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Discussion Starter · #1 ·
So, my Jeep has been throwing the infamous heater circuit low codes, p0031/p0037/p0051/p0057, which I need to fix before I have to do AirCare (Vancouver's emissions testing, the mandate for which is over at the end of the year...)

Searching around, most threads seem to result in a PCM replacement, which sounds ridiculously expensive.. So I decided to dig around and see what I can find. I've got a bit over a month to try to do it on my own before sulking down to the stealership with my tail between my legs...

Bit of a background, the lights started coming on, but I could clear it (with power off) using my ScanGuage II and the codes would go away for a while, before popping back up again. After a while, it began to reliably pop up in the first 15 minutes of driving from the day after clearing it. (Perhaps heat related?)

I probed the voltages: Key On, Engine Off the voltage is ~1-1.5 volts on the heater circuit for O2 Sensor 1/1 -- I'm not sure of this is normal, but necessitated some digging around. Individually unplugging any of the sensors (or combination thereof) results in the code not returning. (They are the stock sensors that came with the Jeep). Engine running, the voltage seems to bounce between ~13/14 volts (Alternator output) and ~6 volts, and does this somewhat randomly several times a second. (I checked that one out with an oscilloscope to figure out why my multimeter reading was jumping. Will try to get a screen-cap next time I lug the 'scope down)

Checked all the cables, can't find a short, or any obvious fray points. Grounds appear good, and I've cleaned the PCM contacts and ground point on the engine. ASD Relay appears to work nicely.

All codes popped up simultaneously, Jeep runs well, and I'm not getting any other O2 codes, so I don't think it's a sensor problem itself, but rather somewhere else in the circuit.

So, after that long winded background, here's where I'm at: Trying to understand how the PCM Oxygen Sensor Heater circuit works...

I cracked open the PCM, and checked probed a bit with some wire attached to my multimeter probes. (I don't want to remove the potting compound yet, as I didn't have anything to replace it with handy)

I've isolated the Oxygen Sensor Heater driver circuit to the circled area:

Circuit component Passive circuit component Electronic engineering Electronic component Hardware programmer


It appears to be an 4827128AD by ST Microelectronics, for which I could not find any datasheets. Instead, I sent to the ST Microelectronics site and after a bit of digging in their automotive components section, found what appears to be a similar (maybe even equivalent?) part based on function, pinout and package: VNQ660SP High-Side driver. Some helpful information in that. Note that there's a "Status" line, I'm not sure how the PCM uses this.

A closeup of the area:

Passive circuit component Circuit component Green Hardware programmer Fluid


This takes from the ASD Relay output, and drives the Oxygen Sensors Heaters, each individually. The interesting part are the four resistors to the right of the driver. Those are 10k resistors going from the Oxygen Sensor Heater outputs to an unknown net, connected to the what might be a small transistor (Potentially 5 volts? Could this explain the jumping signal I measured, if the PCM drives the heaters on/off for some reason?)

Digging around, I found some NGC PCM training material that has a circuit (Figure 24), which I suspect is incorrect, because driving the heater through a 10k resistor wouldn't work, and because what I've probed is different. I suspect the driver drives the heater directly, and the 10k actually connects to the low-side transistor

That's where I'm at today, this is very much a part time project which might not actually get anywhere. I just figure I'd see what I can dig up before shelling out for a new PCM at the dealership.

To do:
- Find out how the PCM senses the output voltage low condition. There might be another circuit that does the sensing. Will require more probing...
- Get some jumpers so that I can wire up the PCM in the Jeep and isolate the heater signals, this will let me know if what I'm seeing on the sensor end is coming from the PCM or from the harness.
- Keep digging for intermittent problems in the harness, or real problems in the PCM. Maybe re-touch some connections or replace some parts if I get brave..
 

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Discussion Starter · #3 ·
Did you have the Sentry Key on your Jeep? (SKIM)

That's an unknown that's keeping me from buying one online, since I don't know if they can program the PIN for that or if it needs to be done through a dealer.
 

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Did you have the Sentry Key on your Jeep? (SKIM)

That's an unknown that's keeping me from buying one online, since I don't know if they can program the PIN for that or if it needs to be done through a dealer.
No I dont have one. The one they sent me doesnt give me a CEL but if I do the key trick shows a couple codes related to the sentry key. Forgot exactly which ones at the moment. So of they can unprogram it im sure they can program it as well. I'd give them a call though
 

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Geez. After this kind of effort going to the dealership should not be done with your tail between your legs...but after checking your profile, I can see how your career choice contributes to your stubbornness to find a solution to what seems to be an electrical problem.

I've also heard people getting pcms online for much less than the dealer.
 

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Discussion Starter · #6 ·
Someone else has gone down the rabbit hole, much further than I intend to:

http://www.cherokeesrt8.com/forums/showthread.php?t=4504&page=11

That PCM is slightly different than mine, but has several commonalities, including the usage of the 4827128AD high side switch. He's tracked another component to the O2 Sensor Heater circuit, the "LM2901Q" quad comparator. This is likely doing some sort of sensing of the heater levels. I've got a good candidate for which chip this is on my PCM (if the circuit is the same), so that's my next lead for when I get around to probing again.
 

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Discussion Starter · #7 ·
Remembered I snapped a shot on my phone of the oscilloscope readout at the Sensor 1/1 position when I didn't have my USB key handy.

Rectangle Font Display device Astronomical object Technology


From what I can find digging around the internet, it's normal for the heater signal to pulse on/off, as the PCM varies how much power goes to the heater as the engine warms up. (Hopefully this applies to my Jeep's PCM too?) What is strange is that on mine, instead of turning off, it hangs at about 4 volts. Perhaps a flaky driver in the PCM? (That would be easy to replace, but I'm not sold on that being the problem...)
 

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I think I've read that the voltage should drop to 0.5v or less when up to temp (maybe 600º F or so). That should happen in the first few minutes from cold startup.

Can't verify this but dropping only to 4v doesn't seem right.
 

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Discussion Starter · #9 ·
Not much of a useful update in terms of real progress, but I got the o'scope on the PCM while it was installed in the computer, to probe around the driver, with the sensors connected:

Hood Automotive tire Motor vehicle Vehicle Automotive design

(Picture just for the fun of it)

It appears that, when functioning, things work well: The driver gets power (and probably ground?) and drives the heaters properly (PWM signal hitting full votlage, and dropping to ~0v when off). When the MIL is triggered, the computer stops trying to even drive the heaters (makes sense, logically) It also doesn't like to stay in closed loop mode even when the engine is warm.

Here's what the driver output to a connected heater looks like before the MIL goes on:

Output device Font Oscilloscope Computer monitor accessory Television set

(I keep forgetting to bring a USB stick to save the traces on, so I keep having to use my phone camera...)

Looks like probing when the sensor is disconnected provides different results because the drive line is biased at ~4 volts, which is probably used in the sensing circuit. When it senses that there's no heater, it drives it between 12v and 4v randomly, which explains what I saw before.

So.. My "best guessed" right now:
a) the sensing circuit is broken/intermittent
b) The driver is getting an intermittent ground that for some reason I can't find
c) There's a short on the heater line somewhere still that for the life of me I can't find
d) the PCM's internal logic is putting into some weird state based on who knows what.

I'm going to sit on it and think for a couple of weeks while I enjoy the summer, but will probably settle on just replacing the PCM -- To find the sensing circuit I'd probably have to pull a lot of the encapsulate off the board and probe around extensively to get a better idea of where it is and how it works. I don't want to waste away my free time all summer chasing this down, especially since I have a bunch of drive-line seals needing replacement in the next little while..

Looking into the online PCM places, I'm getting a bit weary of getting another "dud" and having to mail it back for warranty exchange-- The cross-border shipping fees and taxes could quickly add up and match the cost of going to the dealership. (Plus if I'm really, really lucky, the dealership will find a stupid simple cheap fix that I overlooked?)
 
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