I have a odd overcharging issue with my NB1 Miata. Running voltage at the battery and cigar lighter (is that still what they are called?) Is between 15.5-16.5 volts engine fully warmed up. Voltage at the obd portmeasured via my scan tool is between 13.8 and 15.3 volts. Because of the discrepancy I used 2 different voltmeters at the battery and cigar lighter and came up with similar results. My aftermarket radio apparently shuts off at 16 volts as it would shut itself off whenever the voltage at the battery/cigar lighter reached 16 volts so I am leaning toward that being an accurate voltage. Anyone have any idea why the discrepancy? Also anyone have an idea as to how I make the voltage under control? From what I understand voltage is regulated by the PCM.
Also forgot to add, I swapped out the alternator for new one, and changed the intake air temp sensor.
Hopefully Keith will come - but I don't think the ECU controls the charging on the NB1- IIRC, it was still mostly on the alternator that does the voltage regulation.
Is there an OBD PID that tells you the voltage the ECU thinks it is?
alfadriver said:
Hopefully Keith will come - but I don't think the ECU controls the charging on the NB1- IIRC, it was still mostly on the alternator that does the voltage regulation.
Is there an OBD PID that tells you the voltage the ECU thinks it is?
If it is internally regulated It is possible my replacement alternator has gone bad.
I do have a scan tool which tells me the voltage the ECU thinks it is getting, funny enough it thinks things are mostly normal.
It’s regulated by the ECU.
If the ECU thinks the voltage is correct, then I’d be looking for a voltage drop or bad ground at the ECU so that it is getting an inaccurate number. I don’t recall exactly what everything is supposed to see and with our big event this weekend I’m not able to do any research at the moment - sorry about that.
In reply to Keith Tanner :
No worries! You gave me somewhere to start, which is more than I had a few hours ago.
I would start by checking to make sure your grounds are tight and not corroded, in particular the one at the front of the intake manifold. I would also check the control wiring harness connector going into the alternator, unplug it and see if there's any corrosion or other damage that could result in a bad connection.
As Keith says, the alternator is controlled by the ECU. There is a control wire going from the ECU to the alternator which turns on the magnetic field that makes it generate electricity, and the ECU PWMs (pulse width modulates) it. It periodically measures the voltage that it's getting, compares it to an expected value and adjusts the PWM duty cycle up or down in order to push the voltage in the direction that it wants it.
If you have access to an oscilloscope you can confirm that the ECU is driving the alternator too hard by looking at the wave form on the control wire. If it's on all the time (I forget if that's high or low) then the ECU is commanding the alternator to be on all the time, meaning that it's probably not a problem with the alternator itself. Given that the scan tool says the ECU thinks the voltage is significantly lower than it actually is, this seems likely.
Assuming that's the case, then it's either a problem inside the ECU measuring the voltage wrong (internal circuitry) or a problem with the wiring going to the ECU that's giving it a false signal. You can determine that by checking the voltage on the back of the connector going into the ECU, I think there's a dedicated alternator voltage wire that the stock ECU uses to measure this, but it's also possible it's just reading voltage on the supply power wire (that's what many aftermarket ECUs do). If the voltage you measure there matches the scan tool voltage then it's a wiring problem, if it's the same as the voltage you measure at the battery/etc then there's an internal problem in the ECU.
mtn
MegaDork
6/19/19 9:32 a.m.
drsmooth said:
Also forgot to add, I swapped out the alternator for new one, and changed the intake air temp sensor.
Did the problems exist before the alternator swap?
In reply to codrus :
Thanks for the info. I recently had the upper half of the intake off for cleaning and by default that ground was off Hopefully it is as simple as the ground. Although it usually isnt the easy thing
DIng Ding Ding Ding!!!! We have a winner. It was indeed a ground in particular the one pictured below on the front of the intake manifold. Weird enough it was not corroded at all but when I tested continuity from the bolt labeled 6 to other points around it, there was none, unless I really dug the probe in. I have a theory. I had the intake off for cleaning just after reassembling I took it to get rust sprayed. That connector was loose, I re-tightened it a few days after the rust spray I wonder if the rust spray has dielectric properties and was causing the connection problem. Tonight I took it apart cleaned it with contact cleaner and a brass brush and it was good. 
Happy ending! Nice call codrus.
Once again, the answer is “what did you just do before the problem started?”
From the Krown website.
Krown has a dielectric strength of 40000 volts, meaning it is non-conductive and will not have any impact on the electrical system.
Except in my case it did since it kept the ground from functioning properly.
