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Swapped new m20, ran shortly now no spark

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    Swapped new m20, ran shortly now no spark

    A couple days ago I swapped out my ruined m20 and dropped in a new one. I didn't open the motor up, just new cooling system and some other maintenance items. When I first started the car up, it ran fine but I turned it off after a few seconds because it was open headers and it was late. After that, it wouldn't start. There is no spark.

    I've tried 3 different coils with no change in results and been testing directly from the coil HT lead to eliminate cap and rotor as causes. Coils test positive and receives power. The main relay tested good as well as far as I can tell. Key in the on position ICV hums, everything seems to be getting power. I've tried 2 different ECUs - the one from the car and a known working one pulled from a salvage yard. Ground from oil pan to chassis and valve cover to chassis are new wires and firmly attached.

    At this point, I've run out of ideas. Any suggestions would be appreciated in regards to what the issue may be!
    1988 E30 325iC
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    #2
    Check your CKP. If the PCM doesn't see the CKP signal then you will get no fuel or spark.
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      #3
      Originally posted by YosemiteSam View Post
      Check your CKP. If the PCM doesn't see the CKP signal then you will get no fuel or spark.
      I've checked and tried 2 crankshaft position sensors and 2 pulse sensors as well. One crank sensor reads 555 ohms the other reads 570.

      EDIT: I also smell fuel when I crank so I'm assuming it's getting fuel.
      1988 E30 325iC
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        #4
        Ok, when you say the coil tests ok, I'm assuming you mean when ohming it, not that you are getting spark out of the coil?
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          #5
          Originally posted by YosemiteSam View Post
          Ok, when you say the coil tests ok, I'm assuming you mean when ohming it, not that you are getting spark out of the coil?
          Correct. When I check the resistance it reads slightly higher than what the Bentley says, but I'm not using a super high quality multimeter (it is brand new though) and it's within approximately 10%. No spark from the coil though.
          1988 E30 325iC
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            #6
            Ok, in that case I would try to check coil drive. Take your DMM and go from ground to the negative terminal of the coil. Check the Voltage both while cranking and when not cranking the engine. Then check the voltage of the battery while cranking the engine. What are these three numbers?
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              #7
              Originally posted by YosemiteSam View Post
              Ok, in that case I would try to check coil drive. Take your DMM and go from ground to the negative terminal of the coil. Check the Voltage both while cranking and when not cranking the engine. Then check the voltage of the battery while cranking the engine. What are these three numbers?
              Voltage from ground to negative terminal while key is on: 12.1 V
              Voltage from ground to negative terminal while cranking: 2.5 V
              Voltage across battery while cranking: 11.4 V

              EDIT: I also just tried connecting an LED from the negative terminal of the coil to the positive terminal of the battery and it didn't illuminate when cranking. So I think that's a pretty good indication of the issue. Not sure how to proceed from here.
              Last edited by Malcolm206; 06-20-2017, 02:22 PM.
              1988 E30 325iC
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                #8
                Actually, all those numbers are about what you would expect. Your coil wire could be the problem. Pull off the coil wire and see if you have some sort of jumper wire you can stick down the coil tower and check spark with that. (you might get shocked if you hold it, but it's not dangerous.) Or if you have a known good coil wire somewhere try that.
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                  #9
                  Originally posted by YosemiteSam View Post
                  Actually, all those numbers are about what you would expect. Your coil wire could be the problem. Pull off the coil wire and see if you have some sort of jumper wire you can stick down the coil tower and check spark with that. (you might get shocked if you hold it, but it's not dangerous.) Or if you have a known good coil wire somewhere try that.
                  I used a spare spark plug wire and a spark plug to do that, no spark.

                  I also tested positive battery to negative ignition coil with the key off 12 V, key on position 0 V and 0.5 v while cranking. Not sure if that's helpful
                  1988 E30 325iC
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                    #10
                    Measuring voltage between the + and neg side of the coil wont tell you much. cause the way a coil works is that the negative side is grounded by the computer to cause a spark event. As long as the resistance of the two windings is close to what you expect you can rule out the coil.

                    If you have just swapped this thing in I would be looking at simple things first, electrical plugs not plugged in properly, loose connections. How does the signal get from the ecu to the coil? that would be the first plug to check.

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                      #11
                      Originally posted by e30davie View Post
                      Measuring voltage between the + and neg side of the coil wont tell you much. cause the way a coil works is that the negative side is grounded by the computer to cause a spark event. As long as the resistance of the two windings is close to what you expect you can rule out the coil.

                      If you have just swapped this thing in I would be looking at simple things first, electrical plugs not plugged in properly, loose connections. How does the signal get from the ecu to the coil? that would be the first plug to check.
                      I've unplugged and replugged all the connections, they all seem fine. Grounds seem fine as well.
                      1988 E30 325iC
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                        #12
                        when you tested the spark plug did you put the metal outside of the spark plug to ground? it wont spark if you just have the spark plug hanging out of the spark plug wire, the spark plug itself needs to be grounded. it usually grounds through the head when you screw it in.

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                          #13
                          Originally posted by e30davie View Post
                          when you tested the spark plug did you put the metal outside of the spark plug to ground? it wont spark if you just have the spark plug hanging out of the spark plug wire, the spark plug itself needs to be grounded. it usually grounds through the head when you screw it in.
                          Yeah, I even tried grounding to several different locations with no results.
                          Last edited by Malcolm206; 06-20-2017, 05:42 PM.
                          1988 E30 325iC
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                            #14
                            Are you sure the wire you used is good? If it is not, your cap and rotor could be bad.
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                              #15
                              Originally posted by YosemiteSam View Post
                              Are you sure the wire you used is good? If it is not, your cap and rotor could be bad.
                              I'm bypassing the cap and rotor by testing directly from the coil. The wires I've tried are used but worked when they were installed on my car prior to being replaced without any issues.
                              1988 E30 325iC
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