Replacing engine computer

Level of difficulty is simple, no special tools, the only thing is just like with your home computer be ware of static electricity.

Location if you don't already know is left side front of engine compartment.

i am having this issue when i accelerate uphill it seems to happen mostly my overhead console flashes liftgate open and computer tries locking the doors continually. i have been having problems with stalling but a new throttle position sensor fixed it. i still have idling issues it idles rough as can be. Do u believe the problems are linked somewhere like the PCM? Also if this helps my door locks automatically lock, but after being locked it would continue to lock itself. i am utterly confused. there is a total lack of engine power sound like it is running fine just lacks the gitty up i'm used too with a 4.7

I'm going to break this down b/c I don't see your problems being related. I'm assuming the "liftgate open" light problem is intermitent. I would try liberally spraying the latch assemblies down with some type of spray lubricant and working the hatch and doors a few times. The ajar switches are inside the doors attached to the latches and are prone to sticking. I don't recomend using WD-40 or any other type of "rust breaker" for this. They are corrosive and can make the problem worse.
On your door locks. My thought is you have a lock/unlock switch that is defective and from body vibration may be trying to lock itself. As a rule, these switches are pretty sensitive.
Your performance issue. You can have a shop hook the vehicle up to a perfornance diagnostic unit, but I think you'll find it's just the "nature of the beast". They are not known for excessive horsepower. Hope this helps.

i went and got my jeep inpected and they said that re flash or replace on board computer . what does that mean? and what is the price ranges?

I believe they are talking about the engine computer. It is located behind the drivers side headlight next to the air filter housing. Your better bet would to try to locate a used one. Around here they are in the range of $50- $100

I hit a bump the other day and my check engine light came on . I unplugged my battery to rest it. The next day i tried to crank it, it cranks but has no spark and does not turn over. None of my gauges register, but my lights work? I heard it could be my rollover button? but i am not sure of that, or my ASD relay , i unhooked then rehooked it but still nothing. Under my odemeter it says No Bus? All my fuses appear to be ok?? Can u help me?

Lady in distress

it could be eather one or both or even one of the grounds on the jeep check all the grounds mostly the one under jeep that goes from frame to the exhaust to the trans this is one wire that goes to all 3 but would like to know code for check engine

Cannot seem to find the PCM on my vehicle. Looking into it possibly heating up and causing misfires and going into "limp home mode" constantly.

Here is the location on the PCM engine control module in the diagrams below that also explain how to replace it. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

i just bought my 1998 jeep cherokee 4.0 automatic about 2 weeks ago, was running fine for the first week and now its running like garbage, the CEL came on and i got it checked out. the code was PO171. the truck has 240k on the body and the previous owner said the motor was replaced and now this current one supposevely has around 80k on it. 2 days ago i did a full mopar tuneup, cap rotar plugs wires air filter and i reset the ECU. the truck was running great for about 15 hours and then on day 2 it was back to how it used to be... running really rough. at idle my truck clanks around like it wants to stall but the RPMs are normal. the truck has a very strong fuel smell like it is running very rich and also the MPG are wayyy down. any help on what i should do? this is my only veichle. thankyouu

Is the engine light on again? If so is it the same code? Are you up to testing a couple of things? Did you replace the fuel filter too? You replaced plugs and wires correct? You will need a digital multi meter to run some test ok.

yes check engine light is back on, it is the same code,i cleaned my throttle body today nd blew out all the plugs attached. still runs the same. i did not replace the fuel filter, no idea how to even check it. and yes i did put in a new cap plugs and wires during the tune up, my dad is an electrician and works on cars sometimes so i am sure he has a multi meter.

I think I would start with the fuel filter it just infront of the gas tank. P0171 is lean condition bank 1 and this can be a lack of fuel by having a plugged fuel filter so thats why i say start there and you have never replaced it so it won't hurt to do it anyway and eliminate that as the cause ok. Once that is done see how it runs and let me know and we will go from there.

1997-2001 cherokees have the fulter inside the tank, its part of the pump assembly. cant afford to replace the pump or dump out 16 gallons of fuel with no where to put it all. also the truck runs like garbage till i reset the ecu, once i reset it and start the truck back up it seem to run good for a few hours and then the next day its back to square one.. that makes me feel like its an o2 sensor, i also checked all of my grounds and all the fuses to make shure the fuel pump fuse wasnt blown and its still basically new

You have a standard cherokee or grand? On the grand cherokee it is just infront of the tank as i have 99 grand cherokee, on the standard cherokee it don't have an inline filter. With that yes it could be an o2 sensor. You don't have any vacuum leaks do you? If not then I will walk you though on testing the o2 if you want.

My oem brain needs replaced, can I buy a used one or does it need to be new

A used one is a bad idea if your vehicle does not have the factory anti-theft system. If it does, you can use any computer. If the one you get isn't programmed for anti-theft, it will learn that from the Body Computer the first time you turn on the ignition switch, then work normally. If your vehicle doesn't have it but the used computer you get does, it will teach that programming to the Body Computer. At that point the engine won't start because both computers will be waiting for a disarm signal that is never coming. At that point you will need to replace both computers at the same time with ones without anti-theft programming.

Computer problem
1997 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

i had to buy a used PCM for my 97 cherokee. do i have to get it programed from the dealer or can i install it? and how? just to be safe! it is out of a 97 and was tested and it was good. please help ive put way to much into this jeep for just a work truck. im hoping it will make me the $$$ for another one fer a TOY!!!!

You are really rolling the dice there. Just because it came out of a 97 doesn't mean it's even the right one for your truck and it definitely has to be programmed before use.

changed it twice ' car races and stall . garage said the ecm computor is the problem . I go to from 1997 cluckers and one ran for 3 weeks then statred to do the same.the next one just raced . am I doing some wrong . does it have to be installled a special way i took the terminal off not to spike it ' three computer al know good ? I match the computers what do you think am I doing something. do they loose there info . are they reprogramable ? help
thanks

Are you replacing the ECM? IF so, there is a relearn process, but it does it itself. Explaine what is happening. Is the engine reving or dieing?

It is reving fast ecm 1997 cherokee

Have you checked the idle air control valve or the throttle position sensor? Have you checked for vacuum leaks?

Computer problem
1997 Jeep Cherokee 6 cyl All Wheel Drive Automatic

My lower computer screen is flashing on and off. saying coolant and wiper fluid is low but it is full. all the icons come on at the same time and the warning chime chimes for no apparent reason. It is a Jeep grand cherokee limited. is this screen replaceable and is it expensive. thank you.

I would recommend having someone troubleshoot it as opposed to just replacing a part upon suspicion. There are fluid level sensors that tell may be bad for example that are giving false information.

. The whole screen flashes on and off all the icons light up, then the screen will turn off. when it comes on again it is ok for a few mins then starts flashing. If shows all the doors and hood and rear tailgate open when you are driving. coolant and washer fluid low. all the transmission modes show at the same time. It is as if it is very confused.

Engine Mechanical problem
1996 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

Dear Sir
we have a jeep cherokee which will not come out of limp mode it has been on a computer head has been done new lifter it has the mechanic beat it was originall put in because it would not come out of limp mode so i donot know if this chap is haing me on we owe him to much now to pull the car out and go else where if you fix my problem i will glacy donate $

Hello,

Has the Mechanic considered the TCM (Transmission Control Module) it is located just below and behind the Glove Box. And is the most likely culprit.


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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

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.

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

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.

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?

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.

How can I tell if my c p m is bad on my 1996 jeep cherokee 4.0 4x4 will not start , no spark, no power to fuel pump, replaced with new pump, replaced with new cam sensor, replace with new crank sensor, new battery ,all fuses ,and relays check good, fuse links all ok , what else could it be ????

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!

You've added new problems by disconnecting the battery. The idle speed is going to be too low for starting unless you hold the accelerator pedal down 1/4". Most importantly, you erased any diagnostic fault codes that were stored in the Engine Computer. Without that valuable information, there is no way to know which circuit has a problem. At this point the best approach is to find a mechanic with Chrysler's DRB3 scanner to view live data. The crankshaft position sensor and camshaft position sensor will be listed as "present" or "no". If either one is missing, the computer won't turn on the automatic shutdown (ASD) relay. That's what sends current to the ignition coil and fuel pump.

The only thing you can do without a scanner is to check for that 12 volts from the ASD relay and check the voltage feeding those two sensors. Look for the wire that is the same color at the ignition coil and all of the injectors, usually a dark green / orange. You should see 12 volts there for one second after turning on the ignition switch. What's important is if that voltage comes back during cranking. If it does not, check for 5.0 volts on the feed wire for those two sensors. If it is missing, unplug both of them and measure it again. If the voltage comes back after cycling the ignition switch off and back on, one of them is shorted. If the voltage doesn't come back, the feed wire is broken or grounded.

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.

So you don't think the melted wires that where touching together and grounding out had anything to doe with it?

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.