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Wiring Harness Weak Point

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  • Wiring Harness Weak Point

    Recently had an NA6 that came in cranking and not firing. Showed codes for no G signal for CAS, no Ne signal for CAS, a fault in the AFM, and a fault in the coolant thermosensor.

    Traced the issue back to a bad ECU. It's a 25 year old car, so not too surprising.

    2-3 weeks later it's back for a similar cranking issue, but this time the CAS lost its NE signal.

    Again, an older car with a fairly common failure. I'm looking at seven bad 1.6l CASes (CASi? CASeses?) sitting on my shelf right now and 6 of them have "No Ne signal" on them. The other one lost G signal.

    I'm curious what the odds are of there being an underlying issue frying the components versus unlucky coincidence/timing. I'd hate to put this fella back on the road only to have it kill something else, but poking around checking wiring voltages and grounds isn't yielding any abnormal behavior. Not too surprising since it's currently running fine.

    Any good ideas on what to be suspicious of/where to look?
    Originally posted by Beastinthebushes
    Do lots and lots of reading and research. Lots of vendors sell suspension, not just FM.

  • #2
    There was a page a long time ago that went over refurbishing (replacing) the hall effect sensors located in the CAS, and I'm willing to bet there's probably one for replacing the optical sensors in the NA6 CAS as well.

    As far as weak points - negative battery cable's connection to the PPF can get pretty gnarly. Since that's your engine's main grounding point, stray voltages do weird things (tm).

    In general - given perfect conditions, stable environment - not overheating, not voltage spiking - the only electronic component that "wears" is going to be an electrolytic capacitor. And outside of capacitor plague ( ) I'd say 25 years is a pretty good lifespan for them. In general if I'm repairing something and don't see any obvious burn marks or corrosion, the first thing I'm going to do is desolder the caps and check them with my multimeter. <- replacing the hall sensor in a NA8 CAS.


    • #3
      I'm going carbureted with a Holley 390 :-)
      92 Italia "Competizione Speciale". 91 BRG, '69 Corvette "Mojo". '14 F150 Coyote 5.0. Looking for something blue.


      • #4
        I agree with Iscariot on the grounding issues.

        I replaced the grounds on my NA6 with bigger, braided straps, and added an extra one in the engine bay. With the exception of a faulty Circuit Controlling Relay, I didn't have any deep electrical issues like you describe.


        • #5
          Originally posted by iscariot View Post

 <- replacing the hall sensor in a NA8 CAS.
          Thanks for the link, seeing all those dead CASs made me realize I should probably look into repairing them. I have that pile of dead SRS modules I need to sort through too.

          I've checked the PPF ground you mentioned because I agree it gets pretty grody, but it seems good. Nothing looking too remarkable still at this point.

          Reading the majority of your post I suppose your vote is going towards unlucky coincidence?
          Originally posted by Beastinthebushes
          Do lots and lots of reading and research. Lots of vendors sell suspension, not just FM.


          • #6
            FWIW, one of the three times the Miata left me stranded, it was the CAS failing suddenly. I keep one in the trunk, just in case. You may be in to something to see if they can be reconditioned. Like the IACV and MAF, new ones are pricey and hard to come by in a pinch.


            • #7
              My understanding is the NA8's are hall effect sensors, the NA6's are optical sensors (and a little bit more susceptible to grime / oil mist, considering its position). But for the most part - the sensors are dead simple and all the (more) delicate bits are in the ECU housing. 25 years in a hot engine bay though will do a number on almost anything.

              Wiring in has 3 typical failure modes - opens, shorts, and resistance. You can get opens from the wire vibrating and fatiguing (or rodents chewing), shorts from high heat melting the insulation, and resistance from open air and corrosion. Each is pretty easy to diagnose with a multimeter and inspecting the wiring loom - but mazda did a pretty good job on it. I'm surprised I haven't heard of many headlight failures from the pop-ups cycling the 16ga wire.

              As a test engineer I can tell you this - one is a fluke, two is a pattern. If you try looking for a cause for every failure you'll waste time chasing ghosts when it was probably something as simple as made on a Friday afternoon at 4:30. Now - if the CAS fails again in a month, I'd be probing it closer.


              • #8
                Originally posted by iscariot View Post
                My understanding is the NA8's are hall effect sensors, the NA6's are optical sensors
                Just an FYI, some NA6 sensors are hall type as well. The only way to tell is open them. Most are optical however.