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Alternator suddenly red hot?

 
Old 07-08-2019, 01:09 PM
  #21  
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re: regulator crowbar et al - I don't think our alternators/fans are that smart. I don't really see a regulator failing intermittently - should be working or dead. Also voltage regulation at idle RPM's is difficult, I wouldn't put too much stock in it.
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Old 07-08-2019, 01:26 PM
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That doesn't inspire a lot of confidence, then. I thought I was on to something good.

What about the voltage reference circuit for the alternator? Maybe it is reading from an inaccurate source, a source being dragged down by load and poor connections? Gotta check the diagrams and see where that sucker is conneted.

Ordered:
80a relay listed above
140a NA8 replacement alternator (because why replace with stock amperage?)
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Old 07-08-2019, 01:33 PM
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What I would do - next time it goes from fast to slow, blip the throttle for a second. Alternator is two electromagnets that push against each other, power (and voltage) output is set by the field strength of that electromagnet. Since we're not at a constant RPM, fields have to vary accordingly - which get annoying because if you're trying to suck 5-6hp out of an engine at idle you're putting a significant drag which'll lead to stalling. This is where I put in that whole "don't pay no mind to idle voltages" concept in - alternators aren't meant to self excite too much at idle rpms - and therefore not suck HP. Old GM 1-wire alternators used in big V8's don't even charge until they're nearly at 3000 RPM (granted - in part because of pulley size and V-belt's like to squeal unless tensioned properly, moreso than newer micro-v belts). Newer ECU controlled alternators basically tell the alt's to turn off at idle or steady state cruise (with the heavy consumption accessories off) when battery voltages are good to save gas.

Either way - I wouldn't trust any observations made below 2k rpm - too much handwaving to explain away why - unless it was something obvious like 18v.
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Old 07-08-2019, 01:44 PM
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For my knowledge, what is the difference between an FD alternator and a 140a stock replacement? Does one offer better low rpm voltage?
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Old 07-08-2019, 01:52 PM
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Originally Posted by andym View Post
For my knowledge, what is the difference between an FD alternator and a 140a stock replacement? Does one offer better low rpm voltage?
IDK other than mounting lugs and plug location being slightly different.
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Old 07-08-2019, 01:58 PM
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Gooflophaze, my Miata idles about 1050-1100 rpm because I like it there where it is smooth and not affected as much by loads. I'm not particularly concerned about idle fuel economy, if you know what I mean.

What are your thoughts on my idea of inaccurate signal reference voltage to the alternator?
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Old 07-08-2019, 02:21 PM
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So we get into the details I want to handwave away - because I don't have direct experience with them, just theory. I'm a digital guy with enough analog knowledge to be dangerous. But my understanding is signal reference should be internal to the alt - or at least Savington corrected me a few months ago. However a higher idle might be working against you in this case.

Regulator basically has a target it wants to be at. The further away from it, the more duty cycle (though I'm using a digital term here I'm pretty sure it's just an op-amp output) to the field it pushes, which in return pushes more drag on the alternator and more voltage out to the battery. There's a bare minimum of voltage in the coil, and at that low point it basically says "phhhbt why even try?" - that's your freewheeling voltage. Once that freewheeling voltage goes above a threshold (because RPM has increased) it wakes up and says "oh hey cool I can actually get on with work now" and gives more voltage to the fields which in turn produces more power (and drag) from the alt. As you approach redline, it's backing off that voltage because power = force * speed, more speed less force. If your regulator was bad, you'd get voltage differences consummate with engine speed (well, that's one failure mode - there are plenty of others).

So immediately after startup you have your ASE / cold idle targets. Since the battery is partially drained from running the starter it's lower voltage and the alt regulator wants to charge it cuz it's dumb and sees a voltage diffference - but your tune makes it more transparent. Since you have a higher idle, you might be dipping into that partial-exciting range, which charges the battery up, then it decides "nah I'm good" and voltage/RPM dips below the "feed me Seymore" threshold and you're pulling volts from the battery.

So.. handwaving / shoulder shrugging.
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Old 07-08-2019, 02:34 PM
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Interesting.

Idea!!! The relay and wire were not hot at idle when I was burping the cooling system and initially charging the air conditioning. When I was trying to finish charging the AC system I was manually revving the throttle body steadily for a period of time. Maybe the voltage regulator is bad but it wasn't evident at idle? Maybe revving it up made it get excited and reveal a bad regulator?
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Old 07-08-2019, 02:37 PM
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Alternator reference is the R/W wire, which is tapped on the downstream side of the main alternator to fuse box run. You can see upwards of several tenths of a volt drop across this run depending on load in a healthy application.

Degradation in this path can result in a true alternator voltage that is volts higher than what is reported by say a megasquirt or measured at the battery. If the fan is connected directly to the alternator, it will see this full voltage.

Theory, but easy to test.

You can evaluate pretty much the entire path from the alternator to the fan by measuring the voltage between the alternator and the fan with the fan running. Hot relays or wires mean either the resistance is high (bad connections/contacts) or the current is high. Both of these situations result in power dissipation, with the power going as the SQUARE of the current. Power increases linearly with resistance. IOW, current dissipates more power when increased than an equal increase (%) in resistance.

Last edited by Ted75zcar; 07-08-2019 at 06:24 PM.
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Old 07-08-2019, 05:03 PM
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I doubt it is the relay contacts. Adding resistance to the circuit will not cause high amps at the alternator, but rather low voltage at the fan, causing it to run slow. On a DC motor, voltage is proportional to speed, and current is proportional to torque. It would also cause the relay to get hot due to the extra power dissipated in the contacts.

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Old 07-08-2019, 06:17 PM
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Correct me if I am wrong, but I was under the impression using the alternator output terminal for anything other than the battery cable is a bad idea.

I am not an engineer - just play one on TV - but I believe the battery should be included in every circuit so it can act as a buffer for instantaneous loads, spikes and whatnot.
The primary duty of the alternator is charging the battery. Your ATM for all things electrical is the battery.


Also,

Steve, your idea of checking the voltage reference source seems very logical to me.
I would still monitor system voltage, tho.
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Old 07-08-2019, 06:27 PM
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Originally Posted by Godless Commie View Post
Correct me if I am wrong, but I was under the impression using the alternator output terminal for anything other than the battery cable is a bad idea.
yes
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Old 07-08-2019, 06:32 PM
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Originally Posted by Ted75zcar View Post
yes
I feel validated!
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Old 07-08-2019, 09:15 PM
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Originally Posted by Godless Commie View Post
Correct me if I am wrong, but I was under the impression using the alternator output terminal for anything other than the battery cable is a bad idea.

I am not an engineer - just play one on TV - but I believe the battery should be included in every circuit so it can act as a buffer for instantaneous loads, spikes and whatnot.
The primary duty of the alternator is charging the battery. Your ATM for all things electrical is the battery.
So you're not wrong but not for that reason. The battery acts as a resistive load that the alternator is always "fighting". If the battery disconnects then you get a load dump which basically spikes the voltage and does very bad things - like an arm wrestler that suddenly lets go and gets punched in the face. Wiring-wise, though - 6awg wire alternator - 80a main relay - 6awg starter - 4awg(?) battery. Adding another ring terminal to the alternator doesn't take the battery out of the circuit.. but thinking about it, it does increase the draw across the 6awg cable from the alt to the main fuse if the alternator isn't excited - but if that wire isn't getting hot, I don't see how it could do harm.

Should also mention the main 80a fuse isn't solely for the alternator - there are two common rails it provides for the rest of the fuse box, but I can't recall specifically which fuses are fed off of which side.
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Old 07-09-2019, 06:39 AM
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Would it present a problem if I have added additional wires between the battery and alternator and from the alternator to the 80 amp fuse in the fuse block? After the wiring issue last year at MATG additional wires were run parallel to stock to "help" ensure the paths were adequate.
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Old 07-09-2019, 09:44 AM
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I don't think you need to all the way to the battery itself to wire a fan.

Here's what I did back in the day: I used the 6 mm bolt that holds the main fuse, which gave me unfused B+.
I then made an auxiliary fuse box, and fed that power wire into it.



Not the best pic in the world, but you get the idea...


And, this is my 144 Amp alternator.
I installed a second, 35 mm2 wire (pretty massive by Japanese car maker standards) between the alt and the battery.
Ground side is seriously beefed up, as well.






The second line between the alt and bat has yellow tape on it, so I could identify which is which at a glance. You can barely see the ring terminal of the original wire on the alt lug.
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Old 07-09-2019, 10:10 AM
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I also have a second wire going from the alternator to the battery, and an extra wire going from the alternator to the 80 amp fuse Bolt.

Is the one from the alternator to the 80 amp fuse a problem for me?

I'm trying to see what difference it makes if the power going from the alternator to the battery and back to the fuse is somehow better than going straight from the alternator to the fuse.
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Old 07-09-2019, 12:36 PM
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Originally Posted by sixshooter View Post
140a NA8 replacement alternator (because why replace with stock amperage?)
Where does this 140A alternator come from? A stock Miata alternator is about 60, I thought?

--Ian
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Old 07-09-2019, 12:39 PM
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Simplify. More wires, more joints, more points of failure.
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Old 07-09-2019, 12:47 PM
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Originally Posted by sixshooter View Post
Would it present a problem if I have added additional wires between the battery and alternator and from the alternator to the 80 amp fuse in the fuse block? After the wiring issue last year at MATG additional wires were run parallel to stock to "help" ensure the paths were adequate.
I strongly believe the "chain" should be battery -> alternator -> fuse (box) -> power consuming whatever doodad.

I'll let Joe Perez convince you for the whys and assorted intricacies for such practice.
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