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Crank, no (partial) start. 5.3 Ω at #4 on OBDII Port. ECM?

Discussion in '1st Gen. Tacomas (1995-2004)' started by DB28730, Mar 20, 2021.

  1. Mar 20, 2021 at 4:57 PM
    #1
    DB28730

    DB28730 [OP] Member

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    1997 T100 4x2, MT, Extra-cab.

    Was running fine when parked a few months ago.
    Replaced:
    Control Switch Relay, Fuel Pump.

    Getting 12v at fuel pump when cranking.

    Will try to start on starting fluid but immediately die.

    OBDII Port test shows
    #4 = 5.3 Ω (<1 Ω required)
    #5 = 0 Ω (<1 Ω required)
    #16 = 13.79v (12-14v required, this is on charger)

    Showing 3v at TPS connector with ignition on (5v required)

    No communication with ECM using varied OBDII scanners.

    ECM fuse in drivers footwell is good.

    Check engine light does not come on at all.
    No evidence of rodent/other damage to harness.

    Replaced ground at negative terminal to body.
    Replaced ground from firewall to engine.

    No change.

    I guess I am leaning towards a bad ECM at this point.

    Would it try to start at all with a bad ECM?
    Is there another test I can run to determine if this is ECM or not?

    I have the FSM and gone through what I think I've found to be relative but I am a bit stuck at this point.

    Any input would be greatly appreciated.
     
  2. Mar 21, 2021 at 6:10 AM
    #2
    0xDEADBEEF

    0xDEADBEEF Trash Aficionado

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    Its a little weird that the ECM would just up and die when parked for a while.

    Measuring 1 ohm isn't easy to do, that 5.3 could easy be a combination of the proper resistance and the contact resistance of the probes. I'm used to seeing 3ish ohms just when i hold the probes on my meter together.

    Which engine is it?
     
    Wyoming09 likes this.
  3. Mar 21, 2021 at 8:37 AM
    #3
    Wyoming09

    Wyoming09 Well-Known Member

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    ECM`s just don`t fail .

    The truck parked underwater ??

    Subject to massive Voltage Spikes while parked?

    If Not that Evil Rodents chewing on wires .

    You may just have Fallen Victim to the T100 curse.

    Depending just what was wrong and what signals the ECM was seeing or thought it was seeing No Fuel or No Spark is quite possible.
     
    gearcruncher likes this.
  4. Mar 21, 2021 at 8:52 AM
    #4
    tirediron

    tirediron Well-Known Member

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    I wouldn't worry about those resistance values; a couple of ohms could easily be just a matter of where you chose to connect the other probe. The slightly low voltage on pin 16 could just be corrosion at your battery connections or something like that, and no connection with the ECM; this is a '97; OBDII was only standardized in 96, and I've had a LOT of trouble connecting to '98 and earlier models.

    If it starts on ether, that indicates that spark and [probably] timing are okay. My first plan of attack would be fuel. You're getting 12v to the pump, but are you getting fuel out? Start by cranking the engine for a long time (~20-30 seconds) and then pull a couple of plugs. Are they wet? If they're dry, almost guarantee a fuel issue. Connect a fuel pressure gauge to the fuel rail and check that. If that's good, then check your injectors. If you have a labscope, this is easy. If not, go out and buy a set of noid lights (cheap like borscht, and it's cool to say, 'Noid light') and check your injector pulses. If you've got fuel pressure, but no pulse, it could well be the ECM, but is more like a bad ground, or missing trigger.

    Let us know how that goes, and we'll move on from there.
     
    Wyoming09 likes this.
  5. Mar 21, 2021 at 3:48 PM
    #5
    DB28730

    DB28730 [OP] Member

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    5VZFE

    I would be concerned with the tool if I couldn't get 0 Ω with the probes touching! lol

    I get 0Ω on the one terminal and 5.3Ω on the other, using the same ground point.

    Definitely getting spark bc it will try to run on starting fluid.

    Getting fuel at the FPR return line.

    i just wish I could determine definitively why I am getting resistance at #4, no communication with ECM, and not MIL/CEL, even at start up.
    This should indicate that there is some communication issue with the ECM.


    Low voltage? The range is 12-14v and I'm getting nearly 14 so that should be perfect?

    I am getting fuel at the FPR return. That was one of my first things to consider. Then when I found the issue of high resistance at #4, and no communication with ECM, it seems that has to be some part of the problem.

    Even if it is not THE problem, it's definitely still not "right" and I have found so much information about bad ECM issues.

    The only reason I haven't done a legit fuel pressure test is I am lacking the banjo/Schrader adapter to properly do it. What an unnecessary pita... :/
     
  6. Mar 21, 2021 at 3:51 PM
    #6
    Wyoming09

    Wyoming09 Well-Known Member

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    Time for the Noid Lights is if your getting the injector pulse.

    Clean the Truck side OBII port if you have not done that all ready.
     
  7. Mar 21, 2021 at 3:51 PM
    #7
    DB28730

    DB28730 [OP] Member

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  8. Mar 21, 2021 at 3:53 PM
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    DB28730

    DB28730 [OP] Member

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    I have a scope, I'll check that out while waiting on the FPR adapter.

    Any idea where on the wiring diagrams the path of #4 can be traced down?
    It would be great if it were just a ground but I've yet to trace it from that point to the ECM
     
  9. Mar 21, 2021 at 3:55 PM
    #9
    12TRDTacoma

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    I would check fuel pressure for sure first. 5.3 ohms is not crazy high to the extent where I would say, "oh yeah it's the PCM or your wiring."

    If that registers fine, then check your fuel injectors, you will need something better than just noid lights though, because all that tells you is that you have voltage there, not necessarily how much, or if the injector pintles are firing off correctly. If you have access to a digital oscilloscope that would tell you the full story of the injectors.
     
  10. Mar 21, 2021 at 3:57 PM
    #10
    DB28730

    DB28730 [OP] Member

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    I do have a scope so I can check that out.

    I guess if I am not getting a good signal at the injectors than I'll move on to CPS>ECM, etc?
     
  11. Mar 21, 2021 at 4:42 PM
    #11
    12TRDTacoma

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    Yeah. I would move forward with that. But first thing is first. Check fuel pressure and then make your moves from there.

    You need to check multiple 5v signal outputs from the PCM later on and make sure they are a valid 5 volts or very close to it at the vital engine outputs CKP/ MAF/ etc. But before you go doing that make sure your basics are covered and documented (if need be) first that way you can start a list of items you have checked.
     
  12. Mar 21, 2021 at 5:01 PM
    #12
    DB28730

    DB28730 [OP] Member

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    It just occurs to me that, it runs perfectly when using starting fluid so it almost HAS to be the FPR?

    I just ordered that adapter but it also occurs me that it tests fuel pressure, but not the FPR.

    I could test fuel pressure by splicing into the line, no need to mess with that FPR.

    So the OBD communication is definitely a think but I don't think it could be the ECM or it wouldn't run so well on S Fluid.

    I'd put off just replacing that but, thinking about it this way, there's really nothing else it could be?
     
  13. Mar 21, 2021 at 5:02 PM
    #13
    DB28730

    DB28730 [OP] Member

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    I'm going to try manually applying vacuum to the FRP and see what happens.
     
  14. Mar 21, 2021 at 5:17 PM
    #14
    DB28730

    DB28730 [OP] Member

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    I THINK I GOT IT.


    I disconnected the FPR and that alone made it try to start and idle on it's own, just a little rough.

    I went to pull some vacuum on the FPR to see if it made any change and it's wide open, cannot pull any vacuum on it at all.

    Going to replace that tomorrow, will follow up.
     
  15. Mar 21, 2021 at 5:31 PM
    #15
    DrZ

    DrZ Well-Known Member

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    The check engine light should come on when you first turn the key to ON and you should be able to communicate with an OBD-II reader. That would be my first concern.

    If you can't pull any vacuum from the FPR, then you are pulling gas out of it? I mean if it's not holding vacuum then your vacuum pump is either pulling in air from somewhere or gas. Usually if the internal diaphragm breaks gas will come out from the vacuum port.
     
  16. Mar 21, 2021 at 5:34 PM
    #16
    DB28730

    DB28730 [OP] Member

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    Yes that is a concern but, it'll become secondary if I can get it to run on it's own.

    MIL/CEL does not come on at all but apparently it's been that way for a while.

    Originally I was leaning towards FPR but then the OBDII test results directed me away from that.

    I didn't want to just replace the FPR since they aren't really that cheap, but i am pretty confident from what I am seeing right now that this should be along the right track.
     
  17. Mar 21, 2021 at 6:00 PM
    #17
    12TRDTacoma

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    Excessive and unmetered fuel at cranking will cause a hard start issue indeed. Replace it and go from there.
     

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