What have I done to give myself a P0601? - JeepForum.com
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post #1 of 19 Old 07-26-2020, 02:10 PM Thread Starter
Frambo
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Question What have I done to give myself a P0601?

Hi. I've given myself a bit of an issue.

I was trying to sort out out an issue with my power steering pump and in order to try tightening a nut, I needed to remove the battery tray (mine is a RHD).

So I disconnected the battery and removed it. Then I had to unclip the fuse box so that I could access the bolts for the battery try. To do this I needed to remove the two bolts holding the grounding plate down in the fuse box and detach that ground. I also ended up giving quite a pull on the battery temperature sensor while getting the battery tray shifted out of the way (but did not disconnect the sensor).
I then put it all back together and tried to start the Jeep. No joy. Whoops, I'd forgotten to reattach the ground inside the fuse box. So I reattached that and she started up just fine.

Drove it for 5 days and then a CEL showed up. I did a quick key dance and saw P0601. Disconnected the battery for 30 minutes and when I reconnected, it was gone!
  • The next day, it came back. So this time I hooked up my superchips tuner and cleared the code that way. The CEL went off and the code was cleared.
  • A couple of days later it came back. Superchips couldn't clear it this time, straight after 'clear codes' function was activated the CEL was still on and it was still showing in the 'read codes' menu.
  • Then, a day later, the CEL went out by itself. The next day, it came back and kept coming on and then going off, seemingly at random.

Since then, it's come on and stayed on. Today I disconnected the battery for, cleaned up the only ground that I had disconnected last week, put it back together nice and tight and reconnected the battery, about an hour later. The code and CEL were gone. I restarted 3 times, gave it a rev and then left it. Came out about 45 minutes later, started her up and the CEL is back on and has stayed on today.

I have had a look online and seem some good info from @Uniblurb , @jtec , @OldWJs and others. I do realise that it's normally a bad PCM. However I'm [I]praying[I] it's not that as I am worried that to get a PCM for a export 4.7 HO might be an absolute pain in the backside.

Also it just seems like too much of a coincidence that this all happened 5 days after I fiddled about with the fusebox and battery. Could it be that my PCM isn't actually and the P0601 is genuinely due to incorrect voltage detected?

I don't think I 'fried' the PCM as the CEL took 4 to 5 days to show up. Could I have messed something up with the battery temperature sensor? Can I use a multimeter to check any voltages? Could my superchips tune be affecting things? Can a P0601 that's caused by a bad PCM come and go like mine has been doing?

It's a particular problem for me as although the Jeep is driving fine, I have my MOT in less than 10 days and since 2018, a CEL is an automatic major failure and I won't be able to drive the vehicle for my work or volunteering.

Any help would be greatly appreciated!

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post #2 of 19 Old 07-26-2020, 02:47 PM
jtec
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P0601 is often a death conformation for the PCM.
Lets try and think positive at least try -
the super ships tune is "problematic*" be sure to remove any tunes or adjustments to PCM.
You worried about voltage- Confirm that, grounds are another concern , going over last repair is a good practice when troubleshooting.
Are there any other codes or warning lights?


*its Sunday I am being nice.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 19 Old 07-27-2020, 01:43 AM Thread Starter
Frambo
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Thanks very much! I'll remove the tunes and adjustments to the PCM.

As for checking voltages, where should I be checking?

No other codes or warning lights... actually come to think of it lately the airbag light has stayed on after startup (maybe twice in 60 days) and simply switching off and restarting has sorted that. I don't think that means anything but felt I should mention it.

Cheers
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post #4 of 19 Old 07-29-2020, 11:34 AM Thread Starter
Frambo
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So I tried removing the superchips tune but the tuner couldn't communicate properly with the PCM.

Spoke to superchips yesterday who said that the only option would be to get it reflashed and then they'll also factory reset my superchips tuner for me and update it if I want to put the tune back on afterwards.

I checked battery voltage and it looks good but how do I check input voltage to the PCM? I'm not very experienced with automotive electrics (beyond changing fuses and wiring up relays) but I have a multimeter and I can follow instructions!

thanks

P.S. What's the difference between 'reflashing' the PCM and disconnecting the battery for 30 minutes?
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post #5 of 19 Old 07-29-2020, 12:15 PM
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its a computer SO simply pt-

Removing battery cables - erases all learned and adaptive settings..

reflash is to 'reinstall software' update with latest version.

contact by private message to 'Waterluvr' he is our WJ PCM guru in my opinion, reflashing replacing PCM's he will be your best source for PCM's... maybe have the part # of your current PCM handy .

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #6 of 19 Old 07-29-2020, 03:29 PM Thread Starter
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Thanks so much for the advice on contacting him - I will definitely do that.

Is there any more testing you think I could do, anything to actually ascertain whether there is genuinely incorrect voltage to the PCM or if the PCM is just bad?

I'm just getting swayed by the following facts:

1. I messed with the connection in the fuse box, removed battery and tray etc

2. 5 days later I get a CEL

3. The CEL comes and goes but then seems to be illuminated more of the time

4. A week later it's permanently illuminated.


My gut feeling with that is that if it's gotten worse since initial symptoms, it's more likely a wiring issue - what do you make of that hypothesis?
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post #7 of 19 Old 07-30-2020, 02:54 PM
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Well...I had a 601 very recently. Apparently, a 601 code indicates a volatile memory check failure - essentially RAM inside the PCM. The other PCM memory is non-volatile memory or "Flash" memory - like a USB drive. The actual vehicle software is stored in the flash memory and isn't easily corrupted. It's read-only unless you "flash" it.

You may recall that a PC computer runs through its RAM memory on boot-up and checks it for badness. The PCM apparently goes through a similar routine when it "boots up". Computers are designed to run on a relatively narrow voltage range. Normally, they can accommodate some variance due to multiple sources of voltage regulation involved with their supply. With a PC computer, there is the ATX power supply which is supposed to be highly regulated and conditioned, followed by additional regulation on the motherboard itself.

I believe the PCM is designed similarly in the sense that it probably has some voltage regulation built into it. It also relies on getting relatively correct voltage and .... this is the important part ... CLEAN power from the vehicle generator. In some cases, the PCM's onboard ability to regulate and condition power may not be sufficient or it may have been damaged or otherwise compromised. What some guys who "rebuild" PCMs do is simply replace some onboard power components because that's about all they can do. That probably works in many cases but maybe not all cases if the prior bad power damaged some chips or circuitry, or the rare instance where a chip actually goes bad.

Anyway, if the PCM is not getting clean power because something is failing...say a diode in the alternator or even some other vehicle electrical component is putting noise into the vehicle wiring, then that might cause the PCM to give bad power to its circuits with the end result being a failed memory check. Technically, the memory didn't fail, but the power it was being fed made it sick. And we're talking about potentially very small variances and tolerances where one PCM might puke on the same power that another one fails to throw a 601 code for.

You need to first verify that you're getting *stable* voltage at or near 12v and ALSO that the power to the PCM is not being corrupted by excessive noise or electrical garbage. You need to monitor the voltage output of the alternator for a while....that should give you an idea on the voltage side of things. In terms of power cleanness, I'm not really sure right off the top of my head how you could verify and monitor that on a vehicle but I know it can be done and probably should be done - probably with an automotive oscilloscope of some sort?

Bottom line - it could be a marginal or failing alternator and/or an especially sensitive instance of a PCM. Solid state electronics themselves fail rarely. Chips DO fail, but usually the problem has something to do with the power they're being fed. If nothing is damaged while that is happening, then the problem goes away when the power returns to normal. So, the 601 code is saying, "hey, the memory checked failed!" but what may really be happening is yes, it failed, but it failed because the PCM power components were feeding the circuit junk power - maybe because it is/was/did at some point get junk power from the vehicle.

Anybody who disagrees or thinks everything I just said is $#$%, please speak up. I'm no electronics or electrical expert.
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post #8 of 19 Old 07-31-2020, 03:35 PM Thread Starter
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Thank you so much for this. I will get working on that and report back. Meanwhile if you or anyone else has any further ideas, let me know!
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post #9 of 19 Old 07-31-2020, 04:24 PM
apropes
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Do you have good powers and grounds to the PCM? Any voltage drop? Have you tested to be sure that it's able to generate a 5v reference signal to one of the sensors? May want to scope that.

Any other codes? What does the freeze frame data tell you?

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post #10 of 19 Old 08-05-2020, 04:01 PM Thread Starter
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So the battery and alternator seem fine, next step is going to be to find a wiring diagram and hunt down the location of the PCM power supply and PCM ground.

@apropes , what is a freeze frame?
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post #11 of 19 Old 08-05-2020, 06:04 PM
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Since you mentioned me in this thread Frambo I've been following it but haven't chimed in until I could offer some useful info.

The PCM grounds (2) are on G103 which is the 2nd ground to the rear of the motor mount on the passenger side of your 4.7 block.

The black/white ground wire on the G103 eyelet connects to pin/receptacle #31 of the black C1 PCM connector. And the black/tan wire on the same double eyelet connects to #32 also on the black C1 PCM connector.

Look at the photos towards the end of my ground write-up and the #4 photo below shows what I just mentioned.

PS. It's easiest to access G103 from underneath by jacking up the body behind the passenger side front tire while leaving the axle down.

https://www.jeepforum.com/forum/f310.../#post39199354


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post #12 of 19 Old 08-06-2020, 03:59 AM
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I'm better at grounds than power sources. But the black C1 PCM connector pin-out (04 FSM, 8W-80-113) is showing #22 cavity, circuit A7, wire red/black as being fused B (+) or in other words 12V from battery positive to PCM.

When looking at the PDC (Power Distribution Center) diagram (04 FSM, 8W-10-4) it shows circuit A7 (same circuit as PCM C1 #22), red/black wire connecting to fuse #19 (10a).

You may want to check fuse #19 and the receptacle to see if you're getting 12V of clean power going through them to the PCM.

I've listed all the connections for both grounds (previous post) and power from C1 in case you want to check resistance from the pin/receptacle in the C1 connector itself back to the source.

Below is a diagram from wjjeeps making it easier to see the location of fuse #19 which is labeled as PCM (10a) on the lower part of the diagram. Good luck.
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power_dist_ctr_02.jpg  

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post #13 of 19 Old 08-06-2020, 01:55 PM Thread Starter
Frambo
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This is why I love this forum! Thanks so much, I'll get cracking on this over the weekend!
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post #14 of 19 Old 08-07-2020, 01:04 AM
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You're welcome, hope the info helps, and good luck.

I'll include a diagram of the black PCM C1 connector pin-out below. If you're going to use an ohm meter to check for resistance or full-continuity of the grounds & power PCM wires I'd disconnect both the negative and positive battery terminal connections first.

It' pretty convenient if you disconnect the black C1 PCM connector and look directly at the face of the connector the grounds from G103 for the PCM will be 31 & 32 which are the 2 cavities on the top far right. Then the power from fuse 19 in the PDC to the PCM will be 22 on the top far left on C1. It's easier to look at the connector then to explain. Lol.

Here's the C1 connector pin-out diagram.
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04 4.7 WJ, PCM Black C1 Connector Pin-Out, FSM 8W-80-113, 1154x945.jpg  

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post #15 of 19 Old 08-12-2020, 04:44 AM Thread Starter
Frambo
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So a little update.

Battery (engine off over night, tested in the morning) was at 12.38V

Alternator (turned on engine, checked voltage across battery teminals) giving between 13.5 and 13.9V


Resistance between the red clamp that joins to battery positive and the 12V pin on the PCM connector was 0.02 ohms

Resistance between PCM grounds at the harness and the chassis was 1 ohm (but this wasn't the grounding point G103, just a bolt in the engine bay).

All those figures sound okay, right? Anything else I can check?

P.S. When I reconnected the battery and started after doing these tests, the CEL was gone but came back on the next start. Weird
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