Hey guys. I am having a problem with my cars voltage. I have done quite a bit of engine work to it recently (over the summer) and i dont know if this could be stemming from the lack of a good tune (its getting tuned soon). Hopefully you guys can help.

So i have recently installed a new alternator in hopes to get rid of the problem but to no avail. When i go outside in the morning to start up my beast, it starts up and then i check the voltage. It is always at about 14.2v. My daily commute to work is about 27miles or 30minutes away. By the time i get to my office, i pull into the parking lot and stop my car. Before i put it into park (the car is still in gear) i check the volts and usually i will be pushing around 12.7-13.2v. When i put it into park, the voltage levels off around 13.7v.

What i am wondering is could this be cause by a bad battery or a lack of tune? Also, if it isnt either of those, how can i get steady voltage from my motor? It just bothers me that when i first start my car it feels fine, but when the motor warms up, i pull up to a stop light and it feels like its about to die at idle. It doesnt feel like its camming, (i have an aftermarket cam) it feels like its running rough. Anyway, if you need anymore information, just ask, ill let you know. I would love to get rid of this problem so any help would be excellent. Thanks guys!

 

That sounds like fairly normal charging system operation to me.

The TB and/or IAC may be dirty or need adjusting which is causing the low idle problem.

Of course a bad injector or ingition problem could cause rough idle.

 

In that case, would a dynotune take care of my problems? I have basically had about a whole engine overhaul and it has been driven without a tune since august. I am planning on getting it tuned and put new injectors in it probably in late january or early february. Thanks for the reply!

Matt

 

Voltage depends on load, rpm, temperature (and the resistance in the ignition switch - well at least as far as your dash readout is concerned). Your fluctuations appear to be normal and as you know, very few other cars come with a voltimeter - probably because the OEM's don't want to have a bunch of cars returned to the Dealerships with myriad of non issues.

Weak voltage can cause driveability issues, as can an a/c voltage weeping down the charge line, but I don't think that's the source of your poor idle. You might try to capture some data with a scanner. Otherwise, making sure the throttle body is clean is a good place to start and it would help to know what specifically was fixed or involved in the overhaul.

 

Well, the overhaul (thats what i call it anyway, im probably wrong) was an engine rebuild. It was bored thirty over, new cam and those specs can be found here http://store.summitracing.com/partde...0&autoview=sku. I had the heads port/polished and rebuilt with 2.02" valves. Hooker super comp short tube headers, a new MAF and new injectors that i dont have on there as of right now. The transmission was also rebuilt and i had a 2500 torque converter installed. I dont think the tranny has much to do with it but i figured i would say it just in case. I will definately check the IAC and the throttle body. I am also going to get a new battery before i get it tuned.

 

You can have the battery tested to see if it will hold a charge sufficiently.

 

12.7 is not weak voltage. The voltmeter will show 14.2 after start because you have just put a large drain on the battery. The voltage regulator in the alternator senses the battery voltage and adjusts it's output voltage. That means it is supplying more current to meet the demand and keep the voltage up. We used to use ammeters on cars to show the in and out to the battery. 0 was good because it showed that the system was in balance. I think you need to look elsewhere for your problem.

 

Battery??? sounds like a waste of $$$, charging system sounds normal. Check the IAC and do a fuel pressure test.

 

Your voltmeter readings are normal. The alternator voltage regulator attempts to maintain 14.7 volts across the battery and there is 0.3 volts drop across the ignition switch, so the voltmeter will read about 14.2 volts alternator cold because it measures the voltage after the ign sw. The voltage regulator decreases the alternator output with rising alternator temperature and at full temperature your voltmeter will read 13.2 volts. This is normal operation.
Worry about something else!

 

I'm not sure about the cam, but the ECM/PCM should be able to control idle. IAC acts as a dashpot when the throttle slams shut, so there may be something with that or the throttle body may just be gummed up I'd still scan it and definitely get some datalogged (after you straighten out the idle issue) to make sure it providing all that's been built into it. Then post that info in the Scan and Tune Section.

 

in the morning your alternator can put out 14.2-14.8 volts.There is a temperature compensation circuit in your voltage regulator,
and as it warms up, it allows less biasing of your voltage regulator field. Now as a battery warms up, it has less internal resistance, they drop the volts about a half volt for that phenomena.

at idle,at 600 RPM warm , you get 12.3- 12.5, loaded.

If your RPM drops to 500 RPM, you are lucky to get 11.7 volts.
The alternator is dependent on the internal voltage regulator to support the interaction between the speed of the rotor, stator, and load.