Tell me it is not my PCM
02-16-2018, 03:18 PM
#46
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Location: Charlotte NC
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Year: 1993
Model: Grand Cherokee(ZJ)
Engine: 5.2
The gremlins are starting to get mad at me. So update! Jeep did great for about three days and then one night at work went to start it and no start 
so I went straight to my fuse and looked and it was blown, I know big surprise right, so I swap in a new one and starts right up. All is well but, this happens a few times over the next few days but each time it keeps getting a little worse some times it takes three or four fuses. I know i'm a bit stubborn at times but I needed it and it always came back to life. I kept saying well when it gets worse it should be easier to find. Fast forward to tuesday night playing pizza chaufer once again came out and blew a fuse, threw in a new one and was on my way then about a 1/4 mile from my delivery Jeep dies driving in the neighborhood! Coast to a stop and check my trusty fuse and sure enough blown then the new thing begins I pop a new one in and it would blow almost instantly turning the switch to on, being 1130 at night it was really easy to see it happen! So I get it towed home and start looking into things and it has been bugging me that I don't hear my fuel pump or relay all the time and in all my others over the years I have been able to. On the times I do faintly hear it seems to start right up. However, when I don't hear the relays cycle fuse blows when the relays click she starts. The only time I did try to take it around the block I made it roughly a 1/4 mile before it blew driving again. So looks like I will be studying the FSM a bit to see where power goes 1st.
so I went straight to my fuse and looked and it was blown, I know big surprise right, so I swap in a new one and starts right up. All is well but, this happens a few times over the next few days but each time it keeps getting a little worse some times it takes three or four fuses. I know i'm a bit stubborn at times but I needed it and it always came back to life. I kept saying well when it gets worse it should be easier to find. Fast forward to tuesday night playing pizza chaufer once again came out and blew a fuse, threw in a new one and was on my way then about a 1/4 mile from my delivery Jeep dies driving in the neighborhood! Coast to a stop and check my trusty fuse and sure enough blown then the new thing begins I pop a new one in and it would blow almost instantly turning the switch to on, being 1130 at night it was really easy to see it happen! So I get it towed home and start looking into things and it has been bugging me that I don't hear my fuel pump or relay all the time and in all my others over the years I have been able to. On the times I do faintly hear it seems to start right up. However, when I don't hear the relays cycle fuse blows when the relays click she starts. The only time I did try to take it around the block I made it roughly a 1/4 mile before it blew driving again. So looks like I will be studying the FSM a bit to see where power goes 1st.
02-17-2018, 06:29 AM
#47
CF Veteran
You have what we electricians call a short circuit. Try finding out all the things that fuse distributes 12 volts to, and disconnect them one by one and see when the fuse holds.
02-17-2018, 06:36 AM
#48
CF Veteran
Wingless had a great method here. All your info too. Use the resistance meter to see when the short is removed. Also just wiggle the fuse block to see if that changes.
The PDC Fuse F6 15A provides power to the coils on both the Fuel Pump Relay and the Automatic Shutdown Relay, plus lots of other stuff. It also powers the: Data Link Connector #1 (near the PCM); the Exhaust Gas Recirculation Solenoid; the Overdrive and EMCC Solenoid; the Evap/ Purge Solenoid and is an input to the PCM labeled Ignition Start / Run on pin 9.
My guess is one of those places is a dead short, causing the fuse to blow.
Use a multimeter to measure the resistance to ground at the fuse location, then calculate the current using Ohm's law, I=-V/R.
Start unplugging each of those loads until the resistance becomes good to locate the problem.
Also measure the coil resistance for each load to ensure those are also not the problem.
My guess is one of those places is a dead short, causing the fuse to blow.
Use a multimeter to measure the resistance to ground at the fuse location, then calculate the current using Ohm's law, I=-V/R.
Start unplugging each of those loads until the resistance becomes good to locate the problem.
Also measure the coil resistance for each load to ensure those are also not the problem.
02-23-2018, 09:28 AM
#49
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Year: 1993
Model: Grand Cherokee(ZJ)
Engine: 5.2
So Finally got a chance to do some probing with the meter. So the argavateing part is the intermittent part of this issue. So I looked for the resistance at my fuse and got like 1.2 and the car starts and does fine. So i assumed that was the numbers i wanted then i played with it a bit to try and get it to "blow" when it did finally I got a reading of 191.7 so I started hunting checking my meter after I unplugged each thing I came to. I started with the fuel pump and then went to the coil then every sensor i could think of. no change so I then in an effort to try and narrow down the issue pulled all my relays with no change there I wiggled the PDC no change so started pulling the maxi fuses just to see if it would change by breaking a connection and maybe narrow down my hunt. Pulled everyone of them and the only one that made and difference at all was the 40 Amp IGN Switch I. when i pulled this my ristance fell back down. so i put the fuse back in and then my meter went blank! so i swapped the meter over to volts and sure enough across the two sides of the fuse i was showing 12 volts! now the voltage goes away with the fuse removed as well. Now the things that seem to make it work is when i turn the key some times it will cycle 10 times before it works. what has been "blowing" my mind is the inconsistency well if i cycle the switch some then the issue will resolve and no voltage across the fuse. So could it be my switch that has been my issue?
02-23-2018, 10:50 AM
#50
Senior Member
Finally got a chance to do some probing with the meter.
The aggravating part is the intermittent part of this issue.
I looked for the resistance at my fuse and got like 1.2 and the car starts and does fine.
I assumed that was the numbers I wanted. I played with it a bit to try and get it to "blow". When it did finally I got a reading of 191.7. I started hunting checking my meter after I unplugged each thing I came to.
I started with the fuel pump and then went to the coil then every sensor I could think of. There was no change.
In an effort to try and narrow down the issue I pulled all my relays, there was no change. Then I wiggled the PDC, there was no change. Then I pulled the maxi fuses. I pulled all of them. The only one that made any difference at all was the 40 Amp IGN Switch. When I pulled this the measured resistance fell back down.
The fuse was put back in and then my meter went blank! The meter was changed over to volts and sure enough across the two sides of the fuse it was showing 12 volts!
Now the voltage goes away with the fuse removed as well.
Now the things that seem to make it work is when I turn the key sometimes it will cycle 10 times before it works.
What has been "blowing" my mind is the inconsistency from when the switch is cycled sometimes then the issue will resolve itself and no voltage across the fuse.
Could it be my switch that has been my issue?
The aggravating part is the intermittent part of this issue.
I looked for the resistance at my fuse and got like 1.2 and the car starts and does fine.
I assumed that was the numbers I wanted. I played with it a bit to try and get it to "blow". When it did finally I got a reading of 191.7. I started hunting checking my meter after I unplugged each thing I came to.
I started with the fuel pump and then went to the coil then every sensor I could think of. There was no change.
In an effort to try and narrow down the issue I pulled all my relays, there was no change. Then I wiggled the PDC, there was no change. Then I pulled the maxi fuses. I pulled all of them. The only one that made any difference at all was the 40 Amp IGN Switch. When I pulled this the measured resistance fell back down.
The fuse was put back in and then my meter went blank! The meter was changed over to volts and sure enough across the two sides of the fuse it was showing 12 volts!
Now the voltage goes away with the fuse removed as well.
Now the things that seem to make it work is when I turn the key sometimes it will cycle 10 times before it works.
What has been "blowing" my mind is the inconsistency from when the switch is cycled sometimes then the issue will resolve itself and no voltage across the fuse.
Could it be my switch that has been my issue?
The prior paragraph was IMPOSSIBLE to comprehend. I took the liberty to edit that post to provide normal English structure.
From what I can tell, the multimeter was being used to measure fuse resistance with the fuses in-place, in the vehicle fuse block. If that assumption is correct, then that is an inappropriate and incorrect way to measure the fuse resistance.
There are several ways to measure if the fuse is good. One is to yank the fuse, then measure the fuse resistance, near zero is good, infinite resistance is bad. Another way is to poke the window at the end and measure voltage. Measuring voltage to ground, if the windows at both sides have battery voltage, then the fuse is good. If the voltage at one side is battery voltage and the other side is ground then the fuse is bad. There are other ways to measure fuses too.
What is working / not working now when the vehicle doesn't start?
When the vehicle doesn't start is the alarm flashing the light, or not?
When the vehicle doesn't start is there fuel pressure, or not?
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