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Unread 08-31-2014, 09:30 AM   #1
unkajeffy
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1991 YJ Wrangler 
 
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It just died??

I am searching and posting at the same time. Been off of here for a while as I gave the XJ to the daughter. 91 YJ 4.0, AX-15. Running absolutely fine down the road then is was like somebody shut off the switch. Cranks, has power to all gauges except the tach. Not getting CEL on key turn. I have swapped the crank sensor still no spark. All the relays are good. So I need some ideas. On a lighter note I left it out of gear last nite and the wheel chock sank in the mud and this morning it had rolled down the drive-way and winch kissed my tool shed.

Thanks in advance.
Jeff

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Unread 08-31-2014, 10:23 AM   #2
mike134
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My answer for everything:

Go borrow the fuel pressure gauge from autozone and plug it into the shrader valve on your fuel rail. Make sure your fuel pump is pumping. You should have 31 psi at idle on the 4.0.
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Unread 08-31-2014, 10:25 AM   #3
sprngrider
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Quote:
Originally Posted by unkajeffy View Post
Not getting CEL on key turn.
This is important. If the bulb is not burned out, this indicates that your ecu is bad. Either brain surgery or replace it.
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Unread 08-31-2014, 11:07 AM   #4
unkajeffy
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Quote:
Originally Posted by sprngrider View Post
This is important. If the bulb is not burned out, this indicates that your ecu is bad. Either brain surgery or replace it.
Define Brain surgey?
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Unread 08-31-2014, 11:07 AM   #5
mike134
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Quote:
Originally Posted by sprngrider View Post
This is important. If the bulb is not burned out, this indicates that your ecu is bad. Either brain surgery or replace it.
It seems like the 2.5L engine computer was more susceptible to bad capacitors. His engine is also cranking, which according to most reports doesn't happen with the brain surgery problem.

It may be worth trying though. Whether it is your current problem or not, it is good preventative maintenance to change 25 year old computer capacitors.

Do the fuel test first though. It's free and takes 10 minutes. very painless.
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Unread 08-31-2014, 11:10 AM   #6
unkajeffy
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Forgot to mention in original I am also not getting prime. The relay is not engaging but the relay works the starter when swapped.
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Unread 08-31-2014, 11:50 AM   #7
mike134
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The forum refers to "brain surgery" when you go into the engine computer and replace all the capacitors one by one. The electrolyte gel inside the capacitors dries up over time and causes them to fail. You can get some really wonky results when this happens. This happens to all electronics which use electrolytic capacitors (almost all of them), its not a question of IF, its a question of WHEN. The procedure is worth learning, because you can then repair a lot of old timey electronics.

Brain Surgery Thread: http://www.jeepforum.com/forum/f12/c...rs-ecu-492597/
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Unread 08-31-2014, 12:18 PM   #8
unkajeffy
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Quote:
Originally Posted by mike134 View Post
The forum refers to "brain surgery" when you go into the engine computer and replace all the capacitors one by one. The electrolyte gel inside the capacitors dries up over time and causes them to fail. You can get some really wonky results when this happens. This happens to all electronics which use electrolytic capacitors (almost all of them), its not a question of IF, its a question of WHEN. The procedure is worth learning, because you can then repair a lot of old timey electronics.

Brain Surgery Thread: http://www.jeepforum.com/forum/f12/c...rs-ecu-492597/
Got it, Is there something I should check before I get the soldering iron out? I dont have an FSM for the wrangler therefore no wire diagram. Also it doesnt get the cel after a few minutes either. And I mean like ten or twenty minutes.
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Unread 08-31-2014, 03:27 PM   #9
unkajeffy
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OK an update, went thru the fuses and relays all ok, fuel pump ok. I found a pin out diagram for the ecm connector 12 volts between pin 3 and 11 nothing exists in pin 9. Still doesnt energize fuel pump. Anybody got a pin out diagram for the ecm on the 91 4.o. or should i start hunjting capacitors? No power to the ASD relay circuit either. all the wires in PDC are solid and in place. Is there a possibility its the ignition?
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Unread 08-31-2014, 04:04 PM   #10
unkajeffy
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I found the right ecm pin out. Power to pin 3 and 9 12v.
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Unread 08-31-2014, 06:56 PM   #11
laybackman
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Quote:
Originally Posted by unkajeffy View Post
OK an update, went thru the fuses and relays all ok, fuel pump ok. I found a pin out diagram for the ecm connector 12 volts between pin 3 and 11 nothing exists in pin 9. Still doesnt energize fuel pump. Anybody got a pin out diagram for the ecm on the 91 4.o. or should i start hunjting capacitors? No power to the ASD relay circuit either. all the wires in PDC are solid and in place. Is there a possibility its the ignition?
The ASD relay. There are two sources of voltage going to the ASD relay. One is an ignition source (pin 86), the other is battery source (pin 30). Then you have the out put side of the ASD relay (pin 87) which in turn energizes the fuel pump relay. So how is your fuel pump ok????

So IF the ASD is not getting ignition voltage source neither is the fuel pump relay. Last but not least is a ground wire on the dip stick bracket which gives your ASD relay a ground source. That is pin 85.

If your CEL is not lighting up that is what needs to be solved first. Did you hit bump or something when this YJ crapped out?
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Unread 08-31-2014, 07:44 PM   #12
unkajeffy
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Grounds are good, asd relay and fuel pump relay have 12v to switched circuit, no signal from ECM or ignition to close the relay though. Checked it key on key off. I can jumper the fuel pump and I have good pressure. I was going about 40mph on a fair paved road and it just crapped out. Evironmental condition 95* F. No big hills. Good radiator, fan clutch and water pump. Manifold cracked on one half of the collector. It shut off like someone turned the key. The first thing I did was assume the no spark condition was the crank sensor, so I put in a new one for the one that was a month old. I went thru the PDC, all the connectors were solid.
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Unread 09-01-2014, 06:22 AM   #13
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This pictures show the PCM plug for a 95 YJ. I believe it will help you with your 91 4.0. You have to find out why your CEL won't light up.


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Unread 09-01-2014, 06:56 AM   #14
unkajeffy
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Ok well first question. I have switched power and battery power at the PCM plug, and good ground from the plug to the battery. 3,9,11. The first diagram i had found was mirror imaged and kinda screwed with me till i figured it out. I am leaning toward the capacitors being fried in the PCM. But will a bad speed sensor cause a no CEL situation? with key switched on and under tow i had no tach action in 4th gear and no light off (done for brakes and power steering). Second question....Why Me? LOL. Thanks for the diagram though its the best image I have seen so far. No visible corrosion on any of the electrical connections I have looked at so far.
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Unread 09-01-2014, 07:10 AM   #15
laybackman
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Quote:
Originally Posted by unkajeffy View Post
Ok well first question. I have switched power and battery power at the PCM plug, and good ground from the plug to the battery. 3,9,11. The first diagram i had found was mirror imaged and kinda screwed with me till i figured it out. I am leaning toward the capacitors being fried in the PCM. But will a bad speed sensor cause a no CEL situation? with key switched on and under tow i had no tach action in 4th gear and no light off (done for brakes and power steering). Second question....Why Me? LOL. Thanks for the diagram though its the best image I have seen so far. No visible corrosion on any of the electrical connections I have looked at so far.
Unplug it and find out!!
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