Don Oswald

My son’s 1996 XJ wouldn’t start after sitting for several weeks and he thought it was a bad fuel pump. Towed to my house. Replaced the fuel pump even though the old one bench tested okay. Got fuel to the injector rail but only dribbled out of the Schraeder valve when I depressed it. No spark and the check engine light doesn’t come on. Replaced the coil with no start. Checked the resistance between the crankshaft position sensor B and C pins and it showed infinite, so from what I find on the internet it should be okay. Probed the 5V wire to the CKPS with the ignition key in the on position and found only 0.625V. Did the same for the camshaft position sensor and got the same reading. Don’t know how I would probe the A17 pin on the ECU but I would surmise that I’d get the same 0.625V reading there too. Cleaned the block to battery cable connection even though it looked okay. Removed the ECU from its fender mount and shined the fender mounting bracket in case this was the ECU ground; no change in the voltage readings. Swapped the ASD relay with no change. Swapped the 30A ECU fuse with no change. From what I read the ECU controls the signal to both sensors by opening the ground circuit to them so I’m thinking the problem lies in the 5V power source.

1. Does the ECU take 12V and reduce it to 5V or does the 5V come from a separate source?

2. Does a 1996 XJ have a ballast resister somewhere that I’m not seeing?

3. Does the ECU have a separate ground that could be causing high resistance and the low voltage readings?

4. Are these symptoms of a bad ECU?

5. Any other ideas on what I can troubleshoot?

agreen

1) the ecu takes 12v and creates a 5v ref internally, no separate power supply.
2) I don't see a ballast resistor on the 96 wiring diagram (I could be wrong)
3) The place where the ECU mounts is not the ECU ground. There are numerous threads about cleaning grounds, but the one I think you want is by the ignition coil.
4) possible
5) Get a logic probe. Test for ground pulse at the 2 wire coil plug as you're cranking it. The other wire in that 2 wire connector is 12v key on.

awg

Does the fuel pump audibly whir for 3 secs each time upon turning the key to the "on" position?...it should

if it doesnt, and you have no spark, that would be suggestive of a failed PCM, or associated wiring/parts

Have a look at the diagnostics in my recently posted '96 FSM link for the ASD relay, as nothing will work if that doesnt

Don Oswald

The fuel pump does run for approximately 3 seconds.

Don Oswald

3. The ground I cleaned extensively was the one by the coil.
5. Unfortunately I don't have a logic probe.

What I did do was disconnect the MAP, TPS, and cam position sensors and probed the 5V wire (violet and white) to the crankshaft position sensor (CKPS); these were all the sensors I found on the electrical schematic that are fed by the 5V coming out of PCU pin A17. The voltage raised slightly from 0.625V to 0.775V. Disconnected the CKPS, connected the camshaft position sensor (CPS) and probed the 5V wire (violet and white), voltage was same 0.775v. plugged the CKPS and TPS back in and the voltage remained steady. Plugged the MAP sensor in and the voltage dropped back to the original 0.625V. Checked all the fuses both under the hood and by the brake pedal in the cabin and all were okay.
One thing I did notice was that the 0.775V reads on both the supply and signal wires on the CKPS, yet when I did a resistance check on the CKPS pins 2 and 3, it showed 0.L (infinite). Not sure how there could be signal voltage unless the CKPS trigger just happened to be in the right position, yet I can bump the engine over and signal voltage doesn't change. I found this millivoltage remains no matter if the ignition key is on or off so I'm wondering if that is normal. That is about all the electrical troubleshooting logic I have from my 8th grade electric shop training.
My son wants to put a new CKPS in but before we do that I'm going to put the Jeep up on jack stands and do a thorough inspection for frayed/melted wires.
Do these symptoms prompt any ideas?

Dave51

Are there any check engine codes?

Dave51

Turn the engine over by hand and monitor what you get.

NM looks like you tried that.

NM the NM, try it by hand slowly. The contact may be going by so fast that your DVM can't read it.

NM the NM the NM. If it wasn't making contact the value would be 0.0.

Dave51

I think that's the next step.

Dave51

Specifically, all the power to (K6) stuff:

Don Oswald

Check engine light doesn't come on and when I connect my OBD II reader all I get is NO LINK. Tach doesn't move when I try to start it. Bypassed the ASD and still no start.

Don Oswald

awg that FSM link is awsome. Not sure I understand everything but it does help some in my understanding how this system is supposed to work.

Dave51

Start here:

https://www.cherokeeforum.com/f2/dlc...r-obd2-162775/

Don Oswald

I have power to the DLC. I plug in my OBD II reader, it powers up but after watching the ***** scroll across the screen a bunch of times I get the NO LINK readout.

agreen

Oh you have NO 5v ref at all! I get it now. If you have no CEL and no 5v ref, you could cut the 5v ref wires right at the ECU to see if something is dragging it down. If you get 5v at the cut, you could have a short in the wiring. If the 5v doesn't come back, it's likely a bad ECU.

https://www.harborfreight.com/comput...obe-63597.html

Don Oswald

Thank agreen. Before I start cutting wires I'm going to put the car up on jack stands and crawl underneath to see if the O2 sensor wiring is is tact and inspect for any other bad wires. It is just odd that it had been running fine and then after sitting for several weeks the problem cropped up.
Thank you for the HF link. Of course, the store a couple of miles down the street is out of stock and the next nearest one is 25 miles away. I will get one though.