ECU for Jeep Cherokee

Big difference among the years, IIRC... I'm pretty sure that all the modules have to agree on the odometer miles, among other things, on mine. ECM, PCM, CCCM, DRBII, DRBIII... alphabet soup.

On my '00XJ, buried way down on pg 14-1 Fuel Systems, is the following:

PCM VIN REPROGRAMMING OPERATION USE THE DRB SCAN TOOL TO REPROGRAM THE NEW POWERTRAIN CONTROL MODULE (PCM) WITH THE VEHICLES (sic) ORIGINAL IDEN- TIFICATION NUMBER (VIN) AND THE VEHI- CLES ORIGINAL MILEAGE. IF THIS STEP IS NOT DONE, A DIAGNOSTIC TROUBLE CODE (DTC) MAY BE SET.

The way I read this, you can pop one in and it will work... but your MIL (malfunction indicator light) will illuminate, and I don't have a clue as to whether it will ever go out. There should be some safeguards preventing folks from resetting their odometers.. that's what this is all about. If you don't have a=20 digital odometer, chances are you've got the earlier ECU, and can plug-and-go... it may take a few start-stop cycles to learn all the operating parms for your engine.

Why the new ECU, btw? __ Steve .

__ Steve .

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One letter... how it changes things. Tim probably meant 'no'... __ Steve .

I need the ECU eventually because of a persistent TPS error. My mechanic troubleshooted it and thinks its probably the ECU. But he thinks there has to be some reprogramming to be done, because I tried one from the junk yard before he looked into it and nothing changed (same TPS error, same mileage). It's a 97 inline 6 Cherokee. It works, but it doesn't start at first try and it is not very smooth if you want to accelerate from 65 to 75 without a kickdown.

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One letter... how it changes things. Tim probably meant 'no'... __ Steve .

The TJ's ECU is known to be extremely reliable and just about trouble-free. Have you already replaced the TPS and cleaned/reseated the electrical connection that connects the ECU to the TPS? Bad TPSs are easily replaced and far cheaper than replacing the ECU.

Jerry

reigelser wrote:

So it only throws a 24 code? Do you have a manual or automatic transmission? Any aftermarket parts on the engine? It's never thrown a 15 (VSS)? That causes kickdown problems too...

With the TPS, you can actually patch a digital voltmeter in and watch the voltage as you drive... won't hurt a thing. If the computer's bad, then you'd expect the +V to the TPS to fluctuate... if the TPS is bad, you'd expect the wiper return voltage to be wrong for the rpm, gear, etc. That's what I'd do next... __ Steve .

It is an automatic transmission. What is the wiper return voltage? The other voltage I can measure on the TPS besides the ~200mV to ~4V? I check the message with an OBD Reader. P0123 I think is the number or P0124, don't remember right now. Are you referring to the Check Engine Light flashing when you mension a 24 and a 15?

So it only throws a 24 code? Do you have a manual or automatic transmission? Any aftermarket parts on the engine? It's never thrown a 15 (VSS)? That causes kickdown problems too...

With the TPS, you can actually patch a digital voltmeter in and watch the voltage as you drive... won't hurt a thing. If the computer's bad, then you'd expect the +V to the TPS to fluctuate... if the TPS is bad, you'd expect the wiper return voltage to be wrong for the rpm, gear, etc. That's what I'd do next... __ Steve .

reigelser did pass the time by typing:

Clean the connectors at both the TPS and the ECU. And while you have the plugs off, ohm out the sensor lines as the TPS shares a common ground link with other sensors and that link can go wonky.

All I have is the 93 schematic so you need the color codes for your year.

Yes... too cheap to get the OBD. '124' would correspond to the '24'... TPS. =20

Start thinking about things like the ground wire between the engine and the firewall.. this is a=20 known problem point. Doug has the right idea... make sure all connections are clean and tight.

Problem starting... to start an EFI vehicle, you should keep your foot *off* the gas until the vehicle has started... no need to touch the gas until you take off. You're doing this, right?

Has other maintenance been done recently? Does the engine run smooth once it starts? Describe what you have to do to get it to start (i.e. grind long, grind twice, feather throttle, etc.).

The TPS is basically a potentiometer, with three wires going to it... supply, wiper, and ground. The supply is a regulated voltage, sent by the computer and used by the computer as a reference. If this is moving around, the computer may have unfixable problems (blown regulator pass transistor, etc.) or another sensor or connection may be screwing up your TPS reading. The wiper voltage is the one that changes as the throttle plate opens and closes... it should track the position of the gas pedal steadily. The wiper carries your 200mV to 4V. __ Steve .

To start the car I have to crank it once for a short time. Even If I crank longer it won't start. Then I turn the ignition off completely and start again and runs immediately. There was no service done prior to the Check engine light coming on. The wiper voltage seems very smooth and stable. I haven't measured the supply voltage while having the engine running yet. Everything else seems to run good: idle speed, kick down response. So would you recommend an engine cleaning first (engine compartment wasn't cleaned in a loooong time) and then a rework of the connectors? Thanks for all of your advice.

Yes... too cheap to get the OBD. '124' would correspond to the '24'... TPS.

Start thinking about things like the ground wire between the engine and the firewall.. this is a known problem point. Doug has the right idea... make sure all connections are clean and tight.

Problem starting... to start an EFI vehicle, you should keep your foot *off* the gas until the vehicle has started... no need to touch the gas until you take off. You're doing this, right?

Has other maintenance been done recently? Does the engine run smooth once it starts? Describe what you have to do to get it to start (i.e. grind long, grind twice, feather throttle, etc.).

The TPS is basically a potentiometer, with three wires going to it... supply, wiper, and ground. The supply is a regulated voltage, sent by the computer and used by the computer as a reference. If this is moving around, the computer may have unfixable problems (blown regulator pass transistor, etc.) or another sensor or connection may be screwing up your TPS reading. The wiper voltage is the one that changes as the throttle plate opens and closes... it should track the position of the gas pedal steadily. The wiper carries your 200mV to 4V. __ Steve .

reigelser did pass the time by typing:

What your describing may be a fuel delivery problem caused by either the pressure regulator or check valve failing. Try this;

Turn the key on, but don't crank. Turn the key off, wait a sec Turn the key on, wait for the fuel pump to stop buzzing Then try starting the engine.

If the engine fires up your problem is fuel pressure leaking down. Diagnosing that further is slightly complicated but a cheap fuel pressure gauge screwed into the fuel rail will indicate a leak if it drops more than 5psi in an hour. The rail should hold pressure for more than a day.

You may have more than one problem... try this sometime... turn on the key, let the thing sit for ten seconds, then crank... should go vroom. It this is what you're seeing, you have a fuel pressure retention problem... either the back- check valve, or a leaky injector.

I'm not that big of a believer in engine cleaning... especially when trying to troubleshoot a problem. Don't compound your problems right now.. sounds to me like you've got more than one.

Is the big braided ground wire still connected between the block and firewall? Look good?

Get some Caig ProGold contact cleaner (sold at Fry's) and put some in the connector (both ends) for the TPS... try again. I want to verify that you've cleared the old ECU code, and it keeps popping a new one... am I right? Does it pop immediately, or after normal in-town driving, or only after exhibiting the symptoms you describe at 60-70mph transition? __ Steve .

Lastly, you may have to go through several run/drive cycles for the ECU to learn the curve of the new TPS...

i have not seen in any posts giving a dtc for a tps issue i have seen and replaced many a tps for a glitch in it's range causing all sorts of issues and some serious testing with a drb and a dvom must take place could be there is a couple of things going wrong though??? like the plug wires are shot. carbon on th valve thats causing a highway speed misfire and there was a tsb on new valve springs for this issue and a weak battery h

Stephen Cowell wrote:

I just tried the proposed starting procedure. KeyOnEngineoff for 10sec and then crank but no vroom. I needed a second try. When I turn the key I hear the Fuel pump for approx 1sec (I believe its the fuel pump). With the engine cold there is a little delay in the throttle response. Between Pushing the throttle a bit and the revs coming up is a ~0.5sec delay. When I rev the engine at 2000 and go of the Throttle it goes all the way down to ~300rpm before it comes up to 800 again. I checked the connectors (cleaned them with WD40) and the ground cable: seem fine. Between engine block intake manifold and fire wall I measure ~0 Ohm resistance, so the ground seems good. I can not reset the error right know, because I don't have the reader available. But in the past I tried 50 times to reset the error and it is not erasable. Sometimes the light goes off, but comes back on immediately. Sometimes the device tells me, the error can't be erased. At the moment the Check ENgine Light readt out tells me 12, 24, 55. Don't know what 55 is, 12 means the PCM (or was it PMC) was disconnected from the battery. I am going to get the reader again to reset the error.

Thanks

Jo

You may have more than one problem... try this sometime... turn on the key, let the thing sit for ten seconds, then crank... should go vroom. It this is what you're seeing, you have a fuel pressure retention problem... either the back- check valve, or a leaky injector.

I'm not that big of a believer in engine cleaning... especially when trying to troubleshoot a problem. Don't compound your problems right now.. sounds to me like you've got more than one.

Is the big braided ground wire still connected between the block and firewall? Look good?

Get some Caig ProGold contact cleaner (sold at Fry's) and put some in the connector (both ends) for the TPS... try again. I want to verify that you've cleared the old ECU code, and it keeps popping a new one... am I right? Does it pop immediately, or after normal in-town driving, or only after exhibiting the symptoms you describe at 60-70mph transition? __ Steve .

Lastly, you may have to go through several run/drive cycles for the ECU to learn the curve of the new TPS...

reigelser did pass the time by typing:

Your correct, the pump runs (primes) the system for about 1 sec.

That's normal. Your letting off the throttle is telling the engine to go into braking mode.

WD-40 isn't a contact cleaner. Go to a local NAPA and get some electrical contact cleaner that is safe for plastic. They make two types, one is for cleaning alternators and you don't want that one.

That's good.

Recapping just a bit.

55 = end of codes 12 = battery disconnected within the last 50 power on/off cycles and is only applicable if found with boatloads of other codes. When you find it paired with lots of codes it is usually a bad or loose battery cable. But it can indicate a faulty ground or power wire to the PCM. 24 = Throttle position sensor input above or below acceptable voltage. TPS signal does not correlate to MAP sensor.

With the ignition on, engine not running, backprobe the TPS. One of the outer pins will have 5V on it. The other will have zero. With the ignition OFF you should be able to probe between the ground and the corresponding sensor grounds for the MAP, CPS, O2 sensor, ECT and MAT sensors. Again, it helps to have the schematic with your wire color codes. My 93 has black with light blue stripe as the sensor grounds.

With the ignition ON, engine not running, measure the center pin. Throttle closed > 200 milliVolts Throttle wide open < 4.8 Volts Anything in that range will adjust. The way you adjust the TPS is by turning the key ON (engine not running) then slowly cycle the throttle between full closed and full open about 5-6 times. Then turn the ignition off.

See that second bit "TPS signal does not correlate to MAP sensor". It could be the rubber hose between the MAP sensor and your inake has a leak or the MAP sensor is failing. They can fail without setting codes. They are of course bloody expensive and hard to test.

Just like the TPS sensor, the map sensor has three pins A,B,C

A = Ground B = output (.5v with ignition on, engine off) C = 5V

There is no easy way of testing this doodad.

I disagree... 300rpm is not normal... I feel that the idle air solenoid is closing down when he opens the throttle, then motoring back up after the throttle hits the closed stop. This certainly points to a TPS problem.... let's see what happens after he performs the TPS alignment procedure you gave. __ Steve .

Just a slight expansion on your sequence her, Doug.

I've seen three cases where this problem was actually a dirty throttle body. In one, the map port was clogged solid - took a small dril bit and lots of cleaner followed by a pipe cleaner to get it working.

The other case was a buildup involving the idle air mixture. On the one hand, there was enough buildup around the port to actually keep the pintle on the IAC from seating while the buildup was sufficient to actually grab the pintle and keep it from opening smoothly going the other way. Cure was to remove the IAC motor and clean that idle air port REALLY well.

The third case was on my 88 but I'm not sure the later MOPAR throttle bodies have this and it sounds like yours is the later model. Mine has an idle air bypass with the inlet on the top of the throttle body and a needle valve near the base of the throttle body covered by a tamper proof cover. I had to use an ice pick to remove that cover, take the needle valve all the way out, and clean that entire channel out - it was plugged solid. If you remove the needle valve, first screw it all the way in (counting the turns) so that you can reset it to where it was.

Long and the short of it: clean the throttle body REALLY well before you throw too many parts at it. I finally pulled the throttle body completely off and let it sit in cleaner in an ultrasonic cleaner for an hour or so to get mine straightened out.