Voltage Drop with A/C (Now voltage spike while driving) - Page 3 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 04-26-2011, 06:02 PM   #41
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Do you guys have a "fan airflow g/sec" with the software you use to tune MS? If so, I had to turn mine up from the stock .10g/sec to nearly 1.0g/sec when the Taurus fan kicks on with the AC. Anything less and it would kill the engine.
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Old 04-26-2011, 06:04 PM   #42
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1) Get NB alternator
2) Get my expansion board
3) ???
4) Profit!
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Old 04-26-2011, 06:12 PM   #43
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Do you guys have a "fan airflow g/sec" with the software you use to tune MS? If so, I had to turn mine up from the stock .10g/sec to nearly 1.0g/sec when the Taurus fan kicks on with the AC. Anything less and it would kill the engine.
nah, either on or off, I guess you could pwm the output. but I dont have an issue when the car stalling with a/c activation. I just dont like seeing the voltages drop that low... I idle at 850 and can dip lower when coming off cruise...so the voltage will dip into the tens and have to idle for a while to come back up, once or twice it even causes the stereo to reset...i dont want the ECU to reset/die.

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1) Get NB alternator
2) Get my expansion board
3) ???
4) Profit!
I can build the regulator circuit no problems...just dunno if I wanna go through all the trouble...I am running 20 year old alternators here. I probably just need a good rewound alt.

going to put it through more tests tonight with y8s.
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Old 04-26-2011, 06:14 PM   #44
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I'll make sure to post pics of the results of your testes.
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Old 04-26-2011, 06:16 PM   #45
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wheres my car washing testes pic?!
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Old 04-27-2011, 05:37 PM   #46
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hmmm...

so i unplugged my a/c fan. there's barely any voltage drop anymore, and only during initial activation...holds constant at 13.5v.

It is one of those cheap aftermarket fans, but maybe that just means I need to put it on its own relay like all the 94+ miatas?
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Old 04-27-2011, 05:47 PM   #47
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hmmm...

so i unplugged my a/c fan. there's barely any voltage drop anymore, and only during initial activation...holds constant at 13.5v.

It is one of those cheap aftermarket fans, but maybe that just means I need to put it on its own relay like all the 94+ miatas?
I have two ebay fans, and until recently I had them wired to run full time. Once I got the fan mod working, there was a great improvement in startup, idle quality, transient throttle response, and off-idle hesitation.... when the fans are off.

My stock 1.6 ECU doesn't engage the AC compressor until something like 5 seconds after startup.
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Old 04-27-2011, 05:50 PM   #48
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A relay isn't going to change the total amount of current being drawn... Its possible that fan is drawing an inordinate amount of current, but I would tend to think its more along the lines of the straw that broke the camels back. i.e. you'd have the same effect if you had that fan plugged in but disconnected your blower motor instead.
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Old 04-28-2011, 09:47 AM   #49
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My main fan still turns on with the a/c and everything stays kosher. I can even pop on the headlights and there's barely anymore voltage drop... Everything stays above 13v now. I can even go as far as turning on my headlights and a/c at the same time with my stereo pumping out bass like predator drones drop bombs with very little voltage drop and it always comes back up and stabilizes fast. even at my 850rpm idle speed.

So either the a/c fan I have eats current for breakfast or having the fan and a/c compressor running through the same shitty/weak relay is cause for issue. It's doubtful that just adding the one extra fan to the system is enough to throw it over the edge unless there was a real issue with the motor.

right? probably a reason the 94 went to a seperate a/c fan and a/c compressor relay when they still both activate at the same time.

Last edited by Braineack; 04-28-2011 at 11:54 AM.
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Old 04-28-2011, 02:42 PM   #50
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My main fan still turns on with the a/c and everything stays kosher. I can even pop on the headlights and there's barely anymore voltage drop... Everything stays above 13v now. I can even go as far as turning on my headlights and a/c at the same time with my stereo pumping out bass like predator drones drop bombs with very little voltage drop and it always comes back up and stabilizes fast. even at my 850rpm idle speed.

So either the a/c fan I have eats current for breakfast or having the fan and a/c compressor running through the same shitty/weak relay is cause for issue. It's doubtful that just adding the one extra fan to the system is enough to throw it over the edge unless there was a real issue with the motor.

right? probably a reason the 94 went to a seperate a/c fan and a/c compressor relay when they still both activate at the same time.
Nope. They did it so that they could switch on a second fan if the car is overheating badly (ie the main fan failed).

Using a seperate relay does not reduce the current needed. In fact, a second relay draw more current, about 0.2A. Most probably your "El Cheapo" fan is indeed sucking down a lot of current, and your alternator can't keep up.
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Old 04-28-2011, 02:47 PM   #51
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Looks like I need to whip out my trusty $15 DMM and find out!
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Old 04-28-2011, 02:56 PM   #52
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If your trusty DMM is one of those that can read up to 10A unfused...you've been warned.
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Old 04-28-2011, 02:58 PM   #53
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should be 10A fused. too little for testing the fan's draw?
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Old 04-28-2011, 03:11 PM   #54
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A 60amp alternator is NOT a lot at all.

That 80amp non-turbo rx7 is a good start.

Even an alternator from a honda civic is rated at 70amps.

Just my 2 cents...
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Old 04-28-2011, 03:12 PM   #55
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should be 10A fused. too little for testing the fan's draw?
May blow when the fan starts.
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Old 04-28-2011, 03:16 PM   #56
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drats. Y8s does have an inductive ampmeter, I just need to RTFM.


I wonder if the motor specs are stamped onto the back of it...
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Old 04-29-2011, 09:14 AM   #57
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that fan wasnt the issue, my alt is going bad. it's just random.

today it shot to 20v again while driving and I shut it off, took a while fo rit to drop back down...I guess the regulator is dead.

As far as I can tell the FC Rx7 alternator will fit and its rated at 80A (with maybe a differnet lower bolt). No one local has it in stock, the Auto alternator is rated at 65A, no not in stock. I can pick up a stocker rated at 60A this afternoon. Not sure what I should do, just get it swapped or try to upgrade it as well since I've always seemed to have low voltage issues.
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Old 04-29-2011, 10:05 AM   #58
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cheapness/laziness got the better of me.

found a store that was 9 miles away with the rx7 alt. I called advance auto to make sure I could return a core to any store. That would eat up my lunch break with about 20 minutes of driving each way.

I pretty much always use advance auto for part due to all the discount codes, right now they have a 15% off + $50 gift card code (for purchases over $100), or a 20% off code.

When I called, the dude said he could give me a $40 discount off the alternator, which was 31% off. That brought the stock alt down to $89 after core, and the rx7 one he could only take off $30, so it would be around $115 and I'd have to drive about 20 minutes to go get it.

I figured I didnt want to have to drive out during my lunch break, pay more, and hope it works. Yes the difference was small, but that 23% difference was enough for me to outweight any potential benefits of the FC alt.



but the moral of this story is: order your parts through advance auto over the phone and ask for a better discount than what they are currently offering!
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Old 04-29-2011, 10:22 AM   #59
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cheapness got the better of me.

I pretty much always use advance auto for part

but that 23% difference was enough for me to outweight any potential benefits of the FC alt.
I don't know why you've had so much trouble with alternators in the past.
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Old 04-29-2011, 10:27 AM   #60
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cause they are 19-21 years old and worn out and I keep replaceing them with free 19-21 year old worn out ones?
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