low voltage under load - bad alt. vs bad ground

CILO · 02-12-2007, 09:33 AM

Hi all,

Can someone please tell me where the engine grounds are located on the 3.0.

I have a feeling my lean fuel issues may have something to do with insufficient ground. Yesterday i tested the voltage at the battery with the truck in drive, headlights, and fan blowing a full. I was coming up with a reading of about 12.8 volts. under such conditions, is it possible my amp is on its way out?

Also, I've noticed when stuck in traffic my tach needle will drop about 250 rpm without any change in engine speed.

Thanks

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Flash319 · 02-12-2007, 09:56 AM

You will have low voltage when you have such a draw on the electrical system. A car battery is just over 12V by itself, usually 12.5-12.8. Your alternator will bring that up to around 14.5 to 16 volts. To charge a battery you have to supply a high voltage. When you have everything turned on, expecially your heater fan (big draw) the alternator will not be able to hold the voltage at 14 volts. Thus the drop. Sounds like you do not have a problem with your electrical system/alternator. That is a normal drop when fully loaded at idle. If you rev it up it will climb a bit but not much.
Your tach is a different story.

CILO · 02-12-2007, 11:29 AM

Thanks Flash, but shouldnt the regulator bump up the current to make up for the increased draw? to go from 14 to 12.8 seems a bit extreme. wouldnt that have any affect no the vehicles sensors?

Flash319 · 02-12-2007, 11:49 AM

The regulator can only do so much. All it does is regulate the field inside the alternator shutting it off and on when the voltage gets to high. This keeps it at a regulated 14.5 volts. When you start to draw on that the alternator it can only do so much for the size of it. If you have a bigger alternator then you would see less drop, or spin it faster.
The vehical sensors are made to run on 12V. Most of them come directly from the ECU which is regulated to 12 or 5v inside the ECU so fluctuation should not effect the sensors. As long as you have over 12 volts you are fine.
The heater fan is drawing a lot. Electric motors are huge current consumers especially DC motors. When you are running all that, you are not charging your battery very much. I like to give my alternator a break in the winter every once in a while and turn the blowing down for a bit.

All you need is more then 12V to charge your battery so you are fine at 12.8 as long as it jumps back up to 14 when everything is turned off.

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02-12-2007, 09:56 AM	#2 (permalink)
Flash319 Registered User Join Date: Feb 2006 Location: Barrie, Ontario CANADA Posts: 1,356	You will have low voltage when you have such a draw on the electrical system. A car battery is just over 12V by itself, usually 12.5-12.8. Your alternator will bring that up to around 14.5 to 16 volts. To charge a battery you have to supply a high voltage. When you have everything turned on, expecially your heater fan (big draw) the alternator will not be able to hold the voltage at 14 volts. Thus the drop. Sounds like you do not have a problem with your electrical system/alternator. That is a normal drop when fully loaded at idle. If you rev it up it will climb a bit but not much. Your tach is a different story.

02-12-2007, 11:29 AM	#3 (permalink)
CILO Registered User Join Date: Feb 2006 Location: NJ Posts: 114	Thanks Flash, but shouldnt the regulator bump up the current to make up for the increased draw? to go from 14 to 12.8 seems a bit extreme. wouldnt that have any affect no the vehicles sensors?

02-12-2007, 11:49 AM	#4 (permalink)
Flash319 Registered User Join Date: Feb 2006 Location: Barrie, Ontario CANADA Posts: 1,356	The regulator can only do so much. All it does is regulate the field inside the alternator shutting it off and on when the voltage gets to high. This keeps it at a regulated 14.5 volts. When you start to draw on that the alternator it can only do so much for the size of it. If you have a bigger alternator then you would see less drop, or spin it faster. The vehical sensors are made to run on 12V. Most of them come directly from the ECU which is regulated to 12 or 5v inside the ECU so fluctuation should not effect the sensors. As long as you have over 12 volts you are fine. The heater fan is drawing a lot. Electric motors are huge current consumers especially DC motors. When you are running all that, you are not charging your battery very much. I like to give my alternator a break in the winter every once in a while and turn the blowing down for a bit. All you need is more then 12V to charge your battery so you are fine at 12.8 as long as it jumps back up to 14 when everything is turned off.