1996 Jeep Cherokee fries ECU?

Tiny
JASONSXJ
  • MEMBER
  • 1996 JEEP CHEROKEE
  • 203,123 MILES
My 96 jeep with a 4.0L auto died on me one ay checked for spark off of coil none. So I replaced the crank shaft sensor still nothing. And replaced cam sensor (pick-up) and once again no spark. I checked all grounds, fuses, etc. So I found out the ecu was bad changed it with a used 97 one. Fixed it! But only for a few starts and runs. Then same problem no spark! It seems I am getting to high of volts? Which burns them out? How can I track down the problem with jeep not running? Thanks a lot. Jason
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Friday, July 5th, 2013 AT 10:37 PM

13 Replies

Tiny
CARADIODOC
  • EXPERT
Sounds to me like this has been misdiagnosed. Only two things can cause the system voltage to go too high. The voltage regulator is shorted or the wire between it and the alternator is grounded. Either condition would be pretty hard to not notice. You'd be burning out all kinds of light bulbs, the dash gauge would show the voltage to be much too high, and a diagnostic fault code related to "field circuit not switching properly" would be set in the Engine Computer. By replacing the computer you erased any fault codes so that valuable information was lost. You also replaced the built-in voltage regulator along with the computer. Those regulators cause extremely little trouble, and the chances of running into two bad ones in two computers is very unlikely. I've only run into two dead voltage regulators in Chrysler products since 1990 when I started working at the dealership.

It is also possible you have a broken, cut, or corroded voltage sense wire going to the computer so it won't know actual system voltage. That will cause the regulator to keep on increasing alternator output in an attempt to bring that missing voltage up. Even in that situation the regulator only has so much control and the voltage won't go extremely high, ... Certainly not high enough to damage the Engine Computer.

The place to start is by measuring battery voltage with the engine off and with it running. A fully-charged, good battery will measure 12.6 volts. With the engine running it must be between 13.75 and 14.75 volts. I wouldn't panic if it's a couple tenths of a volt too high for a little while but I don't like more than 15.0 volts.
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Friday, July 5th, 2013 AT 11:38 PM
Tiny
JASONSXJ
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Ok thanks I assume that the ignion control module goes out when the ecu quits? How would you start diagnose that problem? Do I need to have it running to accurately diagnose this problem? Sense I would have to find another ecu. ! To go bad again if I didnt find the right problem quickly. Thanks again
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Saturday, July 6th, 2013 AT 4:48 AM
Tiny
CARADIODOC
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Reread my last paragraph.

The multiple things you're referring to are all one computer module. I call them "Engine Computers". Chrysler calls them "Powertrain Control Modules", (PCM). Other manufacturers call them "electronic control modules", (ECM). I doubt your old one is bad although anything is possible. Problems caused by broken, corroded, or rubbed-through wires often change symptoms or go away when parts are replaced due to disturbing those wires. That gives the false assumption whatever was done fixed the problem for a little while but then it came back again.
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Saturday, July 6th, 2013 AT 9:01 AM
Tiny
JASONSXJ
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Well the reason I was thinking it was to high of voltage killing them was when we put on a snap on scanner it gave me 3 codes o2 sensor coolant sensor and? But they all where high voltage with each code. Plus I was reading on AA1 web site that a voltage overage will cause ecu failure. Before I changed the ecu I shook and went through all the wires while it was trying to be started a nothing changed. It just weird that once I changed the ecu it started. And when it died it restarted stumbled then died then restarted ran for 5 seconds then died for good. All my gauges started acting wired and now I have no check engine light. So I guess now I'll go back through all the wires again and connectors. So do you think the ecu is still good? When I was checking volts at sensors I checked the primary at the coil while it was being cranked over I had 12.5 then it would drop to 0 pretty quick. Is that normal to drop voltage at coil while starting? If it doesn't start in the first 10 seconds?
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Saturday, July 6th, 2013 AT 10:54 AM
Tiny
CARADIODOC
  • EXPERT
First let me expand on a few ideas that may be coming into play. There's a lot of different fault codes that can be related to one sensor and they mean different things. They could mean that sensor is working fine but it simply detected a bad operating condition. They can indicate in which way a sensor has failed, but usually it is the entire circuit that must be suspect and diagnosed. Some code readers give the written code description and those are at the mercy of the person who programmed those tools. They can vary enough to cause confusion, ... And they don't explain how to diagnose the code. The best is to get the actual code number so we can find the listing on a chart.

Oxygen sensors have their own way of working so we'll ignore them for now. Most other sensors run on 5.0 volts. Their output signals are one of two forms; a solid voltage or a square wave signal. In a square wave the voltage in this case would go from 0.0 volts to 5.0 volts, and back again many times per second, like flipping a switch on and off very quickly. Some signals go on half the time and off half the time but how rapidly that happens can change. That is called changing the "frequency". Some maintain a steady frequency but the length of the on-time and / or off-time can change. That's called "duty cycle". By far the easiest to understand is the throttle position sensor. It puts out a steady voltage that varies based on how far you push the gas pedal.

The throttle position sensor is a long carbon resistor. A ground wire is attached to one end and the 5.0 volt feed wire is attached to the other end. If you measured in the middle you'd find 2.5 volts. That's the secret to how they work. A movable contact runs along that resistor that is attached to the throttle blade. As you push the gas pedal more, the movable contact moves closer to the 5.0 volt feed terminal so the voltage it reads goes up. To simplify it, as you run the throttle from fully closed to fully open the voltage on the signal wire will go from 0.0 volts to 5.0 volts. That's how volume controls worked in older tvs and radios.

The secret to detecting problems that set fault codes comes from adding another detail. The sensors are designed with mechanical stops that only allow them to go down to 0.5 volts and up to 4.5 volts. Every sensor is different and you might find a range of 0.4 to 4.2 volts, for example. The point is it will never reach 0.0 or 5.0 volts.

If there is a break in the 5.0 volt feed wire going to the sensor, or a break inside the sensor, there would be no voltage anywhere in it, including coming out on the signal wire. You'd have 0.0 volts all the time which is not an acceptable condition. In my story where 0.5 to 4.5 volts is normal and acceptable, 0.0 volts would be detected as a problem and the code "throttle position sensor voltage too low" would be set. Notice that code doesn't say the sensor is bad. It just tells you which circuit to diagnose, and what the unacceptable condition was.

You have the opposite code. When there's a break in the ground wire or in the sensor, 5.0 volts will be found everywhere including on the signal wire. Since that's more than 4.5 volts, the code "throttle position sensor voltage too high" will be set. That has nothing whatever to do with the car's electrical system that should be running between 13.75 and 14.75 volts. That has everything to do with that 5.0 volt supply voltage which comes from the Engine Computer and is very carefully regulated. You won't find that varying by more than a few hundredths of a volt.

Think of that throttle position sensor as being two parts split right where that movable contact touches the resistor. Your coolant temperature sensor works exactly the same way with two exceptions. First, the sensor is a real simple electronic sensor, not a mechanical device like the throttle position sensor. Second, half of it is inside the computer. That's why the TPS has three wires and the coolant temperature sensor only has two wires. You can still measure the voltage on the signal wire and see it change as the engine warms up. The other wire is the ground wire. The 5.0 volt feed wire is inside the computer.

Now we have to bring in another detail. We talked about a broken ground wire and a broken 5.0 volt feed wire, but the signal wire could also be broken. With no voltage on that wire, it could "float" to some random value just because of all the other circuitry in the computer. To avoid that they add a "pull-up" resistor inside the computer. Under normal conditions that pull-up resistor is so extremely high in value it's like it isn't even there and it has no affect on the circuit. When there's a break in that signal wire though, that's when 5.0 volts appears through it and is detected as that unacceptable condition, and the code "voltage too high" is set. The most common cause of setting that code is unplugging the sensor while the ignition switch is on. The second most common cause is corroded or loose connector terminals, then a break in a wire.

The next thing you should be aware of is that 5.0 volt supply that is produced in the Engine Computer. That is very well protected in case a short circuit occurs. To prevent damage to the internal circuitry the computer will turn it off to protect it. The ignition switch has to be turned off, then back on to reset that power supply. You were wiggling wire harnesses in an attempt to see if the engine would start but if the 5.0 volt supply was shut down already by the computer, no amount of wiggling would have helped until you cycled the ignition switch off and back on.

The last thing is the automatic shutdown (ASD) relay operation. That relay sends current to the ignition coil(s), injectors, alternator field, oxygen sensor heaters, and fuel pump or pump relay. The Engine Computer turns that relay on two times and I think that's what you observed. First, it turns it on for one second each time you turn on the ignition switch. You'll usually hear the hum of the fuel pump for that one second. Next, it turns it on any time it sees engine rotation, (cranking or running). It knows that by the pulses it receives from the crankshaft position sensor and the camshaft position sensor. When either signal is missing, the computer won't turn the ASD relay on during cranking. With no voltage to the ignition coil there will be no spark, and with no voltage to the injectors there won't be any fuel.

Sometimes the computer will set a fault code to tell you which signal is missing. Most commonly the sensor is at fault but the wiring should be checked too. If the 5.0 volts is missing to both sensors, one is usually shorted internally and it killed the supply voltage. You would unplug one or both, cycle the ignition switch to reset the supply, then check again.
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Saturday, July 6th, 2013 AT 2:10 PM
Tiny
JASONSXJ
  • MEMBER
Well I think I found it?? Its the stock fuel pump wiring got cut under the rear seat! I was cutting something underneth the jeep to repair the front spring mount ( rock crawler but all stock wiring) but fuel cell and pump. So the wiring was cut in two and melted on both ends. I got that fixed properly. Tried to start nothing! I guess that fried the computer? Thanks s lot for you're help and patience!
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Saturday, July 6th, 2013 AT 2:26 PM
Tiny
CARADIODOC
  • EXPERT
That wiring would not hurt the computer. The fuel pump gets its current through a fuel pump relay or the automatic shutdown (ASD) relay. Both of those relays are fused. You can jump them with a stretched-out paper clip or you can pop the cover off and squeeze the contact, then listen if the fuel pump runs. If it does, the fuses have to be okay.
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Saturday, July 6th, 2013 AT 3:37 PM
Tiny
JASONSXJ
  • MEMBER
So you don't think the melted wires that where touching together and grounding out had anything to doe with it?
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Saturday, July 6th, 2013 AT 3:42 PM
Tiny
CARADIODOC
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Yup. That would prevent the fuel pump from running. You have to determine now if the pump runs for one second after turning on the ignition switch. The easiest way to do that to squeeze or bypass the relay like I described.
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Saturday, July 6th, 2013 AT 4:26 PM
Tiny
JASONSXJ
  • MEMBER
Fuel pump runs for one second now! But no start! . So I am thinking that it fried the ecu after the wires touching all that time and the computer couldn't take it?
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Saturday, July 6th, 2013 AT 4:34 PM
Tiny
CARADIODOC
  • EXPERT
That's the whole point of using relays. The computer doesn't have to take it. In fact, it couldn't. Think of an elevator. A relay uses a small current supplied by the computer to turn a large current on and off for, in this case, the fuel pump motor. You can't lift ten people, at least not with just one finger, but you can if you use your finger to push the "up" button in the elevator. That's using a small energy to turn a large energy on and off. The computer is your brain and finger muscles. Those don't get damaged if you overload the elevator. The cable snaps, which, in this case would be the fuse!

Now that you know the fuel pump runs for one second, you've proven the pump motor and wiring are okay, the relay contacts are okay, the fuse feeding the relay is okay, the small-current part of the relay is okay, and the computer has control of the relay. All of that circuit is working and it will continue to work if you have spark. I took a jump there but the result is if you have spark during cranking, the same stuff is running that made the fuel pump run for that one second. You need to move on now and check for spark. If it's missing we'll have to diagnose that circuit. I'm basing that on the fact you found 12 volts to the ignition coil earlier while cranking the engine. That means you should have spark but I'll wait to hear back.
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Saturday, July 6th, 2013 AT 5:39 PM
Tiny
JASONSXJ
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I still have no spark.
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Sunday, July 7th, 2013 AT 4:34 PM
Tiny
CARADIODOC
  • EXPERT
Okay, now lets start at the beginning. Step 2 is to determine if the automatic shutdown (ASD) relay is turning on during cranking. I don't have a service manual for your vehicle so you'll have to figure out the wire color we're after. Look for the wire that is the same color at every injector, the ignition coil or coil pack, and the alternator. That is the 12 volt feed wire that comes off the ASD relay. You can measure on any one of those wires. On most car models that is a dark green / orange wire.

You will see 12 volts on that wire for just one second when you turn on the ignition switch. It's easy to miss with a digital voltmeter because they often respond too slowly. A test light might work better. If you don't see that voltage for one second, most likely your test probe isn't making good contact with the terminal. You can back-probe through the back of a connector or you can pull off one injector plug and touch right on the terminal. If you see that voltage, next see if that voltage turns back on during cranking. If it doesn't come back during cranking, the crankshaft position sensor is the first suspect and the camshaft position sensor is the second.
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Sunday, July 7th, 2013 AT 8:34 PM

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