Going through fuel pump failure again on my 94 JGC. No pressure at the fuel rail and no error codes. Got almost 4 years out of this one.
Interesting failure mode this time around. I had been experiencing a miss at idle at stop light for a couple weeks. Shifting to neutral instead of drive with brake on had no effect.
It is dark and cold right now in Colorado so I will swap out the fuel filter tomorrow and check the pump connector for corrosion when it is light.
snipped-for-privacy@mail.com did pass the time by typing:
It might be the pump, but I'd check the fuse and relay first.
I checked the error codes again and it threw a "41": "Alternator field control open or shorted" according to Allpar.
Interesting observation, before I turned it off there was a lot of static pops on the radio, sounded like ignition noise. Never heard this before.
So fuel pump may be good to go. A bit more digging is in order.
With the rough idle, "noise" and no start could bad ignition wire. There was a TSB also, 18-34-95.
Short answer: bad secondary on ignition coil
Long answer: Swapped out spark plugs wires on Thursday morning, $20, at Autozone. Existing wires were the OEM Mopar with 12 years and 140k. Result no start.
Saturday morning: CPS is critical component of Jeep 6 cylinder 4.0L ignition system. Decided to throw a new CPS at the problem even though no diagnosis performed and no error code "11" on the OBD. Picked up CPS, $68, and a Haynes repair manual JGC 1993 through 2003, $20, at Autozone. BTW Autozone cannot test CPS. Forgot about the PITA reaching the 11mm bolt. Reference
Swapped out CPS. Result no start.
Saturday night: crack open the Haynes and load the Popular Mechanics/Alldata 1994 Jeep CDROM I picked up in Kmart bargain bin 6 years ago for $5. Decide to actually test the basics Sunday morning: air, fuel and spark.
Air is pretty simple loosen throttle body clamp from air filter box hose and pop off hose. Fuel is pretty simple remove second quick connect line on rail, first is pressure regulator. Turn ignition switch to on with rag covering line. Fuel was flowing so pump is good to go. Spark is simple use old timing light with induction clamp around spark wire. Removing plug and shorting out can be hazardous to PCM as wells as yourself. No spark.
Next test is see if voltage and "pulse" are getting to coil to test PCM and circuit. Turn to run and there is 12 volts at coil connector. Crank engine with test light connected between both connections to coil. Light pulses. So PCM and circuit are good to coil. Spark plug wires were swapped so they are not suspect. Primary resistance is in specification 1.2 ohm as given by Haynes. This is easy to test using the large wiring connector under the battery tray vs. pulling the coil. Secondary coil test is performed using the large wiring connector and pulling the coil to distributor lead. This test as 5.8k ohms. Very low per Haynes specification of 11k to 13k ohms. Call Autozone and they can test coils. I pull the coil. The 10mm bolts have a free nut on the back of mounting bracket so you have to use combination wrench as well as socket to pull it off. I have the counter man test the suspect coil. The test turns out to be what I just did but I but a $65 CPS serves as a reminder to measure twice, cut once. Install replacement coil and engine fires up.
Another recurring problem on the 94 JGC appears to have been fixed, hard shifts between 2 & 3 on the 42RE when cold. I have not looked at the diagrams enough to see if the TCM has feedback to the ignition circuit.
Very good reference to ignition coil resistance by Chip McCoy,
Who needs an oscilloscope.
For a Jeep all you need is a rag, test light and ignition key:
Rag to catch gas coming off the fuel line quick connect. Test light to hook up to the coil connection. Ignition key to get the OBD codes, if any.
Does not get any simpler than that ;)
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