do I need a new battery or an alternator?

[maxZpak]

my car rpms idle at a lil over 500 rpms seems a lil low to me and when i turn on the lights it drops a lil bit more and starts shaking the car a little bit. Do i need a new battery or an alternator how do i check? and if its a battery what kinda battery would you reccomand on getting?

[SilverTwin]

Sounds more like you need to diagnose the cause of the low idle, it will naturally feel rough that low. The car should be at 750-875 rpm at idle.

Give us a few details on the car, NA or TT and maintenance history. It may be as simple as a vacuum leak, dirty throttle bodies or just that the idle simply needs adjusted up.

Tim

[maxZpak]

alright its a 1990 TT with a 170,000 miles on the odometer but I got the car used from a guy who said the car has a new motor in it with 48,000 k miles on it. I dunno about how he maintained it... seems like he didn't have it for to long after he got the motor put in. But anyways it has 555CC nismo injectors, a fidenza friction fly wheel insert, a greddy profec B boost controller, and greddy cat back exhost. Also new timing belt. I have all the reciepts. ALSO my boost controller is not hooked up properly right now because I noticed there are no hoses that run from the greddy controll valve to the actuators ( there are no hoses ran to anything at all) so I think my wastegates are oppen right now, I cant tell maybee their pluged up. And I checked the RPMs on the car and it does idle at about 750 to about 800 but it seems like it runs slugish and rattlesa little. And when I turn on the lights it gets a little worse.

[evildky]

go to autozone and they will hook up and test your charging system fro free, sounds more like an ailing aternator but they can tell you for sure

as for the wastegate actuators, there should be a vac line going from the outermost hard pipe going back to the wastegate actuators, I dunno how the profec works but I assume it simply needs to go inline on that vac line, and without it hooked up you'll be limited to stock boost, assuming the lines are hooked to the actuators, if not then the wastegate will not open and you will run very high boost levels

[maxZpak]

is ailing aternator mean bad alternator in other words? I did a test on my alternator by starting my car and unplugging the battery and the car ran fine so I'm thinking i might need a new battery or is that a bad way of testing it?

[evildky]

GO TO AUTOZONE AND HAVE THEM TEST YOUR ALTERNATOR AND BATTERY, IT'S FREE!

the old unplug the battery while it's running bit can actually damage the ECU on some cars, and all that proves is that at idle you alternator makes enough power to keep the car idling, the question is does it make enough power to keep the car running and charge the batttery? does the battery have a dead cell? only takes a few minutes to find out for free at autozone and likely many other chain parts stores, modern internally regulated alternators can fail in more ways than one, which si why it's a good idea to have it tested