The voltage readings are normal both with the engine off and with it running. But since the problem wasn’t occurring then, no way to know if the alternator or battery is involved yet. Suggest to re-measure the voltage in those conditions when the problems are occurring.
@sgtock21
You need to raise them to replace the headlight. These are from the days of sealed beam headlights.
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Another reason for popup head lights to raise without being lit is so they won’t freeze shut in the winter.
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Should u use a multi meter in AC mode to measure voltage fluctuations?
M/c put a standard multi meter and showed how the voltage fluctuates across the battery and concluded that the alternator is cutting out - need replacing - as I write this post, I am wondering if there might be a loose connection instead of alternator? -
When I serviced the car, m/c checked the battery and said that ibattery shows symptoms of the car not being using frequently (which is not the case) - so the alternator was not charging adequately. Battery had been changed since then
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Can a faulty alternator/wiring cause incorrect diagnosis of starter motor?
starter motor was recently changed - the shop that re-built starters did measure the voltage but did not report any voltage fluctuation (I did not find any either as reported in this thread but I used it in DC mode)
No, a multimeter in AC mode has a limited frequency response. Use the DC mode and watch the numbers. But the output of an alternator with a bad diode is more akin to DC with ripple, with the ripple frequency varying with RPMs. The meter may be able to see that in AC volts mode. But note that the battery will smooth out most of this, as it acts like a huge capacitor.
Also note that most multimeters in AC mode will also respond to DC. Basically for most multimeters the AC volts is intended only to measure 50-60 Hz AC waveforms with no DC imposed.
You really need a scope to check this.
Who is M/C ?
@“Bill Russell”
m/c = mechanic
So what device would measure the voltage fluctuations like a scope?
m/c used one looks like a multi-meter - it is just about the size of a typical calculator
There are scopes that are pretty small.
The one m/c used had smaller display like a calculator
NYBo: I was just making a lame joke concerning why to open the pop-up headlights without the lights on. Your reason makes perfect sense. I am now intrigued. I have owned 4 cars with pop-up headlights (1981 Mazda RX-7, 1981 Fiat (Bertone) X1/9, 1991 Mazda RX-7, and 1996 Mazda Miata) and don’t recall a switch position, separate switch, or button to raise the headlights without illuminating the bulbs. I was fortunate to not have to replace a headlight bulb in any of them so I have zero experience. I searched on-line for headlight replacement on the Mazdas and found everyone possible trying to sell me bulbs. There was a YouTube video replacing the Miata bulb but it started with the pop-up raised and not illuminated. Of course why would they be replacing a working bulb. You have made a wise choice to come to this forum. There are many very well trained and experienced automotive mechanics willing to help you.
On vehicles without a separate headlight lift switch, if you switch the headlights on then switch to the parking light position the headlights remain up so they can be cleaned or replaced.
Nevada_545: Thank you. Not that it really matters now (although I may purchase another pop-up headlight Miata). I was just puzzled.
Alternator was repaired - the warning lights are gone
However an earlier diagnosis is inaccurate - it is not the headlight but the fan on high causes alternator to fail - the cooling fan on low is fine. The repaired alternator still fails when the fan is set to high but passes when the fan is set to low speed.
The cooling fan, a slider switch, is the issue - or somewhere there. Is this an electrician job? I do not how easy to get into the dash and access the slider switch - otherwise, just don’t turn on the fan to high and become the only non-working component in this classic!
BTW, I just unplugged and plugged the pop-up lights manual switch and it now pops-up fine.
Is the cooling fan the blower motor for hvac, or the radiator fan? If HVAC might just need a new variable resistor.
@Barkydog - it is the cooling fan on the dash - not the radiator fan - how easy to change he variable resistor?
@db4690 - in short alternator was rebuilt
went to a good alternator build shop - the first one they tried put the warning lights (for the alternator and pop up light) permanently red - then they replaced the voltage regulator as well. Now the warning lights are gone. They replace the brushes too. I am sure they did a bench test too but the guy went off and I could not get more details.
Should I remove the new battery and have it charged?
The variable resistor is usually very easy to replace less than $50 for the part, mine ws $18, auto parts store should help with location, if not resistor then motor, or control switch shorting out. start simple. Variable resister most common cause of failure.
Sounds like it’s the blower motor going bad, drawing too much current.
From what I read here it seems things have improved by a change in the alternator but it doesn’t look like the charging system is working 100 percent.
Here is some info about the warning lights. The lights are tied into the exciter circuit for the alternator so they will go into a test mode when the ignition is turned on. So when the alternator has a problem it can cause more than just the battery warning light to come on. You seemed to have that problem here.
When checking for AC ripple voltage you should be able to place the meter in the AC voltage mode and check for ripple voltage. While a lot of analog meters do not block DC voltage in the AC mode the digital meters I have used (mostly Fluke) do indeed block the DC component and pickup AC ripple very well. You should normally see less than 0.1 volt of AC voltage while the alternator is running at normal RPM.
The DC voltages you gave out on the first page look good to me if they were taken at the battery. One good test you can do is to check the voltage drop across the alternator to battery lead. There is a direct connection between the two (including a fuse). There should be less than .3 volts between the alternator output stud and the positive battery terminal while there is a good load on the charging system if the connection between the two points is good.