the relays.

The fuel rail is pressurized, so I assume the pump is working.

I am assuming that the ASD relay is not energizing. Should I hear it click when I plug it in with the ignition switch on? I have switched it with other realys and same problem.

I have not had a chance to get in and clean all the grounds, but from the ones I have checked they all look OK, But will be cleaning when I can.

If the plug on the back of the instrument cluster is not seated, will this cause the ASD relay to not energize?

Thanks in advance, hopefully I will have the manual tomorrow, I feel lost with out one.
 
1. Are you using the same battery that was in there when you bought it? If so, get a new fully charged battery in there before doing anything else. Storage kills batteries. Trying to start your XJ with a battery of unknown condition isn't likely to work well; you need a certain amount of available voltage to start your engine.

2. If you have no fuel or voltage gauge, the crank sensor is suspect and Autozone crank sensors have a history of reliability issues. I use only Mopar crank sensors purchased from Jeep.

3. Test your ignition coil. They can be tested for both primary and secondary resistance with a meter and a manual.

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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I knew the Autozone sensors had a bad rep, but like I said, they were open.

I get the logic flow on the start and ASD relay. PCM making and breaking ground. That helps.

From what I read, I thought that disconnecting the CPS should allow the gages to function again. Mine don't come alive with the sensor disconnected.

I had the charger on the battery the whole time I was working on it and testing it. I was checking voltages in the 14.25VDC range. I did some more reading and I will be cleaning all the grounds s soon as I get a chance. and getting a factory CPS.

This sounds like the CPS problem, but not having the gages work after disconnecting the CPS made me wonder.

This also has an automatic with the 4.0l I guess I didn't state that earlier.
 
Voltage testing is inconclusive when testing a battery. If you want to know the condition of this battery, take it to any parts store and have them "load test" it; they will do it for free. Load testing is the only conclusive method for testing a battery. My bet is that it will fail.....not many batteries will survive 3 years in storage without a battery charger (trickle/tender) hooked up. Getting a solid battery is absolutely job 1. Basics first always. Failure to do so will result in the chasing of your tail. I enjoy chasing tail, just not my own! :)
 
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My guess is what sometimes makes the gauges not work, is the same thing that makes my CEL not light (in my 96) when I have a sensor short. A partial short in the 8 volt, voltage in, feed for the CPS will cause grief. Multiple senors share that 8 volt feed, vehicle speed sensor, and the sync sensor on my 96 and a 97, not sure about a 98 though (sensor wiring sometimes changes, but that orange wire to the CPS seems to be a constant), Any sensor (on that 8 volt feed) that sucks down that 8 volts to the CPS is likely to give you no fuel and no spark issues and may mess with the PCM enough to shut down the gauges and/or the CEL.

Coolant really sucks when it spews all over your motor, the glycol sucks up water from the atmosphere and stays wet. It conducts electricity a whole lot better than plain water and can cause line leaks and shorts. After frying an alternator and pulling my hair out chasing down various sensor issues caused by coolant seeping into connectors, I am now pretty darned conscientious about cleaning up spewed coolant.
 
So after installing a new battery, a factory CPS, and clean all ground points, I should(if still no spark):

Check wiring harness for leaks to ground.

Pull sensor plugs one by one till gages come back to life, then chase the source.

Check/Reseat all connections in the engine compartment.

My thought right now is, if the gages start to work, I have found the problem, is this a reasonable assumption?

Thanks
 
So after installing a new battery, a factory CPS, and clean all ground points, I should(if still no spark):Thanks

1) Check for the 8V supply at the CPS plug. You do have a voltmeter, right? If it is missing, then proceed with the rest of this. If it is present, a shorted sensor is not your problem. The shorted sensor (and no gauges) is only one of the symptoms (at a very low rate) of a failed CPS. The most common is a no start condition. This can be intermittant and usually begins when the sensor is hot, but it may just not start one day, after running every day for years.

Check wiring harness for leaks to ground.

Pull sensor plugs one by one till gages come back to life, then chase the source.

Check/Reseat all connections in the engine compartment.

My thought right now is, if the gages start to work, I have found the problem, is this a reasonable assumption?

Thanks
 
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I checked all the combinations on the CPS plug and got .3vdc. Not even close to the 8vdc stated.

If the alternator fries, will it cause the same conditions that I am having?

Thanks
 
I put the charger on the battery and checked the voltage again. The orange line on the CPS went from .3vdc no charger to .45vdc with charger. The coolant temp sensor also had the .45vdc on it as well. I had no voltage on the coil plug.
 
Looking at the Haynes manual ignition wiring diagram and while it is somewhat helpful, I have no idea what the pinouts on the PCM are. It looks like all the black/lt blu lines run to the TCU, and if the orange line for the CPS is +8vdc, I am assuming that the +8vdc supply to the other sensors is supplied by the orange/other color wires, and the signal wire is the other.

If my reasoning is right, the CPS, Sync sensor, distance sensor, tps, all have an orange wire and could be pulling the +8vdc down.

I see that the blue/yellow is the ground for the ASD and fuel pump relay.

Anybody willing to PM me a pinout of the PCM?
 
Most all of the sensors have voltage in (sensor voltage supply), it varies by model year. Also varies by how the senors are grouped, which ones seem to share the same supply voltage.

I think you were on the right track when you asked about unplugging sensors until the gauges came back. If the short is in the wiring you have to unplug them all anyway to isolate (segregate) the various sensor circuits to check for a wire short.

There are different sensor voltage supply circuits, some with a different senor supply voltage, it is feasible they are wired in parallel inside the PCM and what affects one sensor supply driver may affect the driver right next to it. In other words it may be a temp sender or even an O2 sensor that is causing the voltage issues, not for certain, but it seems a possibility. But that 8 volts in and you are measuring 3 volts, sounds like you have a short in a sensor in the same sensor voltage supply loop and not a neighboring loop. Sync sensor, CPS, and speed sensor are the three in that 8 volt voltage supply group, as far as I know. But the electrical diagrams for the later year XJ's are hard to read and then figure out the associations between various circuits. They have the wiring diagrams broken down into very small bite sized pieces and it takes a lot of page turning (and a little luck) to get the big picture without brain burnout.
 
Thanks 8Mud,

I pulled all the sensors that I could find, while checking to see if my gages come back. Nothing. I probed the PCM a bit and some pins with battery voltage and some with 9 volts.

I don't have an owers manual with the fuse box diagram or a pinout of the PCM.

If anyone could help with that it would be nice.
 
So I have been checking the pins on the black connector on the PCM and the last 2 pins are suppose to be ground. When I check between ground and those 2 pins, I get 1 vdc. When I disconnect the white plug in the middle, I get 0vdc between the pins and ground. I have disconnected the injectors , generator field oil sensor, speed sensor, everything except the Torque Convertor Clutch solenoid. and I still get 1vdc between the 2 ground pins on the PCM and ground. The 5V power at A17 is 1.25V. I have 1.25V at many of the sensors with them all disconnected.

Should I open the PCM and look for shorts, or should I be looking at the harness?

Thanks,
 
I'm far from an engineer, but in my experience ignore the 1.25 volts. Could be bleed over from another circuit through moisture someplace, combined with an iffy ground or even inductance voltage your meter is picking up.

Make sure your battery and chassis grounds are good and make your test to chassis ground (after ohming the chassis to negative battery pole) and ignore the sensor ground, except to check for standing voltage or do an ohm test.

Harness shorts are possibility, a wire burnt on the exhaust manifold or someplace else (the rear O2 sensor is a prime spot to look). But IMO it is more likely to be shorted sensor.

Just a guess, but likely valid in simplistic sort of way. The sensor voltage drivers share a common power source, a ladder or parallel circuit inside the PCM. Every rung of the ladder draws amperage, when one draws too much the others get starved. Exactly how the circuits interact is for an electrical engineer to determine (ad they don't always get it right).

Different years cluster the sensor supply voltage with different sensors, I don't have the 98 specific wiring diagrams. Not to long ago a guy had pretty much the same issues you are having, it turned out to be the temperature sensor in the thermostat housing was shorted. How the drivers interact, on my theoric ladder circuit, are anybodies guess. But if one rung of the ladder is sucking way more amps than the design parameters, it almost has to affect it's neighbors on the the same circuit (inside the PCM). For instance if the CPS shorts the gauges stop working, it is doubtful the gauges share the same power supply as the CPS (but heck most anything is possible), more likely IMO is the gauge circuit gets starved for power because some interrelated circuit (in the PCM) is sucking too many amps and/or generating too much heat which degrades function of some components..

Personally I'd eliminate all possible external factors before I started looking at PCM failure. A few reasons, PCM failure is rare, sensor shorts aren't. Sensors are relatively inexpensive, PCM's aren't. And lastly. even if it is an internal PCM failure, in all likelyhood something external caused the failure and installing a new one may just be a waste of time and money, a new one just might fail again in the same way.
 
I have pulled every sensor connector and my gages still don't come back to life.

I still haven't put a new battery in it, but I am trying to figure out how that would kill the 2 gages. I pulled the dash and reseated the gage cluster and everything under the dash looks clean and tight. I keep testing for 8vdc at the CPS, and will all the sensors disconnected, I only get 1.25vdc.

This is the same voltage I get when I check the PCM ground to chassis ground.

I have cleaned the grounds by the PCM, the coil, and the battery. I think I am going to pull the kick panel off and check the wiring around the fuse block.

This jeep did get hot and blew the radiator, I don't see any chaffed or melted split loom or loose wires. If the alternator got drenched, could there be a problem there? IS there a fuseable link somewhere that could be blown? I am still getting the no buss OBDcode as well.

Thanks for the help. This really shouldn't be this difficult.
 
Reading another post, I noticed a reply that said if the battery was disconnected or reconnected with the ignition on, there could be some deeper problems, what would those be?
 
Reading another post, I noticed a reply that said if the battery was disconnected or reconnected with the ignition on, there could be some deeper problems, what would those be?

From what I've read it says it can scramble the PCM programming. Seems ike it is enough of a problem so they put out a TSB, telling you not to do it. Most times in earlier models it was associated with voltage regulator issues in the PCM.

Have you checked the PCM *in* voltage and the switched voltage from the ignition that turns on the PCM?
 
I just put in a new battery and got the same result. 0.5vdc at the CPS plug. I have 0.5vdc at both the left and right terminals. Since I don't have the wiring diagram or pinout specs, I don't know what I should be reading on the meter.

I have 8vdc on A02 on the PCM pin which is the "fused ignition switch output" as far as my diagram is concerned.

I have 12.5vdc on A22 which just says "Battery 12v"

If I scrambled the voltage regulator, can this be reprogrammed at a dealership or should I find a salvage unit?
 
I have a 97 and a 99 set of diagrams, what I normally do for a 98 is look and see what the consistencies are (what's the same) and make assumptions. One leg of your CPS should be voltage in either 5 volts (mid nineties), 7 volts (early nineties) or 8 volts (late nineties), my guess is your year is 8 volts. But like I said it is just a guess.

I have no idea what programming is affected by arching the battery poles connecting them with the ignition on. I've never seen it myself, just heard the horror stories.

I'd like to help you out, but I just had an eye operation and working on the computer is a real chore for me. My son is calling me Mole man. Doc says it is going to be a month before my eyes are anywhere near normal again.
 
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