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First post....

D

DeepsouthXJ

NAXJA Forum User
Location
Deep South
As much as I hate having my first post be a tech post, I'm in a predicament and need some outside help, so thanks to everyone in advance.

1999 cherokee sport, 4.0, 4x4, not sure about tranny or axles.

My jeep turns over but doesn't fire. Realized that the fuel pump didn't energize when the key was turned to the on position. Checked relay, continuity is good. Jumped the 30 and 87 on the plug, pump energizes. Gauges were working, checked fuses, and other relays, good. Then the gauges stopped working. Unplugged CPS, turned key to on, gauges didn't move, plugged CPS back in, still no fire, gauges haven't moved. Held odometer button in for 30 sec, turned key to acc, released button, nothing. All relays and fuses are good, haven't checked the board they plug into for shorts, voltage and the like. Where should I go from here. I haven't checked for spark, but if fuel pump isn't energizing then spark doesn't matter, but I will do so this afternoon. Any advice is appreciated.
 
Late model XJ's aren't really my thing, but some things are applicable for most years.

What you have, seems to be what some people call a "no bus". It could be something else, but various sensors have a supply voltage. Some sensors share a supply voltage and are wired in parallel. When one sensor shorts or has too little resistance, it effectively shuts down the supply voltage or the supply voltage falls to a point it is useless for the PCM.

The temperature sensor in the thermostat housing seems to be a common spot for this to happen. Depends on the year/model though.

Here is a component tree for one 5 volt supply voltage source. Try unplugging these sensors and see what happens.


gratis bilder
 
Key thing doesn't read codes on 1999. Needs reader.

I assume it is throwing a code? If so, when you get it read, there is a pretty good chance it will take you to the source of the problem.

-Ron
 
Unplug the IAC (P0505) and see if your gauges come back.

The fuel pump relay will normally close for 2-3 seconds to prime the fuel rail when the key is turned to the run position and won't close again for normal running until the CPS gets a good pulse signal while the motor is cranking over.

I was thinking if you have a sensor short and the CPS isn't getting a 5 volt supply, the PCM isn't going to get a good pulse signal and/or the PCM just pretty much shuts down when the 5 volt supply has a short and multiple sensor failures. Though in my experience, you still get the 2-3 second prime (the fuel pump relay closes for 2-3 seconds) when you turn the key to the run position.

The "no bus" often kills your gauges and/or the ability to read the codes.

If your fuel pump isn't closing while the motor is cranking, you are also likely not getting any spark. Don't get fixated on the fuel pump relay, it is not closing for a reason. The reason could be anything from no power to the PCM, an electrical problem in the PCM, a sensor malfunction or wiring problem that is screwing up the info the PCM needs to function. A easy place to start would be to unplug some sensors and see if your gauges come back and/or measure for the 5 volt power supply at the CPS or the cam position sensor (easy to get at).
 
8Mud said:
Late model XJ's aren't really my thing, but some things are applicable for most years.

What you have, seems to be what some people call a "no bus". It could be something else, but various sensors have a supply voltage. Some sensors share a supply voltage and are wired in parallel. When one sensor shorts or has too little resistance, it effectively shuts down the supply voltage or the supply voltage falls to a point it is useless for the PCM.

The temperature sensor in the thermostat housing seems to be a common spot for this to happen. Depends on the year/model though.

Here is a component tree for one 5 volt supply voltage source. Try unplugging these sensors and see what happens.


gratis bilder
Click to expand...


I'm getting 5 volts to all my sensors, CPS is good, relays good, fuses good, still not energizing fuel pump when turned on. Starting to point to the ECM. How can I test to see if that is really the problem.
 
Just had something similar happen with my 01. Turns out it was the wiring behind the intake manifold.

Did you check the ADS relay?
 
The wiring attached to the firewall directly behind the intake manifold, has a "t" that heads down the firewall.
 
8Mud said:
Unplug the IAC (P0505) and see if your gauges come back.

The fuel pump relay will normally close for 2-3 seconds to prime the fuel rail when the key is turned to the run position and won't close again for normal running until the CPS gets a good pulse signal while the motor is cranking over.

I was thinking if you have a sensor short and the CPS isn't getting a 5 volt supply, the PCM isn't going to get a good pulse signal and/or the PCM just pretty much shuts down when the 5 volt supply has a short and multiple sensor failures. Though in my experience, you still get the 2-3 second prime (the fuel pump relay closes for 2-3 seconds) when you turn the key to the run position.

The "no bus" often kills your gauges and/or the ability to read the codes.

If your fuel pump isn't closing while the motor is cranking, you are also likely not getting any spark. Don't get fixated on the fuel pump relay, it is not closing for a reason. The reason could be anything from no power to the PCM, an electrical problem in the PCM, a sensor malfunction or wiring problem that is screwing up the info the PCM needs to function. A easy place to start would be to unplug some sensors and see if your gauges come back and/or measure for the 5 volt power supply at the CPS or the cam position sensor (easy to get at).
Click to expand...


I'm getting 4.8 volts at my sensors, is this good or too low.
 
Ok, with key on ON position; fuel pump relay; 86 has 0 volts no continuity, 85 has no continuity, but 2.98 volts. 30 has 12 volts no continuity, and 87 has continuity and no voltage. Exact same thing with ASD relay.

Cooling fan relay shows continuity at 85, 12 volts at 86. So does starter relay.
 
WB9YZU said:
Probably shouldn't be posting pages out of the FSM.
Click to expand...

I read the fine print, it said reproduction of excerpts for discussion purposes was OK, just don't reproduce or redistribute the whole thing.
 
Last edited:
Alright, last night before I gave up, I pulled all the connections to the board under the hood and sprayed it down with electrical cleaner. This morning I put everything back together, nothing. Just for $&@!$ and giggles I discoed the CPS sensor again, turned on the key, and the gauges come back on. Nice... Replaced the CPS sensor and I am now a happy jeep driver again. Thanks everyone for the help.
 
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