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electrical gremlins

SightlierDuke23

SightlierDuke23

NAXJA Forum User
Location
Charlotte NC
Hey all. I have a 96 I4, aw4. So yesterday my jeep started fine drove to the end of the neighborhood, and poof shuts off. Try to start it and it just cranks. I had no spark anywhere. I have no CEL, I replaced the CPS, cap, rotor, coil pickup, plugs, and wires. I am getting fuel to the rail and it is at 45, so fuel is not the issue. We ran the test for the coil and it checked out fine, but i am replacing that for good measure anyhow. Is their something else i should be looking at as well? Grounds are good and battery tested good too. Im about to strike a match. Any input would be greatly appreciated!
 
I would first give it a few squirts of starter fluid
down the throttle body, just to rule out a fuel
delivery problem.

A scan tool might show a pending code that would
give an idea of what's wrong. The 96 model year
is unique in that it shows both OBD I and II codes.
 
ComPuter? Can you smell fuel at the tailpipe when you crank? You can have fuel at the rail but no computer means no injector action I think. Correct me if I am wrong.
 
Last edited:
Did you test fuel pressure while it was cranking? The fuel rail will prime nd pressure up any time the key goes to or past the run position, which doesn't mean the fuel pump relay is closing, that only happens if cranking (or running) and the ASD relay is closing.

Do you get a CEL when you turn the key to run, a lamp check CEL?
 
Runnin'OnEmpty said:
I would first give it a few squirts of starter fluid
down the throttle body, just to rule out a fuel
delivery problem.

A scan tool might show a pending code that would
give an idea of what's wrong. The 96 model year
is unique in that it shows both OBD I and II codes.
Click to expand...

Yeah, tried the starter fluid, no change. And nothing pending when using scan tool
 
8Mud said:
Did you test fuel pressure while it was cranking? The fuel rail will prime nd pressure up any time the key goes to or past the run position, which doesn't mean the fuel pump relay is closing, that only happens if cranking (or running) and the ASD relay is closing.

Do you get a CEL when you turn the key to run, a lamp check CEL?
Click to expand...

Yes, went up slightly while cranking, and no CEL in the run position. Forgot to mention i replaced the ASDrelay as well
 
Last Tuesday,My 96 XJ died while running 55 down the highway,Everything quit.Had power ,would crank,but not fire.Towed to shop.
Tried to hook up scan tool-no comunication ?
Further testing showed power to all relays,however no signal voltage to any sensors,,,,,,catastrophic computer failure was the cause.
Replaced the computer with re-man from NAPA ----started right up, running fine.
YMMV,,,,its a costly part to throw at your XJ ,but dont rule it out.
 
I still suspect something is drawing down the 5 volt supply voltage to the CPS, this should shut down your ASD and fuel pump relay also, but I doubt I've seen every possible outcome of low 5 volt sensor supply voltage possible. I usually test by unplugging sensors until the CEL light comes back on.

Try volt checking the coil in and out. Best to check while it is cranking. One side should be constant battery voltage in, the other side should be a slightly lower voltage that pulses while the engine is cranking. And maybe ohm test the high voltage wire from the coil to the distributor cap while your down there.

No "check light" CEL with the key in the run position, usually means a sensor short (or a burnt out bulb). A computer (PCM) is an expensive hunch, most sensors can be tested. A shorted sensor and the PCM shuts itself down. This usually means no ASD, but like I said, I doubt I've seen every possible failure scenario.
 
Last edited:
Update, i replaced the coil, still same issue. There was a procedure to test the connector to the coil. Says to set to dc/v and you should read either 0 or 12v while cranking. 12v being correct. I get a reading of .845 not sure if this gives anyone an idea. testing the coil out is nil.
 
The dark green and orange wire going to the coil is the power in wire and comes from the ASD relay, 12 volt +/-. The other wire (I forget the color, grey I think) is the trigger wire and will be about the same voltage as the *in* wire until the PCM pulses (grounds) it. The same wire (dark green and orange), through a splice is the power for your injectors (and other stuff). Have you checked the 30 amp fuses in your PDC? Fairly common for an O2 sensor heater to short out and blow that fuse.

The slight voltage you are reading is likely to be standing voltage from a shared ground someplace. It doesn't mean much.

If your CEL isn't lighting up when you turn the key to run, the bulb is burnt out or (usually) you have some sort of sensor short and the PCM shuts down the bus to protect itself.
 
If you have a CEL, try the key method and see what kind of codes you have.

I really don't know if you have a wiring issue or the PCM is shutting down the ASD relay. The PCM will fail to close (ground) the relay coil for various reasons, one being no CPS pulse signal while cranking.



Pin 30 is power in, pin 87 is power out. You can try an ohm test between 87 and the coil (dark green and orange wire). 86 should be hot with the key in the run or start position. The 85 pin goes to the PCM and the PCM grounds the relay coil to close the relay.
 
Re computer cost - I see the used pulls around here at the wreckers for between $50 and $75. Can always resell it on ebay/craigs list if it turns out not to be the problem.

Of course, I would only do it only after checking other free/cheaper to fix possibilities.
 
UPDATE Figured it out. It was the CPS. original was bad, replaced it with one from the dealership.....which was also bad. a buddy of mind has his engine out and let me try his, and it fired right up. Went back to the dealership, picked up a replacement for the first one and all is good. Apparently factory isnt always fool proof plan, but everything is kosher now! Thanks everyone!
 
8Mud said:
I still suspect something is drawing down the 5 volt supply voltage to the CPS, this should shut down your ASD and fuel pump relay also, but I doubt I've seen every possible outcome of low 5 volt sensor supply voltage possible. I usually test by unplugging sensors until the CEL light comes back on.

Try volt checking the coil in and out. Best to check while it is cranking. One side should be constant battery voltage in, the other side should be a slightly lower voltage that pulses while the engine is cranking. And maybe ohm test the high voltage wire from the coil to the distributor cap while your down there.

No "check light" CEL with the key in the run position, usually means a sensor short (or a burnt out bulb). A computer (PCM) is an expensive hunch, most sensors can be tested. A shorted sensor and the PCM shuts itself down. This usually means no ASD, but like I said, I doubt I've seen every possible failure scenario.
Click to expand...

this.
Just like "no bus" in a later model one usually means a 5 volt sensor has checked out.

start unplugging them one at a time and cranking it.

the last time it happened to me it was the oil pressure sender. Shut it down like I had turned the key.
 
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