PCM Bypass and External Voltage Regulator

I have a 2008 Hyundai Elantra with an aftermarket 200a alternator. My voltage is all over the place. Dropping into the 12's and spiking into the 15's. I was told that my vehicle's PCM is controlling the alternator and I need to bypass it to help maintain steady 14.xx voltage. I have a charge wire and a two wire plug on my alternator. I owuld like to keep my voltage in the 14.4-14.8 range. How do I properly wire an external voltage regulator? Which external voltage regulator should I use?

After more research a Transpo 911-02R looks like a popular external regulator. Will this work well with my alternator that has a built in IC regulator?

Deleted and reposted after reading your 2nd post: You don't need that external regulator - your alternator has its own regulator. Those regs are for direct rotor control, totally unsuitable for use in internally regulated alternators (unless you bypass the internal regulator, but why?). I'd assume the 2 wires are for a charge light ("L" - which may be required to guarantee charging; a local resistor or bulb from IGN +12V can be used instead), and a Sense "S" that goes direct to the battery's +12V post. I don't know if your PCM needs tricking into thinking it has the correct alternator connected, but you'll get better fuel economy with a non-PCM controlled alternator (albeit with less acceleration).

I was told that my pcm controls the alternators output. And that bypassing the pcm will help me maintain 14.xc volts. Why would bypassing the pcm help my alternators ouput? I think the two wire plug is for the field voltage and ground.

edouble101 wrote: Why would bypassing the pcm help my alternators ouput?

Because a PCM cannot control an alternator for which is was not designed. (Google how it works in your car.) Thinking what 2 wires are isn't good enough, but whoever manufactured the alternator should have the appropriate info. And one will NOT be ground. If the other is field, what type of regulation does it need, and how many Amps? My bet is it's S&L, though others are possible. Externally regulated alternators usually have more than 2 outputs (excluding the B/B+ 12V output and power/chassis GND), and why would you not have been sold or at least told about the appropriate regulator? If there is some legend or terminal labels, or you have an alternator model number, reply with the details.

oldspark wrote: Thinking what 2 wires are isn't good enough, but whoever manufactured the alternator should have the appropriate info. And one will NOT be ground. If the other is field, what type of regulation does it need, and how many Amps? My bet is it's S&L, though others are possible. Externally regulated alternators usually have more than 2 outputs (excluding the B/B+ 12V output and power/chassis GND), and why would you not have been sold or at least told about the appropriate regulator? If there is some legend or terminal labels, or you have an alternator model number, reply with the details.

I looked at the factory alternator and it is labeled. The connections are B+, S and F. L is present but is not connected. Scratch the idea of an external regulator. I should have done more research before I posted that. My voltage spikes up to over 15v and dips into to the high 12v range. I assuming that the internal regulator on the alternator is good. What can I do to prevent my voltage from spiking and fluctuating so drastically? My battery is a little over a year old and rests at 12.7v Is my PCM bad? I can not image that bypassing the pcm would be a good idea either.

Exactly as I thought and wrote above. You have your answer except that you will have to find out what the F is for. Why would your PCM be bad? Doesn't it control the original alternator (not that that is relevant now)?

oldspark wrote: Exactly as I thought and wrote above. You have your answer except that you will have to find out what the F is for. Why would your PCM be bad? Doesn't it control the original alternator (not that that is relevant now)?

From my research F is a field terminal. This terminal is an output to the pcm. S is an input to the alternator from the battery. I could very well be wrong. Yes the pcm did control the factory alternator. The issue was that the factory alternator did not produce enough amperage for the additional loads I placed on it. I wonder if the internal regulator used in the aftermarket alternator has the F terminal an I terminal which would be an input versues an output.

So, get the appropriate voltage regulator for that alternator - unless it is inbuilt and F is merely a "Field tap" for external monitoring. And don't connect your PCM to it - you risk blowing your PCM (aka EMS in normal parlance). Last time: Your PCM cannot control that alternator. (Unless you have the appropriate interface for it.) I wonder just what you will end up destroying...