Question about Mazda MX-5 Miata

5 Answers

Situation - 1998 NB engine installed in a project car using MX5 looms. In the MX 5 the alternator plug has a grey and a grey and grey/red lead. The grey/red lead goes from the PCM to the rear of the car and then back to the alternator. The grey lead goes from the alternator to the PCM.This describes the current flow as depicted. My Mazda wiring diagram, produced by Mazda, does not indicate what the grey/reg wire does at the rear of the car.I suspect that it samples voltage status from something, as when the two leads are connected directly to the PCM the alternator is unregulated. Connecting the grey/red to an ignition feed source fixes alternator regulation but seems to cancel out the charge warning light fed from the PCM. Regards Bill Moore

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  • wilmor760 Feb 05, 2010

    The grey/red line does not go to the fuel pump input. I have installed this part of the loom almost without any change and it is as per the 1998 MX5.



    The computer involvement is to is to reduce charge rate when the engine is at high RPM and with a high charge rate. In theory this provides a little more power. The alternator regulator is integral and has been bench tested ok. The wiring diagram for the grey wire indicates that current flow is from Alternator to PCM so it is not the computer controlling regulation.



    I believe that the problem of not having the warning light go out is because the grey/red line from PCM to to an ignition feed source is open. This is easily rectified but am usure of the effect to the PCM if 12V feeds back to it. There is probably a diode somewhere but the position is not shown, only current direction.



    If you can tell me what the grey/red wire does at the rear of the MX5 and if the PCM is diode protected at the unit then I will be able to get the system fully operational.



    Regards,

    Bill Moore

  • wilmor760 Feb 05, 2010

    If you refer to Mazda Wiring Diagram 1/98 5423-10-98A page B1b you will see that the GY/R line travels from PCM to rear, to engine which is what the 1998 loom that I have did when it was on an MX5. The wires are easily identified. Before installing the looms in the present car I did continuity checks on the GY/R and confirmed the above. The wire had been cut just beyond the rear connector so was unable to find if it had a pick up or otherwise. From the rear the wire travelled back to the engine compartment via the PPF.

    My diagrams and checks confirm the positioning of the leads on the PCM plug as you state, the difference being the routing.

    With the rear section of GY/R out of the loop the alternator runs unregulated, pushing battery up to 15.5 V and over.

    With an ignition fed source pick up in the system the alternator appears to regulate normally with Battery Voltage 14.2

    In both of the situations above, the charge light goes out on initial start and then comes back on after about 40-50 seconds. The alternator seems to be still charging with Battery voltage remaining at 14.2V

  • wilmor760 Feb 05, 2010

    I did not see the solution comments first time around. The CPS lead is in an adjacent plug and loom. Fairly hard to get them mixed up. If there was no continuity between CPS and PCM I doubt that the engine would run.

    I will try the fix after I get a diode.

    Yes I have done ground checks The earthing is pretty much the same as the original MX5

  • wilmor760 Feb 06, 2010

    You obviously are not viewing the wiring diagram from the correct MX5 wiring manual as if you were you would see that the colours I have stated are correct and are what I physically see in the actual loom along with routing.



    The original question was what does the GY/R lead from PCM to the rear of the car, back via the PPF to the alternator plug do or pick up on? It almost certainly picks up on something (why run it to the rear and back to the alternator) as the alternator runs unregulated if wired up as suggested by the Mazda wiring diagram.

  • wilmor760 Feb 07, 2010

    The harnesses on the car are almost a complete marriage. Engine harness and front looms from the same donor car with the rear loom the same model and year. The only difference being that VDO instruments have been used versus Mazdza.



    Because the instrument and warning lights have been rewired to some extent I have been assuming that the fault would lie in that area when in fact the charge light coming back on after 40-50 seconds was doing what it is supposed to do. Indicate a fault. Initially the alternator showed some charge with the warning light on which tended to mask the fault. It has now ceased charging. I believe that with a servicable alternator/regulator the problem will be eliminated. I have been able to confirm the circuitry as applied to my car from another builder who has installed the same model engine and looms as mine.



    Thank you all for your efforts,

    Regards,

    Bill Moore



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Hi
I know this may sound a little obvious but Have you checked all the earth connections? Also as the PCM wire is travelling to the rear of the car then back again it will be losing power over that distance and the amps will drop off. Try running the wire direct to the front to see if anthing changes.

Regards
Dan

Posted on Feb 08, 2010

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Well bill seeing as how you are building a project car we have to toss out the idea that this can all be solved by a wiring diagram. One of the biggest problems with custom wiriing/ engine installs and one of the main reasons I stopped doing them is the fact that you need to make a complete marriage of both wiring harnesses, and signals that come from the body wiring harness on one model come from a engine harness on the other.
that being said,
what are you meaning by unregulated? as in the voltage is not 14.2V or is very rapidly changing?
please get back to me so i can give more info

Posted on Feb 06, 2010

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Not too sure about the colors you have i see different colors, however the large lead should (normally red) should go back to the battery for charging. the other connector i see only has 2 wires. one goes to the pcm to operate the charge light (other side to meter fuse). and the second wire goes to a main relay which will supply rotor feed voltage/ current to excite the rotor and start the alternator producing voltage. hope this will help you.

Posted on Feb 05, 2010

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Hello! The Gray/Red lead runs from the PCM on Connector #1 pin O, directly to the alternator...The Gray lead runs from the PCM on Conn. #1 pin T to the alternator...The Brown/Red lead runs from Conn. #1 pin Q to the charge light...

That Gray/Red lead is a "straight runner,"as in no pick-off points...It goes from the PCM directly to the alternator. I do not know why, in your loom, it heads to the rear of the car...Are the wire colors easily identified; Not faded?

I would suggest opening off the Gray/Red run to the rear and only allow a straight connection to the alternator...

Let me know if charging system operates normally...


Guru.......saailer

Posted on Feb 05, 2010

  • 2 more comments 
  • David Burbee
    David Burbee Feb 05, 2010

    Hello! I believe I have a solution to the mystery of the Gray/Red wire. Tell me; Is the loom for a 1998 or 1999 MX-5?

    Guru.....saailer



  • David Burbee
    David Burbee Feb 05, 2010

    Hello! I have a solution for you...There are two Gray/Red leads in the loom. One is a run between the PCM and the alternator...The other is a run from PCM Conn. #2 Pin J to the Crankshaft Position Sensor...Check for continuity between the center pin on the CPS to PCM Conn. #2 Pin J...If you look at the CPS you'll see the Gray/Red lead...

    The PCM appears to have a problem controlling the alternator when the proper Gray/Red lead is connected...You can proceed with your fix...If you add a diode to block +12vdc from feeding back into the PCM you'll solve your problem...If the PCM is diode protected having two in series, both to block +voltage, will not harm anything and gives you peace of mind...Also (you probably have done this) Check the ground from body to engine and engine to Negative Battery Terminal, on the X10 scale, Read Zero ohms...

    Let me know how you make out...

    Guru.................saailer


  • David Burbee
    David Burbee Feb 06, 2010

    Hello! Here is a link that sells software which recovers lost data from flash drives. I've got a mail out to them to see how a USB port could be connected to the flash memory in your answering machine...


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    saailer


  • David Burbee
    David Burbee Feb 06, 2010

    Hello! sorry about that. ignore the comment. system burped and sent comment to wrong address............saailer

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I believe the grey/red,goes to the positive input on the fuel pump,so the alternator is activated only on start,and run mode only.The e c m regulates the alternator voltage threw the one grey wire,while the other is for the fault light,it should work,unless the two grey wires are swapped in the wrong place.

Posted on Feb 05, 2010

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