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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Jeep
Satisfied Customers: 1516
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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Not sure how this works, but this is a follow-up, the latest

Customer Question

Not sure how this works, but this is a follow-up, the latest pcm I've been given is a: P56044810AG shows a code that there's no voltage at the map sensor, and according to my old snap-on scanner there's no voltage at the tps either the engine fires, then
idles for a few seconds and dies. it won't rev either.
Submitted: 1 year ago.
Category: Jeep
Customer: replied 1 year ago.
There is voltage everywhere with the map sensor disconnected, and the engine will run, all sensors read proper at 1500 rpm, P0108 map volt high comes on, let it idle for a bit then dies. Will restart every time and the is 5 volts at the sensor with engine running or just ign on with
Map sensor unplugged
Expert:  Juan Crespo replied 1 year ago.

The fact that this was not happening with the previous PCM leads me to believe the current unit is also not working properly. The reason I say that is because both HO and regular engines have MAP sensors; therefore if there was a problem with the MAP circuit, code P0108 would have been present with both computers.

Customer: replied 1 year ago.
the p0108 only showed up after I unplugged the map sensor....I didn't try running unplugged with the other pcm, but there was the 5 volts there with the key on and the sensor unplugged.
so I presume the code would have been triggered had I attempted running the engine.
Expert:  Juan Crespo replied 1 year ago.

As you remember, I first came in to the conversation at the point where the main issue was a wrong PCM part number. The reason I'm saying this now is because, at that time I assumed someone had previously diagnosed the problem as being a bad PCM. Did that in fact happen, or has my assumption come back to bite me in the rear? ;-0

Customer: replied 1 year ago.
Since I am dealing with autozone and already have theory tied up, I kinda need to prove to them the power "ain't right" so, is there a schematic available that I may verify circuit integrity?
Expert:  Juan Crespo replied 1 year ago.

I can certainly upload the pertinent engine performance wiring diagram for you, however that might not solve your issue. Also, Autozone employs people who sell parts, not qualified technicians who know how to diagnose and repair cars. If someone in the Autozone store is saying that you need to verify circuit integrity before they can exchange the PCM or refund your money, tell them you will as soon as that person shows you his/her ASE L1 certification.

I think we have a couple of different needs here:

  1. You need for Autozone to get you the correct PCM, part 56044810AD - if they can't get it, get your money back and buy it some place else.
  2. I need to know the reason why the PCM is being replaced - this will help me understand what's going on with your engine and will put me in a better position to help you.
Customer: replied 1 year ago.
started running really rough, computor originally showed a P0600 code along with a bunch of others, then just the p0600
I was out of town and a friend's kid put a scanner on it and decided to change the IAC cause that was one of the codes.
didn't help.
I get back to town discovered the drivers side head dropped a valve seat....tears
Take care of that, computer still shows the 0600 so's we order the computer.
first computer does the same thing.
get a second computer, that's when we got the "ho" computer with the map sensor volt low and dual knock sensor codes
get the third computer with a 801AG ending part number, and we are back to the low map sensor voltage with an initial ignition then dies within a few seconds.
unplugging the map sensor, the motor runs reasonably well, but shows a "map sensor high voltage code.
Customer: replied 1 year ago.
also, our debate is not about az employees, it's verifying circuit integrity, which wouldn't be that bad of an idea just in case a pinched wire is knocking out the pcm, or if it is a pcm problem at all. I too have been an "ase certified tech." kinda guy, I do industrial maintenance and electrical work now days. Since I do not keep up with the nuances of the assorted vehicles I thought would be a reasonable way to get semi-reliable information from someone a tad more familiar with the jeep 4.7 and it's nuances than I, and just how critical is the "ad" to this specific model when the store sources claim several other ending letters will serve as well.
thank you very much
Expert:  Juan Crespo replied 1 year ago.

I'm sorry if you misunderstand my statements and purpose. My goal is to help you solve your issue, not engage in any kind of debate or contest to see who knows more.

Being myself a retired Chrysler Master Technician, I would encourage you to go to your local Jeep dealer and ask them what's the correct PCM/SBEC part number for your application as they might have a supercesion that I'm not aware of. Look at this way, it won't cost you anything and at least will ensure you get the right part regardless of your choice of supplier.

Back to the issue at hand, the computer learns engine vacuum value by interpreting an input signal from the MAP sensor. If the sensor is disconnected, the computer will look at other sensors to try to keep the engine running and set code P0108 as it "sees" no change to the 5 volt reference signal it sent to the sensor.

In reading your last post, I notice that you consider the information I provide as "semi-reliable". As a former professor of Automotive Technology at JS Reynolds CC, and Senior Technician Trainer for the largest used car retailer in the world I assure you it doesn't get more reliable than this. However, if you so desire, I will opt out and re-open your question to the field. How would you like to proceed?

Customer: replied 1 year ago.
I'm impressed that my question still isn't answered. may I get a schematic, and, in your humble opinion, (ideally fact based) is the "ad" on the part number critical to my particular vehicle's performance?
Expert:  Juan Crespo replied 1 year ago.

In case you haven't noticed, I've got an ego the size of Mount Everest - which makes my opinion on technical matters anything but humble ;-). However, as luck will have it, we won't have to rely on such opinion because it is a matter of record that Chrysler uses letters at the end of some part numbers to indicate a certain production batch or software release that fits a particular application - as per my database (circa 2009), the AD designation fits your specific vehicle.

Many of these AD units are still available for sale by several outlets that advertise on the internet. However, if you wish to know if there are any SBECs that supersed the AD, only the Jeep dealer can answer that.

In addition, I'll be sure to upload a wiring diagram for you tomorrow after 2PM ET.

That should definitely answer your question regarding the PCM. Once you have the right PCM/SBEC plugged in, I'll be more than happy to answer any other inquiries you have concerning this driveability issue.

Best Regards.

Expert:  Juan Crespo replied 1 year ago.

Wiring diagrams attached as promised.

Best Regards.

Expert:  Juan Crespo replied 1 year ago.

Just found the latest SBEC release for yor GC:

New: 56044810AJ

Rebuilt: R56044810AJ

Supersedes: 56044810AD, 56044810AG, 56044810AH, 56044810AI

Autozone part number: 79-4810V

Best Regards.

Customer: replied 1 year ago.
thanks for the wiring diagrams, and a fourth computer has been installed, along with a tps. pig tails have been inspected, wire harnesses as well, grounds have been cleaned and verified.
the engine starts and runs with the map sensor electrically disconnected(unplugged) there is 5v at the orange wire, black with light blue stripe is ground, and the green wire with the red stripe has 5v. When reconnected(electrically) the engine dies...the only code shown is the P0108 due to it being disconnected and out of the loop.
what do you think?
Expert:  Juan Crespo replied 1 year ago.

The voltage at the Dark Green/Red wire is the signal return wire and should read 0 Volts with the MAP disconnected. The fact that is reading 5 volts tells me that either the return and reference signals are shorted together somewhere in the harness, or the 4th PCM/SBEC is internally shorted. Disconnect PCM (C1 connector) an MAP, then check continuity at the MAP connector between the Orange and the DkGreen/Red wires. If there is continuity (0.00 Ohms), the short is indeed in the harness. However, if there is no continuity (open circuit), then the PCM is internally shorted.

Repairing that short should take care of the P0108, then we can take it from there.

Best Regards.

Customer: replied 1 year ago.
With cam, crank, map, and tps sensors disconnected, there is no continuity or high resistance between the Orange (5v sensor voltage) and the dk. Green w/ red stripe map sensor wire. Connect plugs at pcm and apply power, 5v at both wires at map sensor. Cut the wire at terminal 27 in C1 and voltage is lost at the map sensor's green/red wire, but remains at terminal 27...so...
Another trip to the "zone"😜
Expert:  Juan Crespo replied 1 year ago.

There has to be either continuity or high resistance/open circuit between the Orange and the DkGreen/Red wires. Continuity is the opposite of high resistance. In other words, if you look at your Ohmmeter screen, when both Ohmmeter probes touch, whatever the reading means continuity; when you pull the probes apart, whatever the reading means OPEN CIRCUIT. I don't mean to belittle you in any way, but if there is any doubt as to how to set the Ohmmeter to test these circuits, please give me the brand and model number and I'll help you set it the correct way.

Best Regards.

Expert:  Juan Crespo replied 1 year ago.

Uh, oh... Wife just informed me I'm not supposed to work on Sundays! Will log back in tomorrow after 2PM ET.

Best Regards.

Expert:  Juan Crespo replied 1 year ago.

Just wanted to check and see how things are going with that Jeep.

Best Regards.

Customer: replied 1 year ago.
first off, explain to the bride that even Jesus encouraged people to help get oxen out of the ditch, even on the seventh day, or first day, which ever calendar your using...:-) and I do appreciate the help.
as for a lesson on meter interpretation, I appreciate the offer, and I'm thinking it's a case of semantics as in, when I was in tech school, a long time ago, continuity meant 0 ohms of resistance, and a high resistance could be due to reading through diodes or other components involved in the circuitry to ground or bleeding over to other circuits because of component breakdown, pinched wire or it could even be normal depending on what normal is.
Now, a fifth pcm has been obtained and installed with the same results, as in it will run as long as the map sensor is unplugged.
Customer: replied 1 year ago.
I do appreciate the difference between high resistance and infinity in reading the ohm meter....
Expert:  Juan Crespo replied 1 year ago.

With all due respect, I can't see where semantics are involved in my offer to help you understand how to use an Ohmmeter to diagnose this issue.

In any case, I really want to help you, but can only do so if you do the procedures I indicate and provide me with the information I request. For example, I recommended doing the following: "Disconnect PCM (C1 connector) an MAP, then check continuity at the MAP connector between the Orange and the DkGreen/Red wires. If there is continuity (0.00 Ohms), the short is indeed in the harness. However, if there is no continuity (open circuit); then the PCM is internally shorted"; to which you replied "With cam, crank, map, and tps sensors disconnected, there is no continuity or high resistance between the Orange (5v sensor voltage) and the dk. Green w/ red stripe map sensor wire." As you can see, your reply still doesn't tell me whether or not the PCM connector C1 was disconnected; or what the resistance value was as shown on the Ohmmeter screen. Can you provide that information?

Customer: replied 1 year ago.
the statement concerning the other sensor connectors being disconnected (me not knowing the internal workings of the pcm)is to verify no current flow or resistance read through the sensors..
now to your suggestion and reading between the orange and the green/red at the map connector with c1 disconnected, and there is no continuity .
do you really think I can run through 5 pcm with internal short?
Expert:  Juan Crespo replied 1 year ago.

Once again you don't provide me with the information I specifically requested; i.e., was the PCM C1 connector disconnected when resistance was measured between the Orange and the Dk Green wires? what was the resistance value shown on the meter?. Would you rather I opt out and re-open the question so other experts can try to help you?

Customer: replied 1 year ago.
With C1 disconnected, and meter leads touching the connector pins to the MAP sensor(unplugged, disconnected as well) the analog ohm meter needle does not move, and the digital meter Showa a reading of:"O.L"
(when the digital meters are touched together the screen reads:
000.0 ohms)
Customer: replied 1 year ago.
uh that should have read:
" ..when the digital meter's leads are touched together.."
Customer: replied 1 year ago.
as in the Red lead and the Black lead...
One of my favorite authors, Augustine of Hippo, made this observation of Cicero after he
Had spent some time with him:
"For he was not ignorant of his ignorance, the modesty of a genius mind.."
Thus I strive to be "modest" myself.
I am certain that between my tenacity and your genius this
Problem can be diagnosed and maybe even fixed.
Shal
Customer: replied 1 year ago.
om
Expert:  Juan Crespo replied 1 year ago.

The truth is that I am just a Master Technician - one of about 300,000 in the USA - who is trying to help you solve your car issue.

Back to that issue; with ignition off and C1 connector disconnected, measure resistance between PCM/SBEC terminals 17 and 27. If continuity is shown, then the PCM is bad and installing it will result in the same problem with the MAP sensor.

Best Regards.

Customer: replied 1 year ago.
30.24 k ohm between terminals 17 and 27 at C1
Customer: replied 1 year ago.
is the reading normal? And it reads the same when reversing the meter leads.
Expert:  Juan Crespo replied 1 year ago.

With C1 disconnected, it should show "OL" (or same as what the Ohmmeter reads when probes are not touching anything) between terminals 17 and 27.

Best Regards.

Customer: replied 1 year ago.
I order another pcm.....:-)
Customer: replied 1 year ago.
another pcm from another source came in today, at C1 reading between pins 27 and 17 reads 30 k ohms, just like the others that have been checked.?
Customer: replied 1 year ago.
that is fresh out of the box nothing connected meter leads touching terminals 17 and 27...
Expert:  Juan Crespo replied 1 year ago.

If the new SBEC/PCM still gives you the same bad running condition and the same code P0108, then yes, it is also bad.

I've never experienced this situation before where multiple PCMs all test bad. I'm going to opt out and re-open the question so other experts can chime in. Please don't reply until another expert responds or the site will revert the question to me.

Best Regards.