blowing a fuse at high rpm and wide open throttle

[alwayzAMC]

kind of stumped here, and not sure where else to try

99 xj 4.0 auto.

while driving during hard acceleration, i blow the under hood fuse for coil and pcm, fuse number 21

it also does it while not driving in neutral if i put the accel pedal to the floor.

when it blows the fuse i do get a 1389 code, no ASD relay output at PCM, i dont believe the code is the reason for bliowing the fuse, but rather the code that is generated when this fuse blows. after replacing the fuse the code goes away with only a few restarts

i have tried replacing coil, ASD relay, fuel relay, the ignition key switch, CPS, TPS, and cleaned grounds to the coil and replaced the negative terminal to the battery.

i have a 97 i have been using to swap parts back and forth, except the coil, which i had to buy

the first time it did this was when i had my rear shocks replaced, and went from 235 to 31" tires. not that i think this is the culprit, but thought i would mention it.

thanks in advance for any help.

RD

 

[GrimmJeeper]

have you tried the obvious and looked under the dash to see if your accelerator pedal is interfering with any parts of the wiring harness under the dash?

 

[old_man]

Turn on the key, open the hood, grap the throttle and rotate it to WOT and see if the fuse blows. If it does, unplug the TPS and try again. In theory you could have a bad TPS.

 

[alwayzAMC]

have you tried the obvious and looked under the dash to see if your accelerator pedal is interfering with any parts of the wiring harness under the dash?

good thought, disconnected throttle linkage, and tried at throttle body, same thing

Turn on the key, open the hood, grap the throttle and rotate it to WOT and see if the fuse blows. If it does, unplug the TPS and try again. In theory you could have a bad TPS.

tried that as well, it does not blow if the engine is not running, but does blow the fuse when running, if i have the tps connected or not.

 

[sidewaysstarion]

Check the harness where is goes behind the fuel rail, make sure the end of the fuel rail hasn't worn a hole in the harness and is shorting out wires.

 

[Talyn]

x2 on the wiring harness

 

[alwayzAMC]

i checked the wiring harness over a good bit and dont see anything that might be suspect.

i tried unplugging a few sensors and see if i can trace it more, no such luck there.

i played around, as i had more spare fuses tonight, it seems to blow with no load in neutral at abouut 4200 RPM if i rev slowly, if i stomp it, i can get to redline, but blown fuse as well. not sure if that helps at all. it sure does not help me much in diagnosing, but maybe the engine sends out a different signal to somewhere at WOT or about this RPM, i may try swapping the distrobuter this weekend when i have more time on my hands. really wish it would throw me a code that helps more. the ASD part of the code gives me a bit of insight, is that part of the computer? or a different entity??..

 

[Shorty]

I would inspect everything that the ASD relay controls to see if there is something that would turn on and off based on engine speed.

I would also swap or check the ASD relay just to rule it out of the equation.

 

[alwayzAMC]

tried the relay already, and have no idea what the ASD controls i am almost starting to wonder if it is something dash related, as i have been having troubles there also.

 

[old_man]

Is the idle a bit rough? I'm wondering about a shorted injector or injector driver in the ECU. Both would pull pulses of power but not enough pulses per minute to blow the fuse until the RPMs get up high enough.

Do you have a noid light you can lay hands on to test the drive to each injector? Either way, I would pull the injector connectors one by one and read the resistance of the injectors. IIRC they should be around 16 ohms, but the real check is to see if one is radically different from the rest.

 

[XJFrankenstein]

ASD is Automatic Shutdown. If the ECU stops getting a signal from the Crank Position Sensor, the ASD relay trips and that shuts off power to the fuel pump and the ignition. The purpose of ASD is prevent a fuel explosion after a major collision.

 

[91to99]

Just kind of throwing out a thought...
It seems logical to me that maybe the fuel pump may be having issues. Could it be shorting out with a high demand?
We had a ZJ fuel pump take out a engine control 30 amp fuse this weekend. End result was a bad fuel pump.

 

[alwayzAMC]

Is the idle a bit rough? I'm wondering about a shorted injector or injector driver in the ECU. Both would pull pulses of power but not enough pulses per minute to blow the fuse until the RPMs get up high enough.

Do you have a noid light you can lay hands on to test the drive to each injector? Either way, I would pull the injector connectors one by one and read the resistance of the injectors. IIRC they should be around 16 ohms, but the real check is to see if one is radically different from the rest.

idle is ok, i do have a stall in acceleration once in a while, hit the gas, and nothing for a second, then goes. fuel milage sucks abit, and not a whole lot of power.

and not sure whata noid light is, last time i heard noid, some lil purple guy stomped on my pizza, will try a resistance test, not much of an electrician though, guess is time to learn. will see if i can find a write up.

ASD is Automatic Shutdown. If the ECU stops getting a signal from the Crank Position Sensor, the ASD relay trips and that shuts off power to the fuel pump and the ignition. The purpose of ASD is prevent a fuel explosion after a major collision.

fuel pump is suspect. i have a couple extras, i really hate to change one on a hunch, but knowing now it is controlled some by ASD it has become highly suspect

 

[alwayzAMC]

and the stupid answer is........



alternator, took off the pigtail on the alternator, drove it a ways, not a problem, drove it all the way till i ran the battery low enough to need a boost ( oops) never would have suspected it.

but all up and going, and off to the next on my to do list.