Mazda mx5/miata 1999 not charging

Hey Guys,
I have a Mazda Mx5/Miata (1999) with an Adaptronic ECU, its not charging. Ive done voltage drop checks and wiring is all good, earth and supply to the alternator. I did a current draw measurement and the alternator is not drawing any current. Now Mazda has decided to put the regulator in the ECM, so the alternator only has a 2 pin plug. Both of them go to the ECM. From some recourses I am assuming one is for the ECM to monitor voltage and the other wire to regulate the voltage to the alternator. (and yes the wiring from the ecu to the alternator is good)

Now when I have it plugged in I get a weird reading on one of the pins, voltage slowly increases from 1.5 volts. So i am guessing this is the monitoring circuit. I was getting weird readings on the other wire as well (12 volts), so I started experimenting and put my test light to bat + and back probed this other wire. The car would idle really slow and the battery voltage would slowly increase as the alternator started charging and was drawing around 60 amps (I did this step at the ecu aswell) So would you be confident in saying the ECU driver/regulator is faulty. Im not really sure what voltage i should be reading with the monitoring circuit?

Thanks Mitch

This may help, though it is a 97 Miata

99 mx5 (right hand drive conversion, Australia) is slightly different. There are so many different drawings even the manual was different to the car. Anways......

My idiot light was on the whole time until I did that bypass test (as you did in the video) and would start charging again (idiot light would turn off during this test) and battery voltage would keep climbing up to 16 volts, so i had to stop this test rather quickly. So now Im thinking the input from the idiot light to the PCM is good and that the earth side of the idiot light just goes into the pcm and comes out of the pcm to the alternator to excite the alternator possibly? I guess i will check voltages coming and out and see if they're the same.

Ill have to keep testing, I will have to remember exact measurements and configuration on the plug and report back. This is for a cash job so i am not keen on telling the customer to go buy a $1500 computer if Im not sure what voltages i should be seeing coming out of the computer.

Thank you a lot!!

Okay, this 99 is completely different than the 97. On the 97 (see video) the PCM is not involved at all.
Which wire did you connect your test light to?
Tread carefully with this. I would not be in a rush to replace the engine computer. We need to see the voltage signal or control on the wire that you bypassed with it plugged in AND unplugged from the alternator.
Also, you should have trouble code that could give us some good direction.

ScannerDanner wrote: We need to see the voltage signal or control on the wire that you bypassed with it plugged in AND unplugged from the alternator.

Second this. With the voltage readings, we can get a much better direction for testing.

I've been into this same system on newer makes/models, but it's always a command/feedback setup as you suspected. The grey/red is the PCM's control of the field regulator, and the grey is the feedback to the PCM from the stator. Both will be pulse width signals of some kind, unless there's a fault (and there is).

My suspicion is that the 12V you're seeing (on the grey/red) is a near full-field signal from the PCM, because it's expecting to see system voltage rise and isn't. Why the test light produces a result, I'm not sure, but I wouldn't say the test light bypass means that the alternator is OK. You may be overcoming some kind of resistance issue inside the alternator by using a test light, as opposed to the pulse width signal the alternator usually sees from the PCM. This might also explain why the alternator output goes to 16V. :lol:

For my info, has the alternator been replaced recently? Mostly wondering if somebody has installed a self-regulating alternator before this came to you, and you're chasing a parts problem.

That what Tyler is saying, happen to me also once with a 1999/2000 MX5 Mazda ...
Other shop installed wrong generator ... and i had the same problem to diagnose

The wrong generator wasn't working with ECM's singal but different ...

Solution was to replace again the generator with the correct part ... the one that
uses pulse from the ECM to start charging and turn battery light off.

P.S Battery light was turning on from the ECM when key ON, even
when we had the wires unplugged

Great info on this Tyler, got me over some headscratching for sure.
The grey/red is the field control which rises from 1.5v to 4.0v which by that time the light turns on in the instrument. I then jumped 12v through an led test light from BAT+ into the grey wire which then turned off the warning lamp. The voltage recorded was 4.2volts for reference . So clearly the feedback signal from the alternator is missing. As soon as the test light was disconnected the warning light returned

Many thanks Scannerdanner forum

Nice usage of the LED test light. Thank YOU for adding to the thread!

No problem at all Tyler. I was going round in circles thinking I was missing a turn on 12v from the computer and that it was potentially faulty! or broken wire but whilst google searching, this SD Thread popped up and I knew I could 100% rely on it . I was quite pleased with the feedback wire bypass test myself , saved a heap of time. Customer has authorised new unit for tomorrow