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post #1 of 15 Old 03-30-2012, 06:44 AM Thread Starter
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Question PCM Questions

Damn newfangled computerized engines!! What, in caveman terms does the PCM do?

Here's why I ask.

As some might remember my almost Jeep-B-Que:

https://www.jkowners.com/forum/showthread.php?t=41815

At that time, the dealership did not have a PCM in stock to replace the fried one. The technician stated that I really didn't need it. It would drive fine (and it has since the above mentioned almost disaster) ...but my check engine light would remain on.

The technician then said that I wouldn't need it till my state (Maryland) emissions testing came due. I got my notice yesterday.

So, to my next question. If, per the technician, the PCM wasn't necessary ...does it do something that would require me to replace it before having my emissions testing done?

I only ask cause the dealership quoted me about $500 to have it replaced ...and that's $500 that could go to corners, beer, bacon and hookers.

Anyway, thanks in advance for any information.

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post #2 of 15 Old 03-30-2012, 07:07 AM
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Quote:
Originally Posted by e-jeep View Post
Damn newfangled computerized engines!! What, in caveman terms does the PCM do?
In caveman terms EVERYTHING related to running the engine and then some.

Why is the light on? What code(s) is/are set? That's where I would start. Post up and we can help. Codes make it easier. You may not need a new PCM.
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post #3 of 15 Old 03-30-2012, 07:09 AM
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From my experiance, they dont like any warning lights on the dash on when you do your emissions testing even if its not related to emissions.

But that was when I lived in Cali
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post #4 of 15 Old 03-30-2012, 07:51 AM
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Check engine light on is immediate failure for most any state inspection. What code is it throwing?

I didn't read your other thread, but if it let the magic smoke out, it'll probably need replacement and coding. Which can take time to order a vin specific PCM, don't wait until the last minute.

PCM does everything mostly through CAN bus communication between modules, to checking the correct key, a/c, share info with the trans control module, you name it. The other things it can control directly to the TIPM for
fuel activation, fan, a/c clutch, etc etc.


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post #5 of 15 Old 03-30-2012, 09:26 AM Thread Starter
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All are sensor heater circuit low.

0031
0037
0051
0057


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post #6 of 15 Old 03-30-2012, 09:37 AM
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All four sensors are showing low resistance for the heater circuits? Having all four O2 sensor heaters fail would be pretty uncommon. However I'd also expect to see more codes than just that for a "bad" PCM...but it's possible. Those O2 faults will lead to longer warm-up times before they cycle, which leaves the engine in open loop...resulting in more fuel being used than necessary until they warm up enough. You can bet on the inspection failing you for that, IMO..

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post #7 of 15 Old 03-30-2012, 10:05 AM
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You will fail in maryland. In obdII and newer vehicles the MD test only logs for mil codes and readiness indicators

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post #8 of 15 Old 03-30-2012, 10:20 AM
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Quote:
Originally Posted by Goodysgotacuda View Post
Those O2 faults will lead to longer warm-up times before they cycle, which leaves the engine in open loop...resulting in more fuel being used than necessary until they warm up enough. You can bet on the inspection failing you for that, IMO..
Absolutely which means the tech lied to you telling you it would be fine.

The purpose of the MIL is to alert you, the driver, that the vehicle is operating in a non certified state. ie: emmisions are impacted.


Like Goody said, unlikely all O2 sensors went bad. It's also unlikely that all 4 of their signal return wires are shorted to ground or open (normal that is true) However in your case it's possible those wires burned and didn't get repaired. However, its also not all that unlikely that whatever IC inside the PCM that does that detection is damaged due to a short or an overload. Or a wire trace burned up. It happens quite a bit, especially when there are external (external to the PCM) forces at play that it wasn't designed for.

Pretty easy to test everything though. You can measure the resistance across the O2 sensors at room temp and you should have between 2 and 30 Ohms. Measure the resistance in the wiring from the sensor to the PCM, should be minimal. Measure resistance in the wiring to ground, should be infinite on the signal return wire. Check the connections at the PCM.

However, with all that said, if you've already dumped $2000 into it for new wiring and they did their job right then it probably is the PCM. If its not the PCM then they didn't do the first job right and they need to fix it.
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post #9 of 15 Old 03-30-2012, 10:20 AM
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Quote:
Originally Posted by e-jeep View Post
All are sensor heater circuit low.

0031
0037
0051
0057


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All four heater circuits share the same ground. (pin 2 in each connector)

It's the only common point among the sensors' heaters.

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post #10 of 15 Old 03-30-2012, 10:28 AM
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All four heater circuits share the same ground. (pin 2 in each connector)

It's the only common point among the sensors' heaters.
Yes, good catch. This is on the heater control circuit NOT the signal side as I was leading towards. Some of what I said still applies (would have to be an open circuit on the signal return, not a short to ground) but they do share the same ground.

the ground is the black wire on each of the sensors. check the resistance there to ground.
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post #11 of 15 Old 03-30-2012, 10:29 AM
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Quote:
Originally Posted by ronjenx View Post
All four heater circuits share the same ground. (pin 2 in each connector)

It's the only common point among the sensors' heaters.
Grounds through the module I assume, so it can measure the resistance. Certainly worth checking that pin.

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post #12 of 15 Old 03-30-2012, 10:39 AM
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Quote:
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Grounds through the module I assume, so it can measure the resistance. Certainly worth checking that pin.
Nope. G104 or 102 don't have the schematic now.
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post #13 of 15 Old 03-30-2012, 11:02 AM Thread Starter
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Many thanks everyone. End result is that it needs to be fixed before my 30 May inspection deadline. It is what it is ...time to deal with the dealership again. Going to try a different one this time.

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post #14 of 15 Old 04-04-2012, 02:55 PM Thread Starter
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WINNING!!!! The PCM replacement was covered under warranty due to an earlier recall. Vehicle inspection in the U.S.S.R of MD ...passed!

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post #15 of 15 Old 04-04-2012, 05:16 PM
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Quote:
Originally Posted by e-jeep View Post
WINNING!!!! The PCM replacement was covered under warranty due to an earlier recall. Vehicle inspection in the U.S.S.R of MD ...passed!
Nice!

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