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914World.com _ 914World Garage _ Alternator Output not keeping up with audio system
Posted by: Kansas 914 Oct 25 2011, 11:14 AM
I installed a Boston Acoustics GTA-504 and since install I have not been able to charge the battery during long drive with the music cranked up. I really thought the 55AMP alternator would have been fine for the small amp.
I am running 6.5" separates on 2 channels and bridged the other 2 channels to mono for the sub. All speakers are 4 ohm.
There are two 25A fuses on the amp and I added a single 60A in-line at the battery for the power to the amp. No fuses have ever blown - but I wondered if the alternator is actually cranking out 55 Amps. If I don't drive with the music on it works and charges the battery fine. I think I might just install a bus alternator (75Amps) unless others think it is a mistake.
Link to install manual: http://www.bostonacoustics.com/PDFs/manuals/GTA_2_4_Ch_Man.pdf
Quick Specs:
Posted by: kg6dxn Oct 25 2011, 11:24 AM
You can try adding additional grounds from the battery to the engine.
Bottom line, alternator to small to keep up with demand.
Posted by: Vacca Rabite Oct 25 2011, 11:26 AM
Lots of things here
Corrosion at connectors force more current draw then expected. It may not be enought to pop a fuse, but a little will add up when the alt is at the limit.
The alt may not actually produce 55 amps any more.
If you have better headlights, horns, fuel pump, oil cooler, AC, they are all sapping more power then was designed.
Voltage regulator may be slightly wiggy.
Too many variables. Do a load test and see what kind of power you are really making.
Posted by: Kansas 914 Oct 25 2011, 11:27 AM
You can try adding additional grounds from the battery to the engine.
Bottom line, alternator to small to keep up with demand.
Hi Mike - thanks - I was hoping I would be okay - moving to plan B (Bus alternator). I don't think spending the big bucks to have the alternator re-wound makes sense for this small system.
Posted by: Kansas 914 Oct 25 2011, 11:30 AM
Lots of things here
Corrosion at connectors force more current draw then expected. It may not be enought to pop a fuse, but a little will add up when the alt is at the limit.
The alt may not actually produce 55 amps any more.
If you have better headlights, horns, fuel pump, oil cooler, AC, they are all sapping more power then was designed.
Voltage regulator may be slightly wiggy.
Too many variables. Do a load test and see what kind of power you are really making.
Connections are all new - regulator is not. I will get an ammeter and see what I am truly working with.
Thanks Zach - cheers!
Posted by: kg6dxn Oct 25 2011, 11:40 AM
You need to add up all the differet demands from your system. The factory alternator was sized just big enough to run the car with a cheezy AM radio. Once you add load you need to upgrade. First, you can up size the charge wire from the alternator. This will help current flow more freely to the battery. Upgrading or adding grounds will also help. Car manufacturers size all the wiring just big enough to prevent fires. Not for electrical performance.
I don't have a T4 engine. I run a 185 Amp Powermaster alternator. I never run out of juice.
Posted by: stugray Oct 25 2011, 01:12 PM
Who drives a 914 while NOT listening to the engine? ;-)
Just curious?
Stu
Posted by: Spoke Oct 25 2011, 01:39 PM
Not charging the battery with the amp can be tested with a voltmeter on the battery.
You can test this by running the car at about 2k RPM with all electrical appliances off and a voltmeter on the battery.
Monitor the battery voltage and turn on electrical appliances one by one; lights, amp, etc. and see when the battery voltage drops.
Posted by: messix Oct 25 2011, 02:09 PM
the head lights draw more power than that amp would, does your batt drain when driving with the lights on?
Posted by: Rod Oct 25 2011, 02:29 PM
That amp should not flatten the battery whilst driving, when driving the amp takes it's current from the alt, not the battery so I reckon there is actually something up with your battery - what does it float at??
Posted by: stugray Oct 25 2011, 02:47 PM
"when driving the amp takes it's current from the alt, not the battery"
The amp will take the current from whichever supply is at the higher voltage.
When running the whole system is at the same potential, so all share the load.
If the car pulls 60 amps, and the alt is only putting out 50 amps, then 10 amps is coming from the battery until loads are shed to below 50 amps.
Even then, if the alt puts out 50 amps, an the load is exactly 50 amps, then there is not enough left to charge the battery.
Stu
Posted by: Rod Oct 25 2011, 03:08 PM
That amp will be drawing 26.6 amps when driven really hard - no more. His alt surely still puts that out??
I have a theoretical 133 amp drain on my battery using the same watts/voltage equation (1600w system) and never struggled with the 55 amp alt once I changed to an Optima battery.
Check earths (especially on the amp to chassis cable) and make sure your batts upto scratch 
Posted by: Cap'n Krusty Oct 25 2011, 03:28 PM
Good luck finding anything other than the 55amp version. The major wholesalers of Bosch remans don't even list it, and it was pretty rare when it was more-or-less available.
Posted by: Drums66 Oct 25 2011, 03:43 PM
......What about a 911 alternator?...but I think everything in the circuit
have to be comensurate no? 
( may be over KILL) try some of the suggestion's before you get drastic!
Posted by: Kansas 914 Oct 25 2011, 03:51 PM
Who drives a 914 while NOT listening to the engine? ;-)
Just curious?
Stu
I was on a 5 hour drive from MUSR. Car was great at the event - no issues. Drove home 5 hours listening to tunes and the battery was dead in the driveway...
Posted by: hcdmueller Oct 25 2011, 04:02 PM
The bus/912e alt is 70amp and available from places like Autozone or NAPA. I can't guarantee if they are great quality. You are supposed to use a different pulley with a cooling fan but they are NLA so you will have to use your regular 55amp pulley. The cooling boot is different but available from Bus Depot. I have an old Bosch 70amp in my 914. The biggest difference I noted was when I added a ground wire straight from the alt to the chassis. I would try that with the 55amp first.
Posted by: stugray Oct 25 2011, 04:18 PM
"The biggest difference I noted was when I added a ground wire straight from the alt to the chassis. I would try that with the 55amp first."
OR a ground straight from the alt to the Amp.
If the Amp is pulling the highest current then that would make the biggest difference.
Stu
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