car doesn't maintain charging voltage

This is a problem I could remedy if I simply increased the idle but basically since I have the xp cam and reduced the idle speed to 700rpm, the pcm seems to disable the alternator after a little bit if the car is idling in park or in gear, then cycle it back on a couple seconds later, and then it does that over and over. I can see it happening exactly like that all the time if I look at the analog voltage gauge or the aeroforce. I would presume this is because the voltage fluctuates too much as the engine speed is not as smooth is it would be with a factory cam and 800rpm idle speed.

I see nothing in that area I can edit with the hpt so I'm just wondering if this is something that an alternator booster like what zzp sells would fix. If it ups voltage about a half a volt, maybe that would keep it above the threshhold to keep the pcm happy?

Do a search on here.
AA bought one of those from ZZP awhile back and found it didn't work on our cars due to the alternator wiring. He did come to a conclusion about wiring in a resistor that boosted voltage by about 1 volt, IIRC.

I thought you would want to increase idle RPM with an upgraded cam?

AA wrote:

I thought you would want to increase idle RPM with an upgraded cam?

I think I've also seen that but have never seen why. If you can supposedly use the cam with no tune and have no problem other than the P0300 code, it seems silly to raise the idle.

My guess is with a cam optimized for high RPM operation, it is harder to keep it running smoothly at idle, so you would raise it slightly. Not the case with your cam, though.

I've bumped mine up a little even with the stock cam. Just seemed to idle smoother, and interior vibrations went away.

I've seen the same when I wanted to hear the cam lope and lowered my idle.

In HPT there's no access to the tables for alternator cut out etc. DHP has the tables of when the alternator should cut out and not generate power. IIRC it's 600rpm. Anytime the pcm sees less than 600rpm your pcm turns off the field to the alternator.

I've thought about finding an overdrive pulley and lowering that setting. Except I'm currently on a 99 Bonneville PCM and Charles never unlocked that year/model. Otherwise I'd make some changes and then use my HPT to change all the other settings back where I'd like them.

If you know of someone local that could make the change to a pcm before writing with HPT it theoretically should work. Or if you have a stock bin (DHP) and a pcm...I would be willing to try it for you. I've got a DHP.

BillBoost37 wrote:

I've thought about finding an overdrive pulley and lowering that setting.

I always get a little confused regarding the use of the words "Underdrive and Overdrive" when referring to pullyes.
Is Overdrive a smaller pulley (so as to turn the alternator faster) for more output.
and Underdrive a larger diameter pulley for less drag or parasitic loss?
If so then why doesn't everyone just install a slightly smaller pulley to get more out of the alternator. Other than the obvious issue of wear on the alt.?

Quote :

Is Overdrive a smaller pulley (so as to turn the alternator faster) for more output. and Underdrive a larger diameter pulley for less drag or parasitic loss?

This is correct.

Quote :

If so then why doesn't everyone just install a slightly smaller pulley to get more out of the alternator. Other than the obvious issue of wear on the alt.?

The wear issue is pretty significant. Just reducing the pulley size a small amount can raise its RPM enough to cause an alternator to run hotter, which causes it to fail prematurely.

Also, you can't have "free" energy. For any pulley driven accessory, the smaller the pulley is, the faster it spins and the harder it is to drive. So you will lose some horsepower by sizing down. This is why you see underdrive pulleys that advertise a horsepower "gain". Their larger diameter makes them easier for the engine to turn, so you get less out of the accessory, more out of the power source (engine).