I'm helping a friend with his 92 Cherokee. The computer keeps putting it into the shutdown mode, and kills the power to the fuel pump. The car runs fine on starter fluid down the throat of the carb. We have replaced the ballast resistor relay, the auto shutdown relay, and the fuel pump relay. No help.

We have replaced the O2 sensor, the air temp sensor, the throttle position sensor, no help. I checked the output voltage from the Map sensor, its 4.7V so that's okay, and that also says the power to the other sensors is okay. When we turn on the key, we get a very short (~0.1 sec) pulse on the output of the shutdown relay that sends power to the injectors. The dark blue/yellow wire from the computer that controls that relay sits at 12.2V, indicating it does not want the fuel pump on. The ECU does not report any trouble codes.




What else do I need to check in the diagnostic procedure to find out what signal does the computer think is out of spec and thus must prohibit running?

I have the online cherokee shop manual, But I don't find an electrical diagnostic procedure chain anywhere in it.

You knowledgeable help is greatly appreciated.

 

Yes, the center plug wire from the coil to the distributor could be bad. But also check the cleanliness of the interior of the distributor and the rotor. There should be absolutely no grease or oily feeling on that interior. Clean it with alcohol, not thinner or gasoline - both of which will leave a conductive residue. The conduction can then short out or weaken the spark to individual plugs.

 

Honestly, the best thing that you can do is to verify the condition of parts before you put them in. Even a new part in the box can be bad. Before you put any parts on, test them as if they were suspect (cps, tps, etc.)

I would go component by component through the ignition system from the distributor all the way to the plugs. Make sure the distributor inside is clean, like ls4 said. Then make sure that the wires are seated, etc. etc.

This is one of those issues that can give you alot of trouble.

I would do the test with the starter fluid while you are at it.

 

As I said in the original call for help, the engine starts and runs ok on starter fluid down the throat. But the signal from the ECU that controls the auto shutdown relay forces it into the shutdown mode immediately. And there are no trouble codes. So I'm looking for some kind of diagnostic procedure to follow, since we have already replaced a lot of parts without any luck on finding the source of the problem. And that includes replacing the ECU.

Just a coincidence, my friend's Jeep is also a red 92 Cherokee.

 

Crankshaft position sensor. 5 volt pulses.

 

Wouldn't the crankshaft position sensor have to be functional for the engine to run at all? Remember, the engine runs on starter fluid, but doesn't get any gas due to the shutdown relay position, which is caused by the ECU.

 

(At least in the later XJs) the engine will run for 3 seconds. If the PCM fails to see a CPS signal then it will shut everything down.

 

Besides, checking is free.

 

For 1993:

And it looks like it's 8 volts back then.

 

Sorry, I meant my response for the op. In your case, I would agree with Dave, check the CPS. It sounds like something is telling the ECU to shut it down. I don't know what else could tell the ECU to shut down.

The red is really a nice color on the XJ.

 

The 3 second rule goes back to at least 1994. But even if CPS is bad, it would seem that there's still another issue as the fuel injectors should have been working (since fuel pump relay and ballast resistor are bypassed cause voltage is supplied to the
fuel pump through the starter relay) until spark shut down.

 

Doesn't that mean it DOES want the fuel pump on?



That said, it would appear you never get to that point.

 

No, look at the schematic again. One side of the coil for the relay is tied to ignition +12V. For that relay to actuate the other side of that coil must be low. The computer pulls that pin DOWN to turn the relay ON and supply power to the fuel pump. If the computer simply releases that pin, and lets it float as an open circuit, it will then go to +!2V, as seen with a high impedance volt meter. That then shuts OFF power to the fuel pump and injectors, since there is virtually 0V ACROSS the coil of the relay.

If you ground that Dark Blue/yellow wire with the ignition on, you will hear the fuel pump turn on, but that is defeating the purpose of that safety circuit, so its not a long term solution.

 

The starter fluid would bypass the injectors, since its atomized fuel right down the throttle body. Dangerous to spray it there very long. But it would run if the spark is present. At that point the issue then becomes does it run even a few seconds but smoothly, or popping and snorting and very rough, which would indicate presence of a wrongly timed spark - back to the ignition circuitry - AFTER we first find out why the ECU wants to shut things down.

 

How exactly are you grounding DB/YL? Because if you have spark, ASD has not yet tripped so fuel pump should be running.