hotshot27

So to give a little background info, my 1999 sc300 has been giving me some grief with voltage issues at idle.

When the car is on and in park, I will see 13.6 volts on a hot alternator. But by the time I put it into drive, have the AC on, and my HID's (fogs and headlights) I see 11.6 volts. But if I rev the car just a little I am able to jump back to 13.6 instantly.

THEORY:

the alternator is internally regulated so there is not an adjustment to increase voltage. My idea was to find a way to safely trick the alternator to putting out just a little bit higher voltage, which in turn will mean less current draw (volts x amps = power) to achieve the same output. I remember coming across some GM threads about using a diode or two in order to cause a slight voltage drop on the alternator sense wire. This causes the alternator to always read a slightly lower voltage than what it puts out. A single Diode will cause a 0.6 volt drop. So in theory, I place a Diode in line with the alternator sense wire, causing it to see a voltage drop, and adjust output accordingly. The result is the alternator seeing the usual 13.6 volts, but in reality by battery and the rest of the car is receiving 14.2 volts.


IN PRACTICE :

This was strictly temporary and for testing purposes, not for a long term.

The alternator sense wire is fused in the engine bay fuse panel by a 7.5 amp fuse. so I simply replaced the fuse with a 6amp Diode (on my first try I had the Diode backwards. car started fine but threw a battery light on the dash. turned car off, reversed the diode and then looked for results)

Before I did this, car was fully warmed up and idled for roughly 20 minutes. Charging voltage was 13.6 volts.

AFTER started the car, no battery light on dash, and volt meter reads 14.2!!!!!!!! I let it run for an additional 30 minutes and it slowly creeped to 14.4 and then leveled off. My guess is because the battery was getting a better charge the alternator was slowly able to put out a higher voltage.

I will be placing the fuse back in its spot, but cutting the sense wire on the alternator and soldering the Diode into place for long term.

I've played through several "what if" scenarios to see what the worst possible thing from this might be. The only one I can think of MIGHT be a little extra wear on the internal regulator. But Its not operating outside the realm of a regular alternator. some cars charge 14.7-15.1 stock. And if my math is right, my cold start voltage used to be 14.4 and should now be 15.0 (acceptable range)



I've driven for about an hour with it like this and saw zero issues, just a nice healthy voltage. My voltage sitting at a stop light with headlights, fogs, AC, etc has increased from 11.6 to 12.6

Please feel free to provide some feedback on this, I don't mind using my car as the guinea pig since it isn't my daily.

P.S. I also discovered a lot of landcruiser guys also do this when they add an additional battery to their car.

chnk

iTrader: (12)

interesting replacing a 7.5 fuse with a 6amp one?

hotshot27

negative. I replaced the 7.5 amp fuse with a 6 amp diode. and that was strictly for testing purposes, not long term. for long term the diode would need to be soldered in line with the sense wire.

over40driv

I agree with your plan. I think when our cars were new the fast idle circuits which raised engine rpm when using A.C. at an idle worked better. Years ago the Grand National guys had a part that you plugged in to the alt. wiring and a vacumn line that would boost battery voltage only at high rpms thus not always over charging the battery.

over40driv

I would also be interested in the part number and ohm of your diode.