Cherokee won't stay running.

My 87 XJ 4.0 auto dies after starting as soon as you release the key from the start position on the ign. switch. I replaced the switch and it does the same thing. If I hold the switch on the start postion the engine will run,albeit with the starter engaged and running, as soon as I release the key to the run postion it dies. Any ideas would be greatly appreciated.

John

The usual answer to this is fuel pump ballast resistor. However, the fact you have an '87 complicates this. The original 4.0L models in early 87 didn't use the ballast resistor. Some people complained that the fuel pump was too noisy, so a ballast resistor was added to drop the voltage, which reduced the pump speed and thus the noise.

This was a mid-year change made during 1987, and there was also a TSB (technical service bulletin) on it, so some vehicles were retrofitted. What you're looking for is a whiteish ceramic block about 1/2" high and 2-1/2" long, mounted on the inside of the driver's side fender next to the air box. It will have two fairly heavy wires coming off it. If you see that, try jumpering the two wires together. If it solves the problem, you can either buy a new ballast resistor, or bypass it permanently.

The other thing to check is the starter relay, which is on the passenger side in the relay bank just behind the battery.

OK,I was hoping it would turn out to be simple like the resistor but nope. I've got power to one side of the resistor when cranking by my test light but it goes off when I release the key. Should I have power at the resistor when the key is in the on position? I have a Haynes manual but it doesn't have any mention of the relays or resistors or a wiring diagrams of the fuel or ignition.

I'm not sure which relay is which but they're four of the same so I switched them one at time and everthing remains the same.

John

To test the resistor, just use a clip lead and short across the resistor and see if it runs. The resistor is just there to quite the pump and make it last a little longer. If shorting out the resistor makes things run, its the resistor. If not it is most likely the relay.

If you are unfamiliar with the component locations here are a couple of pix to help

http://www.4x4getaways.com/XJRelays.jpg
http://www.4x4getaways.com/XJBallast.jpg

Skyhawk1 said:

OK,I was hoping it would turn out to be simple like the resistor but nope. I've got power to one side of the resistor when cranking by my test light but it goes off when I release the key. Should I have power at the resistor when the key is in the on position? I have a Haynes manual but it doesn't have any mention of the relays or resistors or a wiring diagrams of the fuel or ignition.

I'm not sure which relay is which but they're four of the same so I switched them one at time and everthing remains the same.

John

Click to expand...

One side? Which side, upstream or downstream?

I don't even know if you should have power to the resistor when cranking, because the crank circuit bypasses the resistor and sends full voltage to the fuel pump for starting. That's why an XJ with a bad ballast resistor will start, but quits as soon as the key is released to the RUN position.

With the ignition on in the RUN position, you should have power on BOTH sides of the ballast resistor. After the vehicle has sat for awhile, when you first turn on the key (to RUN, not all the way to START) you should hear the fuel pump running for a few seconds while it preesurizes the system. If you're not hearing that, don't have 12 volts on both terminals of the resistor with the key in RUB, and/or don't measure continuity across the resistor with a multi-meter ... you probably have a bad resistor. As I wrote before, just jumper it with a 14 or 12 gauge wire as a test.

Thanks for the ideas and the diagram. It's nice to know what's what. It appears that the resistor and the relays are OK. Someone suggested the ignition module but I don't know if I can check that and they're a bit pricey at the auto parts store, might have to see if I can get one at the bone yard and give it a try. Thanks.

John

Eagle, first thing I did after your post was jump the wires on the resister, then I checked for power on either side of the spades with the wires connected and the key on run, then probed them each disconnected with the key on run, then each while cranking. The only power shown was while cranking and it was the wire closest to the front.

John

Skyhawk1 said:

Eagle, first thing I did after your post was jump the wires on the resister, then I checked for power on either side of the spades with the wires connected and the key on run, then probed them each disconnected with the key on run, then each while cranking. The only power shown was while cranking and it was the wire closest to the front.

John

Click to expand...

??????

Impossible.

If you jumped it and power got to either terminal, power would HAVE to get to both terminals. Either I'm not explaining correctly how to check it, or you're not explaining correctly what you did.

1. Power (ignition) off. Set a multimeter to the OHMS range and put one lead on each terminal of the ballast resistor. Is there continuity? (i.e. do you get any reading at all?)

2. No jumper: key in RUN position. Is there power to either terminal (or both terminals) on the ballast resistor?

3. Jumper the two terminals on the ballast resistor. Start the engine. Does it quit when you release the key?

4. Leave jumpers on the ballast resistor. Crawl under the back of the vehicle. The fuel pump is mounted in the front of the gas tank, high. There are three wires coming out of it. About a foot to 18 inches forward of the fuel pump is a weatherpack connector. Unplug it. Key in RUN position. Multimeter set to DC VOLTS, 0-50 volts scale (or 0-25 volts if your meter has that). Connect the multimeter to the pink/black wire (+) and the black wire (-). Is there a reading? Should be about 12 volts.

5. If the answer to #4 is "Yes," now remove the jumper from the ballast resistor and probe the connection at the fuel pump again. Is there voltage? If so, it should now be approximately 9 volts, because it is now being reduced by the ballast resistor.

Let us know what you find when you follow this protocol.

Eagle said:

??????

4. Leave jumpers on the ballast resistor. Crawl under the back of the vehicle. The fuel pump is mounted in the front of the gas tank, high. There are three wires coming out of it. About a foot to 18 inches forward of the fuel pump is a weatherpack connector. Unplug it. Key in RUN position. Multimeter set to DC VOLTS, 0-50 volts scale (or 0-25 volts if your meter has that). Connect the multimeter to the pink/black wire (+) and the black wire (-). Is there a reading? Should be about 12 volts.

5. If the answer to #4 is "Yes," now remove the jumper from the ballast resistor and probe the connection at the fuel pump again. Is there voltage? If so, it should now be approximately 9 volts, because it is now being reduced by the ballast resistor.

Click to expand...

Clarifications:

In #5, he may still have the harness unplugged from step 4. If so, he will see 12v, not 9, because there is no load. He'll have to reconnect and backprobe to read 9v at that connector. However:

The ECU only turns on the fuel pump in key-on mode for 1-3 seconds to prime. It turns it off and then turns on again at cranking. [RENIX manual] In #4, he won't read 12v but for 1-3 seconds. Now if he wanted to stay there while someone cranked it..... If it doesn't stay running he may never see the 9v by backprobing either.

I hope that helps.

skipc said:

The ECU only turns on the fuel pump in key-on mode for 1-3 seconds to prime. It turns it off and then turns on again at cranking. [RENIX manual] In #4, he won't read 12v but for 1-3 seconds. Now if he wanted to stay there while someone cranked it..... If it doesn't stay running he may never see the 9v by backprobing either.

Click to expand...

Good point on the priming. But if he runs the check with the fuel pump unplugged before priming the system, the ECU won't register pressure at the regulator and the current should stay on. He could hit the Schrader valve on the fuel rail to be sure the pressure has been dumped (be sure to catch the gas that squirts out in a rag).

OK, sorry if my post was a little confusing yesterday all I had was a test light. Got my digital VOM back and checked for continuity across the resistor and it is good. Released the prime on the fuel rail and turned the key on while the resistor was jumed and volt meter hooked to the jumpers, no voltage. Cranked car and after a couple of seconds cranking the car started and voltage showed 11+ as soon as I released the key the voltage stopped and the car stopped running.

I was told to check fusible links, the only wires I saw that looked like they might be fusible were the hot wires coming off a common terminal in the relay center with fat black rubber between two gauges of red wire, all these checked good for continuity.

As I said yesterday I switched the fuel pump relay with all the others from the relay center so I'm guessing the relay is good. However if I pull all the relays it will still start and run until I release the key to the run position.

I was told of a similar problem that turned out to be the ignition module but I'm not sure if it would have anything to do with the votage to the fuel pump. I'm not getting the initial power to prime the pump when the key is first turned to the on position. I'd be inclined to think it was the ECU or a bad wire from it but I don't have a decent wiring diagram in the Haynes manual for anything but the lights.

John

Another idea. Check the yellow wire coming from the ignition switch going to the fuse block. You can also check it at the yellow wire going into the 3 pin connector at the ignition module under the hood. Should have 12v+. If not, trace it to the ignition sw. Should have 12v in 'run' and 'start'. Perhaps it's faulty.