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Thread: 1997 F-Body ECM

  1. #226
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    Schematic for the ESide goes slowly, I realize how much info is missing. I wonder if anyone can help me with the part marked:

    16383Y37
    27474
    DELCO 466

    This part is a 28pin PLCC and along with it's neighboring part (144436 also 28 pin PLCC) handle limp home functions and ignition. I have noticed that the part has both analog and digital functions. It *looks* like the part monitors the ignition control analog and makes changes to the control path if it sees an error. I think control by the TPU is disabled under certain conditions and the 144436 part takes over with some default timing. The part is clocked or perhaps just inputs re-evaluated based on a relaxation oscillator. This is set up using an external r and c connected to pin 18. The components installed on my board result in a 2KHZ oscillator. The pin discharges the cap to 1.6V... resistor re-charges it to 3.4V--> results in a saw-tooth wave at the pin.

    Note[Frequency is: f = 1/ ( -1 * ( ln(1.8/3.4)*(R*C))), This works out to 2148hz, measured is 2khz]

    I know this part will reassign control of fan relay 1 in the event of fault. Perhaps just turns it on? Same with the fuel pump relay.

    The operation of the part isn't clear to me. I don't quite know what it tries to do or how it detects a switch. Best guess right now is that the TSide is involved over the ribbon cable.

    Can anyone lift a bit of the fog here? If no, there is significant testing to be done in order to uncover how the system works.

    -Tom
    Last edited by Tom H; 07-28-2020 at 04:54 PM.

  2. #227
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    These are the same on the 94-95 pcm. I guess the function they have must be identical on both pcms.

    Code reveals there are some unknown memory ram regions[that are present in the code] and also some jumps to spark and afr correction based on some bits [???].
    Really interested to find out how that limp mode works and I suppose it is related on internal failure within some pcm components, since we will know what happens when low res signal is lost.

    Maybe a pinout of the 2 chips can show some of the functions.

    The ignition control module have some hardware error handling in the pcm that is only reported to the cpu. Maybe that is part of it.

  3. #228
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    Kur4o: I believe that all the code you speak of is for support of a development computer attached to the 40pin header of both T and Eside boards. I am sure that the 28pin parts, there is no cpu involvement. There are connections to the PRU and TPU along with the ribbon cable. There are four pins on the PRU I have not figured out. I think these handle wholesale faults, one may be an indication if the standby voltage has dipped below a threshold. I have mostly worked through the pinout of the two chips but in many cases it is difficult to make sense of the circuit. When I get enough figured out I will post a preliminary schematic.

    -Tom

  4. #229
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    Continuation of earlier post:

    In my schematic I have named the 27474 part U50 and the 144436 U51. Too difficult to write with the long p/n.

    Guess is that U50 is the limp home chip. It receives the low resolution signal and from this it may make a determination if the engine is running. Using the locally generated 2khz it can count low res pulses and if too few, turn off the fuel pump. If confirmed, this could also be of use to turn off the fuel pump in a crash (?) This aspect should be easy to test. Both low and high res signals connect to U51. It is likely that error detection in the pulse stream is found here. Again more testing to determine.

    The Opti-spark signals are received by the following circuits. More as I figure stuff out.

    List1.jpg

    -Tom

  5. #230
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    How about some filters for the opti signal that generate digital data to tpu.
    I think u51 is some NCR chip similar to the injector driver. Can share some similar limp home functions.

  6. #231
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    Hi,

    I wonder if anyone can help with a riddle regarding the "limp home" system?

    It looks to me as if Tside IC1 (pin 40) is not used in the '97. Do you know if it was used in the earlier (94/5) code?

    Signal originates (I think) in the "limp home 27474" part on pin 10. Also connecting to the "ignition chip 144436" pin 9 and PRU pin 61, last to the ribbon cable pin 9.
    I think this signal is used to check for electrical short/open causing mis-fire. The later code may have substituted the crank position sensor and thus I don't see it used on the Tside.
    All info on this will be appreciated!

    The limp home chip takes control of fan, fuel pump and perhaps with the help of the ignition chip, engine spark. Does anyone know the factors that place the system in limp home mode? I know some of the DTCs might do it. Just looking for the bit that makes the switch to limp.

    -Tom

  7. #232
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    Here's a link to diagrams, but I'm not sure which of the colored PCM connectors pin 40 T side is.
    http://shbox.com/1/4th_gen_tech2.html#1995_pcm_wiring

  8. #233
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    Thank you for the link... good information here but not quite the level I seek.

    There are three or four ICs I have no details on. I need to get hold of some samples, remove them from the board and make up test boards to figure out the operation. Can anyone help me figure out what more common boards have the chips? I think that some of the parts may be common to any engine that has opti-spark. LT1 yes but are there others I might find in the wreckers? Did Northstar engines have opti-spark? What about some of the four and six cyl pcms around the time OBDII came out?

    I could probably find dozens of Chev Cavalier ecms before finding F/Y body ecm. Best for me is if I can get my hands on a board that contains both 27474 and 144436. At least then I could get the ignition structure figured out. TPU and mosfet driver for the injectors would also be a score.

    I will try to find pictures that document some of these ecms here on this site (but it's hard to search here)

    Suggestions?

  9. #234
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    I have heard that Nissan Xterras have an optical sensor under the distributor and they work similar to the LT1 setup. The sensor is definitely not the same but I wanted to throw that out there in case you can get your hands on an Xterra ECM.

  10. #235
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    the xterra ecm looks like it's from mars compared to the GM ecms and im sure wouldn't help much

    plenty of engines have optical slot wheel sensors for rotational position... but nothing but an LT1 (or the L99 that was really just an LT1) ever had an optispark

  11. #236
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    Quote Originally Posted by steveo View Post
    the xterra ecm looks like it's from mars compared to the GM ecms and im sure wouldn't help much

    plenty of engines have optical slot wheel sensors for rotational position... but nothing but an LT1 (or the L99 that was really just an LT1) ever had an optispark
    Should I hold out for an LT1 PCM or do other GM modules have the same chips for limp home/ignition an so on. I can get a couple of Olds 6cy pcms for $10 each. Suspect the ignition is all different because of the multi coil arrangement. This would suggest that most of the module would be different. Probably share most code and perhaps things like the idle stepper motor control, power circuits, Output control modules.....

    Bone yard suggests to me that most of the LT1 cars up here were past the "best before" date 10 years ago. They don't see too many...

    Any pictures of the PCB for the 3.8 V6 PCM available? I could read the chip #s

  12. #237
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    In the end, for just a few bucks and a 3hour drive I came up with a couple of PCMs. Vintage around '96 or 7 for a 3.8L Olds.

    This is a bit of a flyer: looking at these to see if the chipset is similar to the LT1. It isn't. Still, worth the effort though to see how the evolution of GM's PCM happened.
    Mechanically the case is quite different. The LT has two half shells that bolt together. This PCM is a pocket with an end that bolts on. Sorry, I didn't take a picture of the
    assembled unit, but it is a common form factor for that era. I am most impressed at how GM changed the heat sinking.Each of the SIP packages (and there are a bunch of them)
    stands up from the board and is captured on to a heat spreader/sink. There is a thermally conductive sheet placed between the aluminum and the package to make up for any
    surface irregularity. The chips are held down with a beryllium spring which if forced down by plastic wedges. Whole thing is covered in conformal coating to eliminate effect of
    condensation.

    I won't spend too much time looking at this all but a few things jumped out at me. Not sure if there is interest but...

    IMG_1748.jpg

    The "clear" side looks to be an I/O board. It contains I/O drivers which I believe are the same as used in the LT1 except with different lead forming. It has three O2 sensor
    interface chips and tracking for three more. I believe this might make the board I have '96. Probably legal stuff made the cat output O2 sensor necessary by '97 (?)
    There is a 68HC11 on board which I think has an internal rom or eeprom. This is not a 'F1 part like the LT1 has, but an 'E9 or 'A8. This part along with the usual analog
    muxes connects to all the usual suspects: TPS, ECT, MAP, knock filter ....
    There is the same part as used on LT1 driving the MOSFET injectors. The actual MOSFETs are different though, these are in a TO220 package & stood up/heat sinked.

    Inter-board cable is soldered in: no IDC connector that caused me a bunch of pain. Also 48 pins wide not 30.

    IMG_1749.jpg

    The "Blue" side holds the main processor. So much effort is spent in the LT1 boards communicating between the two processors and keeping things in sync. This looks like a
    much bigger processor. I believe it may be a variant of 68332 (I am interested if you can confirm this and or add info). The FLASH is still an Intel part, but is 16bits wide. If I
    am right about the processor, the instruction set is completely different and hails back to the 68000 series circa 1980... Good things don't die easy.
    The OBDII interface LOOKS to be the same part used in the LT1, but carries a different number. I wonder if there is a different part that is driven by serial data, perhaps on
    the SPI. Not enough hours in life to figure all this out. This board has all the power supply circuits and filters. I think it has the part that drives the stepper for IAC. This all
    looks the same as LT1

    This all didn't bring me much closer to understanding the LT1 (my goal) but was an interesting side track.

    -Tom

  13. #238
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    Some 96-00 v8 diesel pcm might have some of the same chips onboard since it uses the same optical sensor for the fuel pump position with the same resolution. It might be only the high res signal utilized but the chips that convert the signal can be similar.

    The processor on the v6 you have is definitely 68332 variant. Gm uses 2 or 3 different version of it. The first design have the tabs on the edges of the processor and the pins are scrambled. Later are without the tabs and have more common pin arrangement.

    Most of the chips are carryover or upgraded versions till the can stuff. You can find similar design, drivers and chips, just upgraded over the years.

  14. #239
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    It's nice to see some forward motion in my project. This morning I finally was able to re-assemble my PCM and start verifying a bunch of earlier projects. Some four months after ordering parts they came in and I have now put them back together. I just hate that parts can't be found locally...

    One earlier sub-project was to build a bench simulation of the distributor. The Opti-simulator had a number of problems. First up, it turned out I needed to take care of VATS. I re-wrote the software such that it now produces the needed 50HZ signal. Then there were all my questions about polarity of the high&low res signals. With these two things sorted, I fired up the bench for a test. Trouble followed was that there is a significant leakage current on the injectors. This is done by GM so that the software can diagnose open circuits to the injector. Adding eight bias resistors and the bench is now up and running.
    I can now work with Ignition and Fuel in a controlled environment.

    I will continue to look at the "limp home" system. This was one reason for putting the bench together. All the circuits interwoven that pass through the two 28pin PLCCs are just about impossible to figure without running the system. I now have that.

    Plan to test out the bootstrap access of the FLASH at some point. I thought that there was interest in this to recover modules without removing parts from the board. I will keep this all on the back burner for a while and press on with the rest of things.

    -Tom

  15. #240
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    Does anyone know the correct relationship of crankshaft position sensor to Opti's low resolution? I believe there to be four lobes on the CPK reluctance wheel, which will result in one pulse for every low res pulse.

    Thinking I need to add that function to my opti simulator.

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