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What is the safe voltage for a car at idle??

19K views 36 replies 7 participants last post by  Institor  
#1 ·
Okay so I have been problem solving with my car for a little while now and have come across a issue where I believe the alternator maybe going out. The other day the car had a hard time starting and I had to replace the battery. It was 4 years old and in Florida that's good for a battery. I tested the voltage while idling and it comes up at 13.4v without the AC or lights on. I feel I might need a new one because with newer cars if the voltage is off there can be problems. Is it normal for it to run that low? I don't have I loop and the voltage doesn't really come up c revs. The car has 80k and maybe the load of the supercharger is too much. What are your guys thoughts??

Thanks
 
#4 ·
@arathol yeah thats what I'm afraid of. It seems it's a little on the low side and I feel It may be causing the cut out under accel. I feel 13.4 is low and when the AC kicks it drops to 13.1
Guess I'll have to buy a new alternator. Question will the 150amp alternator work for peace of mind or should I stick with the 100amp?
 
#11 ·
Alright will do. I have the 150amp alternator on order. The guy brought up a good point but can anyone confirm that the non i-loop and i-loop alternators have the same size pulley?

I look at this way. It probably would have happened even if I didn't supercharger my car because 80k is a lot and Florida is not friendly to electrical components. The only real downside is that the SC has to come off to clear the alternator.
 
#13 ·
I feel I might need a new one
Wow wow slow down man! Charging systems on relatively new cars (including Mazda3 3rd gen) are "intellectual". Voltage and current are regulated depending on battery condition, drive cycle, temperature, SOC, various other parameters.
Voltage on battery doesn't mean anything! Get ELM327, check alternator and battery params (current, SOC, voltages etc.) before throwing out perfectly normal alternator....
 
#14 ·
Yep, what you wrote is exactly what I was thinking as well. However the OP ordered a new alt before I even had a chance to post an opinion, so that bus already left the station. But I did want to let you know that there's at least one other person here who believes the same thing that you do.
 
#15 ·
Oh I just had advance auto order it. I haven't paid for anything. On my cruise into work I saw it held at 13.6v around 2k. I saw it dip to 13 with certain loads but only for a moment. Temp outside was 80f but with humidity felt closer to 90F. I did read some post about the adaptive alternator but should it really run that low? I was using the torque app and voltage (control module) to monitor it.
 
#17 ·
I don't think that a momentary drop to 13V means that a smart charging system alt has a problem. You can have the charging system tested free at a parts store, however I'm not sure how valid the results from those testing tools are for all of the different smart charging systems in use these days. One DIY load test that I like and have used over the years would be an easy thing for you to do, and might give you another perspective on the health of your charging system.

To run the test, you'll either need a helper, or else multimeter leads that are long enough to be able to read the meter from the driver seat while it's connected to the battery. The test starts with a fully warmed up vehicle - a good time would be after returning home from a 1/2hr+ drive. I've always done this test with the engine at idle, never touching the gas pedal. Some flavors of the test call for a constant, higher RPM (1500, ...). However, that would require blocking the gas pedal into a fixed position, because no one can keep their foot steady enough on the pedal to produce a continuous, non-variable RPM for the length of time it takes to do this test. I don't believe it makes a big difference what RPM is used, provided it's constant, but that's JMO.

You start the test by turning off everything electrical in the vehicle that's switchable, and checking the running battery voltage. You can record voltage readings, or not as you choose. Then you start turning the electricals on, one at a time, and monitor the meter readings the entire time you're doing this. On my vehicles, there is typically a very brief voltage drop followed by a quick bounce back for some of the higher draw electricals. If anything produces a voltage drop that doesn't bounce back, I'd wait for at least 30 seconds to see if it does eventually bounce back to at least the starting voltage reading.

You don't shut anything back off during the test, so by the end of it everything that can be switched on will be running concurrently, including the blower on high with A/C on (or whatever 'max' setting is available for auto climate controls). Save turning the wipers (on max) for last because you'll need to keep the windshield wet to avoid damaging the wiper system, and will want to use as little water as possible with the hood open. If something didn't go right or doesn't seem to make sense, just start over and run the test again from the beginning.

Even though I wrote a lot, it's really an easy test to do, and it may very well make you feel more comfortable about the condition of your charging system. However, if by chance your vehicle's charging system begins to struggle, or shows unexpectedly low voltage (which doesn't come back up) as the testing moves along, then that would be an indication that there may very well be an issue with it.
 
#16 · (Edited)
Check estimated state of charge (SOC) of the battery and battery current. PIDs - BATT_SOC and BATT_CUR in PCM (power control module).
If SOC is relatively high (I don't really know precise numbers, perhaps > 70%), Mazda will try to save fuel, ergo low voltage, low current. My Mazda Axela (2017 with i-stop) keeps approx. 0-1 Amps to battery on idle (when SOC is above 75% estimated).
It also may try to charge the battery when coasting and braking (low engine load situations) to save fuel, which complicates diagnosing (you have to monitor these parameters when driving, or record them alongside with acc pedal position, engine load, speed, gear etc. for further analysis). And different cars may have different PCM firmware with different charging strategies.
And on top of that, your battery may have some issues that may lead to PCM's incorrect SOC estimation, that's another story.
Car's electrical systems are not so simple these days...

P.S. It can be some corrosion after all, or bad alternator or battery. All i'm saying is do not throw parts in it, this can end up really costly both in time and money :)
 
#18 ·
Maybe somebody should read the service manual procedures for checking the charging system.......the reality is its not so complicated. Pretty simple actually.....test for voltage (using a proper diagnostic DVOM) should be 13 -15 volts at idle, minimum acceptable current should be 70% of nominal at 2500 rpm, tested at the battery, nothing to do with the PCM or anything else. You need to disassemble the alternator for further testing. Crash44s alternator is at the minimum spec for output, so its probably due for replacement.
 
#19 ·
Maybe somebody should read the service manual procedures
This:
283765

and this
283766

renders the service manual instructions somewhat... inconclusive.
70Amps from defroster and lights? especially LEDs? I doubt it.

should be 13 -15 volts at idle
It is, according to OP.
tested at the battery, nothing to do with the PCM or anything else
Seriously? Read it youself first:
283767


Crash44s alternator is at the minimum spec for output
Is it? Can not find where he said that. Voltage means nothing, as i explained. 13-15 from service manual does not mean that 14 is ideal, it simply indicates voltage boundaries.
 
#25 ·
So I drove on the highway and to honest I have to go with arathol on this. I appreciate all your guys help and will do the test but to be honest I think need to just replace the alternator with a replacement stock 100amp. I drove home this evening and the highest voltage was 13.6v with headlights, a/c and driving on the highway I saw more drops under 13 which makes me think the system is failing. I think the main factor is that I have a supercharger on it which requires the fuel system, throttle, and all the other electric components to work harder. On a NA car it probably would be fine but from my experience with newer cars if the voltage is unstable or low. The injectors don't fire properly, TPS can lag, and to be honest I only have the base tune and it still runs rough with boost which it really shouldn't based on the other 2 people (@WoodinvilleDan and @abou824)
I'm not saying it couldn't be the tune but considering my battery died I think it leads me to the fact that the charge system is worn and I should probably replace it before something else breaks. Plus it could relieve some symptoms that I'm experiencing.

Again thank you guys for the help and I'm sure this will help others too.
 
#34 ·
The only time my Mazda3 system voltage approaches 14V or higher is in cold weather. I live in a semi-tropical climate in Louisiana, so I rarely see a voltage that high.

I've got notes indicating that on the highway in ambient temperatures ranging from 60F to 100F, my voltage runs at 13.5V, plus or minus 0.1V. At 100F ambient, I recorded 13.3V on the highway. Of course, the AC is on. I might as well epoxy the AC switch in the "on" position where I live.
 
#35 ·
Okay so I went for about an hour drive with the sun blazing. I used car scan instead of torque to monitor the voltage. And I have to say if Mazda thinks that 13.2 to 13.3 volts while cruising is efficient. They may want to rethink. I think I'm moving forward just replacing the alternator because at this point i dont see any other solution except to replace it. I know it is technically still with spec per the manual, but barely. Metorgray thanks for the info and I think others will find it helpful. Honestly though the voltage drops below 13v way too many times and especially on higher rpm with low boost(1-2psi. I feel it is struggling to recover even coming to a stop and than the ac kicks on and drops to 12.5v with a couple sec of recovery to 13.2v. I am hopeful this will fix my issues. I have checked the grounds and cleaned the positive terminal. This really is the only other thing I can think of.