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Old 04-08-2010, 06:37 PM   #1
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Default Voltage Drop with A/C (Now voltage spike while driving)

What do I need to do to solve an issue I the voltage dropping below 12v when the compressor triggers, which causes me to idle to drop really low (300-400RPM), and sometimes leads to stalling if I don't intervene by blipping the throttle? I'm still tuning my idle (just went standalone) so does come into play, I would never stll with the stock ecu controlling idle, but the rpms would still dip low.

Looking at logs from, it seems I would hold 13.9v cruising and maybe drop to 13.5v in boost...time for a new alt?


Don't say remove A/C.
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Old 04-30-2010, 12:24 PM   #2
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I have the same problem when my radiator fans turn on ( drops from 13.9 to 12.9-13.1) until they turn off and then goes back to 13.9.

From the research i've done the 1.6 alternator is 65 amp.

Has anyone used an alternator out of another car that would mount up to our miata's that puts out a bit more amps?
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Old 04-30-2010, 12:38 PM   #3
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I'm almost thinking of just installing a large 2 farad cap. I need it for my stereo anyways as my lights dim when I pump up JayZ.
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Old 04-30-2010, 12:44 PM   #4
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FD Rx-7 alternators are bolt-on, just add a couple washers. That still might not make enough amperage at idle to help.

This is relevant to my interest as my laptop will stop charging when RPMs drop to idle.
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Old 04-30-2010, 12:53 PM   #5
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Quote:
Originally Posted by Bryce View Post
FD Rx-7 alternators are bolt-on, just add a couple washers. That still might not make enough amperage at idle to help.

This is relevant to my interest as my laptop will stop charging when RPMs drop to idle.
FD Rx7 alternators put out 100 amps. Im going to probably go that route, thanks.
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Old 04-30-2010, 02:08 PM   #6
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It seems to happen pretty consistently with any good sized electrical load. A/C, headlights, fans ect. The idle control will compensate by raising the idle but at the same time increase the airflow causing it to lean out and stumble again. I'm sure no one wants to run 12-1 AFR at idle just to get a 13 or 14-1 when there's a electrical load.

Did you try adjusting the idle settings yet, Brain? Mine idles solid at 950 RPM until it gets a load on it and then same problem!
It recovers good unless I'm in a full decel, then it seems like its too lean to recover.

A higher amp alternator might work, I don't know. Idle and fuel compensation would be the ****, I think. Mabey time for MS2 or MS3.Mabey better idle control with them.
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Old 04-30-2010, 03:19 PM   #7
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I just haven't had time to tune it good. This was with MS-II, the MS-III is going back in and it has different code. Also playing on the bench and I have a better grasp of how the code works so I can tweak it more. And it's very important to have your idle cells keep the AFR steady, if you go lean when the load hits, it makes it that much harder to stabilize the idle, so make sure your low rpm, but higher load cells are tuned well.

And Ms-III has an "idle up with a/c" feature (as well as fans). So you wire the a/c button into the MS, then back out to complete the circuit. So when you turn on the a/c, the MS can idle up and then activate the compressor. I just gotta figure out how to implement this, any I'll be the first as far as I can tell. There's no documentation written on it yet.
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Old 04-30-2010, 03:48 PM   #8
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Yes, They're tuned pretty good. I can see almost a full AFR point drop on the lc1 when I turn on the accessories due to the extra duty cycles on the IAC valve though. It was really bad with the lights, A/C and fans all at the same time. It almost leaned out 2 AFR points! I know that's not helping my idle recovery.

Maybe I'll go down and mess with it. Madhatter is coming over anyway so we can redo his grounds on his MS2. He's got mad tyte signal noise, yo. He owes me a daughter board for building his MS2 anyway, so I'll get to try the MS2 idle control and see if it's any better.

On a side note, the MS3 looks pretty cool. The A/C compensation looks very interesting along with all the other features. My mouth is officially watering uncontrollably!
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Old 04-30-2010, 03:54 PM   #9
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is it stupid that it's just sitting on my desk for the past 2 weeks?
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Old 04-30-2010, 04:51 PM   #10
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i had a custom 110 volt Na alternator build for my MSM and its been plenty good
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Old 04-30-2010, 05:14 PM   #11
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i think 110v may be a little high for a car electrical system.
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Old 04-30-2010, 05:31 PM   #12
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Take it in to an autoparts store, most of them will test it for free. When I had the generator on my '53 chevy rebuilt it was $60 and came back looking brand new. 6 volts of fury.

You should just remove the AC.

I get idle droop with fans and lights, but I have no AC to try out a hard load, and my idle is already 1000rpms, so I actually use lights and fans to control idle sometimes, I should probably do something about that.
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Old 04-30-2010, 05:31 PM   #13
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its been working great since. thats when its cold. it evens out in normal operating temps to about 80 amps
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Old 04-30-2010, 05:41 PM   #14
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Quote:
Originally Posted by Braineack View Post
is it stupid that it's just sitting on my desk for the past 2 weeks?
No, you have probably been busy playing with your kitten mittens.
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Old 04-26-2011, 10:06 AM   #15
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Im bringing this back form the dead. Been running the a/c a little and yet again, the car starts acting queer.

I normally see 13.5v without the a/c. If I turn it on and run past #2 on the fan speed **** it will drop to around 12.8v.

now, its not a bring deal it's stable and doens't drop off.....UNLESS, the idle speed drops below aorund 700, then the voltage will nose dive to the 11s, the MS then has to deal with not only afrs and idle speed, but now a bouncing voltage. Once it stablizes, it slowly comes back up to 12.8v.

Here's the bigger issue, if I start the car with the a/c on, the voltages struggle to get greater than 10v. The battery stays around 12v with key ON, but dorps to 10v during cranking and stays there...the car idles very low, and throttle input almost kills it, but it'll slowly get back up to speed and then stabilze. Its annoying, but I try to just make sure to keep the a/c off when I start the car.

Today, I filled up my tank, and happened to have my laptop with me. I started my car after fill up and the a/c was on...I was fooling with PID idle control and making sure it could handle a/c load...I looked down and my voltage gauge showed 20v and the car couldn't idle. I had to let it sit off for a few minutes where finally on key ON it showed 12v again, then idled fine. and the above situtations held true. This is the first anything like that has happened, where the voltage skyrocketed...I have no clue what caused it, I couldn't replicate.

Any clues?!

are there better alternator options for the 1.6L and/or NA?
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Old 04-26-2011, 10:17 AM   #16
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I've got a hunch: alternator going poop

Not sure why, maybe something to do with the voltage.

You have a reason not to suspect the alternator?
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Old 04-26-2011, 10:26 AM   #17
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That's three different alternators that do similar things. The last I replaced gave me similar voltages, but when the a/c was on the voltage would drain out. The current is off my 93 with only ~80K on it.

I mean obviously I suspect my alternator, but I'm wondering if this is normal...the crazy 20v readout wasn't....i dunno dawg. I want something better.
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Old 04-26-2011, 10:53 AM   #18
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Confirm that the voltage reading on the MS is actually correct with a DMM. I would say a good, calibrated DMM but those are expensive and I know you are allergic to spending money.

If they don't match, calibrate your voltage tables. R3 and R6 are not 1% resistors so you may indeed need to.
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Old 04-26-2011, 11:10 AM   #19
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Last time I had checked, both the MS and my DMM showed the same voltage when the car was off and then again idling.

I could use y8s Fluke and try again.

Is there a way to install an external voltage regulator on an NA alternator?
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Old 04-26-2011, 11:40 AM   #20
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Are we quite certain that falling voltage is causing RPMs to fall, and not the other way around?

At first glance, it seems to me that effect and cause are being mistaken for one another.
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