Voltage drop. What is normal?

[Larry Callahan]

Engine at idle, headlights on and dual radiator fans running, fuel pump, computer, coils, injectors, dual o2 sensors, electric crankcase vacuum pump e.t.c.

I have 14.16 at the output of my one wire alternator (soon to b 3 wire). I see the same thing If I attach the ground to the case or some place on the engine.

At the batteries in the truck I see 13.8

Under the dash I see 13.2



My Autometer AFR gagues won't start the heaters in the o2 sensors until they see 13.5

Also, raising the RPM doesn't help. It still won't reach 13.5 under the dash to kick off the AFR gauges at 4,000rpm

I did rewire the car over the weekend for a 3 wire setup that I will swap out next week. I will hook the sensing wire to a junction block I have under the dash that sees 13.2

Anyway, is nearly a one volt drop enough that I need to start looking into my grounds e.t.c.?

[Vegas69]

Seems excessive to me. You can do a voltage drop test on your wiring or battery cables by using your leads on each end of that perspective wire/cable. It will show how much voltage is dropped from point A to B. I'd start with your positive and negative cables. You are losing .3 from your alternator to the battery and over .5 back to the junction block. Is the battery grounded in the trunk or in the engine compartment?

[SLO_Z28]

Seems excessive to me. You can do a voltage drop test on your wiring or battery cables by using your leads on each end of that perspective wire/cable. It will show how much voltage is dropped from point A to B. I'd start with your positive and negative cables. You are losing .3 from your alternator to the battery and over .5 back to the junction block. Is the battery grounded in the trunk or in the engine compartment?

^this. .3DCV per wire is the MAX. ideally you'll see something like .175 or less on something like a battery cable. Id check the Voltage drops on all your grounds.

[Bryce]

The voltage drop is due to the resistance in the wire. What gauge wire are you using? I use 1 gauge when i mount batteries in the trunk, I do not have a voltage drop between the altenator and the battery. i never measured voltage during starting but I have no starting issues.

[Larry Callahan]

I'm running I think 1/0 or 2/0 wire from battery to battery and a positive from the battery to the starter. I have several grounds running from engine to frame, from engine to chassis but the one thing I don't have is a large ground wire running from the trunck all the way up front. I just grounded the batteries to the support plate the roll cage is welded to.

I'm thinking to try and run a large ground wire from the batteries to the engine?

[Bryce]

Larry,

I grounded the battery locally and the engine locally, no ground cable all the way to the front. Are you sure the ground attachments are good?

[Larry Callahan]

I "think" they are. I will go over all of them tomorrow night.

[JRouche]

Hmm, it is interesting. It sounds like you have a large enough + cable. I use a 0000 welding cable, but 00 or 000 is well large enough.

And it sounds like you made sure to have plenty of grounding cables.

But whats really curious is why you are loosing voltage when the alt is supplying its full potential right to a lug in the engine compartment right? You prolly have it wired up so the alt supplies the 14.16 volts to a lug in the engine compartment right? Like the starter lug or some other lug that the battery cable connects to.

So knowing that the cable size for the battery doesnt matter, thats only large for starting when the alternator is not in play yet. I have a major disconnect switch in my system to remove the battery from the circuit ( I use it to disconnect the battery during long sitting times so it wont drain).

But if you measured 14.16 volts from the alt then that potential should be at the lug that you connect the single wire from the alt to the lug. When you measure 14.16 volts are you measuring right at the alt? If so take a reading at the lug where you have the one wire going to and ground. If it is lower then that says the one wire from the alt to the system is dropping the voltage. Id think about looking at the gauge wire from the alt to the lug. I think that might be a point to look at.

And if you dont see any voltage drop from that wire I would chalk it up to the wiring harness wiring dropping the voltage.

I dont see the battery as being any part of the voltage drop. You arent running the one wire from the alt to the battery, one long run right. You have the alt feeding a lug (may be the starter lug, may be a separate lug, but you arent running the alt line to the rear so the rear doesn't matter. It wont drop your voltage at the lug, even if its a bad battery setup. You will still see the full alt output voltage on the lug if you measure before the lets say bad battery bank. And your fuse block should be connected to that first point of electrical output from the alt. to get full voltage to it.

The thing about voltage is its not like current..

The voltage is the potential. And the potential (the force) will drop off the further down the line from the source you go. The source in your case is the alt. But unlike current it will not be affected with the other loads down the line from where you are checking and using it. If you use the energy right from the source you will see the full potential, no matter how severe the drain is down the line.

Where current is different. Its the volume of energy. There the circuits down the line will affect the circuits ahead of them because the circuits down the line can remove the volume, its like a major water leak in a water system.

And yes, some folks like to describe electrical systems with water systems. And it kinda works for a simple description. But not completely. For the current side yeah, pretty close. But the voltage side not so much. Where in a water system the supply side pressure IS effected by whats happening down range. If you have a major leak you will see a pressure drop at the supply, the water flows easier due to the leak and it will show a decrease in pressure (the potential, or voltage when talking about electricity).

Not so with electricity. Electricity doesn't act like that. Think of electricity as like having a 500 hp water pump supplying a half inch line with water. If you get a small or even large leak in the pipes down the line you wont see any pressure drop at the head of the pump. You may see a pressure drop futher down the line, well past the pump. But that will be due to some resistance from the lines, not so much from the leak, its a massive pump.

Voltage is a first come first served deal. Im not sure where you measured the 14.16 volts from. The back of the alt? The lug? But everything after the back of the alt will drop some voltage (potential, the driving force) due to resistance. And the wiring harness will be a major load, all the small wires.

Ok, now back to what could be the issue. First, 14.16 volts measured out the back of the alt IMO is a lil low. Now.. If you measured the 14.16V after the one wire then I suspect the wire to be a lil small. JR

[H2Ogbodies]

Larry, have you done an amperage calculation on your system yet? Take the total max. amp draw of all of your circuits and you (ideally) want to have that number be no more than 70% of the max. amperage output of your alternator. I suspect that since you have a runaway thermal effect going on where the amperage draw is so high that the voltage is dropping...with the added voltage drop and continuous high amperage draw you are building more heat in the wiring which leads to further voltage drop..and so on...and so on. How are all of your power leads connected? One big single buss bar type plate, multiple 12v lugs, etc.? I can tell you for certain Larry there is a night and day difference between parts store alternators and genuine GM ones-the regulating window switching time in OE GM alternators are much higher quality than parts store "lifetime warranty" ones. And which online bought 1-wires from "name your company", you never know where they source their alternators internals from either. I think if you opt for a new GM Delco CS144 140-amp alternator and wire accordingly most of your problems with voltage will disappear at least for the supply-now as for how your total system is wired, that may/may not be ideally configured...I'd like to see a detailed wiring schematic of your car to pinpoint problem areas. I design and provide vehicle wiring diagrams on everything I build for customers just for that very reason.