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1950 Chevy Pickup custom!
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Discussion Starter #1
Here is what is going on:
Voltage will (sometimes) hold at 13.8-14V on a 3 wire alt (140 amp) chevrolet 350 setup.
As soon as the headlights come on, you see a voltage drop of .5-1 V, the compressor for air ride, another 1-2 V and so on. I also see a voltage drop with an increase in engine rpm, while parked or driving. Checking the voltage directly at the alternator yielded the same results.

-Here are the things I have tried, and done:
-Replaced voltage regulator
-Replaced alternator with a standard 3 wire out of my other pickup ( I know it works properly)
-Flip flopped the two wires for field and idiot light/ignition
-Checked grounds, and ran a cable from the bell housing to the negative batt terminal
-Ran heavier 8 ga cable from the alternator to the battery
-Removing the pos cable from the battery with the vehicle running proved the alternator IS charging but yielded the same result.

I am at a loss here fellas, any ideas?
 

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I'm not seeing the issue here...... 13+ volts with the engine running assy off is normal, as you turn assy on you get a voltage drop by .5 to 2 volts which is also normal. Did I miss something?
 

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I believe with a three wire setup for the alternator, the wires have the following functions.
1. Obviously, Battery plus to the battery, a fairly heavy wire.
2. Idiot light wire. this wire goes to the idiot light, it grounds the idiot light when the alternator is not turning, and when the alternator turns, the alternator puts charging voltage on this wire. The idiot light gets 14 volts applied to both sides of it, and goes out. This wire may also supply the initial current to the field coil to make the alternator start to charge in the first place.
3. A voltage sense wire. This wire "senses" the system voltage where it is hooked up to the cars electrical system. On some alternators, if this wire was hooked up constantly, it would drain the battery. It is usually switched on by an ignition source.
4. You should have a ground wire to the alternator as big as the battery positive wire. This is even more important if you have a 100 plus amp output alternator. I know the alternator is bolted to a metal alternator bracket, that is bolted to the cylinder head, or block, that may be bolted to the negative battery cable, or to the frame (body) of the car, and then to the battery negative, but your electrical system will be much happier with a dedicated ground wire that directly goes from the frame of the alternator to the battery.

Here is an article about checking voltage drops in active circuits. it is a long read, but worth it. It will show you how to check the voltage drop in individual wires.

Voltage Drop Testing
 

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1950 Chevy Pickup custom!
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60 Posts
Discussion Starter #4
I'm not seeing the issue here...... 13+ volts with the engine running assy off is normal, as you turn assy on you get a voltage drop by .5 to 2 volts which is also normal. Did I miss something?
If all it dropped was .5 to 1 V total I wouldn't be concerned. But as I turn on my headlights then blower motor and so on it keeps dropping, down to 11 V, then 10 V etc.
 

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1950 Chevy Pickup custom!
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Discussion Starter #5
Daniel that is probably the one thing I have not tried is to run that heavier ground from the alt case to the frame or batt. Thankyou for the good information there!
 

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Daniel is right..... with exceptions IMO.....

I don't agree the #2 wire should be switched on....... On most GM cars this wire is tied directly into the pos side of the elec system and connectly indirectly to the POS post on the battery. The reason GM connected it on the drivers side wiring harness at the factory splice/fusebox feed, was to take advantage of the long run of wire (resistance) to get a truer sense of the elec demand on the system.

I don't agree you need a seperate ground for the alt....... unless it's bolted to painted surfaces etc..... you do however need that large ground to the eng block, and a not as large ground to the frame, fender, fire wall and radiator support. I use a bolt on the lower pass side of the block/head for my main ground connection.
 

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"I don't agree you need a seperate ground for the alt....... unless it's bolted to painted surfaces etc."

This is really easy to check. Start your car, and run it at a fast idle. Turn on the lights, high beam, the heater motor on high, or even run the battery down a little bit, so the alternator is putting out a reasonable amount of current. Take your digital volt meter, and connect one lead to the alternator frame, and the other lead to the battery negative post. Ideally, this value will be zero, but it will not be. Less than .2 volts is good. Less than .5 volts, not so good. If it is higher than the .5 volts, you probably need to run that thick ground wire to the alternator frame, and back to the battery.
 

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And if you get that voltage drop, you could clean the mounting surfaces to get that good ground........ or as you stated you could run another cable directly to the battery


You have your opinion ..........
 

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I believe with a three wire setup for the alternator, the wires have the following functions.
1. Obviously, Battery plus to the battery, a fairly heavy wire.
2. Idiot light wire. this wire goes to the idiot light, it grounds the idiot light when the alternator is not turning, and when the alternator turns, the alternator puts charging voltage on this wire. The idiot light gets 14 volts applied to both sides of it, and goes out. This wire may also supply the initial current to the field coil to make the alternator start to charge in the first place.
3. A voltage sense wire. This wire "senses" the system voltage where it is hooked up to the cars electrical system. On some alternators, if this wire was hooked up constantly, it would drain the battery. It is usually switched on by an ignition source.
4. You should have a ground wire to the alternator as big as the battery positive wire. This is even more important if you have a 100 plus amp output alternator. I know the alternator is bolted to a metal alternator bracket, that is bolted to the cylinder head, or block, that may be bolted to the negative battery cable, or to the frame (body) of the car, and then to the battery negative, but your electrical system will be much happier with a dedicated ground wire that directly goes from the frame of the alternator to the battery.

Here is an article about checking voltage drops in active circuits. it is a long read, but worth it. It will show you how to check the voltage drop in individual wires.

Voltage Drop Testing
thanks for the link on Voltage Drop. I wish i had read this about 20 yrs ago. Excellent info. clear, plain and simple.:)
Al
 

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When you first assemble, and install an engine, you can get away with not having a ground wire on the frame of the alternator. For a while.
Here is the problem.
The alternator frame is probably made of aluminium. It might even be chrome plated. The bolts are steel. The brackets holding the alternator may be steel, or aluminium. The cylinder head may be aluminium, or steel, the block will be cast iron. All these different metals, when exposed to any water will corrode where they contact each other. This process is made worse if the parts carry electricity.
So what is a good, nice, low resistance connection when first made, will deteriorate over time. Then the "extra" ground wire really helps.
 

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I understand all you are saying....... but why don’t the manufacturers install them that way? Been wrenching for 30+ years and have never added a ground cable to the alt. So I’ll agree to disagree.
 

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EOD, I completely understand what you are saying. I have metal to metal contact all the way from the alternator to the battery.

Do not laugh, this is what I have been working on.

It is a 1970 Datsun 521 pickup. It has a negative battery cable from the battery, to a bolt on the cylinder head. The lug this bolt goes through, has a second wire that drops down to the alternator. It is connected to the "E" terminal on the alternator. ("E" for Earth) Also on that terminal, is a second wire, also heavy gauge, that is in the harness. and goes to a bolt that holds the voltage regulator to the inner fender. There is also a separate ground wire that goes back to the headlight plugs, and the headlights are grounded to this wire, and not the cab of the truck.
The alternator on a stock 1970 Datsun pickup puts out 35 amps. Why did Nissan put all this extra ground wire in the truck? They also wired the charging circuit of the 510, and the 240 Z the same way. I could understand the extra wire on the higher profit 240 Z, but the 521 truck was a utility vehicle.

My experience with the 521 Datsun truck is after 42 years there are a lot of electrical problems, but usually not in the charging system, unless it has been hacked. The wires I described, if in good shape ground the battery, the engine, and the cab of the truck. Unfortunately, sometimes, you have to add an extra ground from the engine to the frame of the truck. Quite often, the tail lights lose a ground connection, and you have to add an extra ground between the tail light assembly, and the bed or the frame of the truck.
All the parts of a Datsun truck, (engine, frame, cab, bed, tail light assembly) have metal to metal contact when the truck left the factory. But still after 40 odd years, if you are having wacky electrical problems, I suspect the grounds first.
 

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1950 Chevy Pickup custom!
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Discussion Starter #13
Well gentlemen, I appreciate all of the replies and information. I will test everything tomorrow and post the results. A little more information I could have added upfront though: The batt in the stock location is under the passenger side floorboard. ALso i powdercoated my frame.. good idea at the time, however i have had to run ground wires back to the battery for EVERYTHING.
 

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There are a few threads on pdr coated frames and grounding points...... I don't remember what the titles were. But someone suggested using a cutter hooked to a drill and simply removing the paint down to bare metal and attaching the wire then painting over that area to seal it back up etc....
 

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1950 Chevy Pickup custom!
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Discussion Starter #15
So here is what I checked today and the results:
I tested the voltage drop from the alt case to the battery negative terminal. I did this with and with out a load on the alternator, and again with an 8ga ground directly from the alt case to the battery negative terminal and results were identical. The voltage drop was 30-34 mV which works out to be a .03 V drop. The voltage change on the positive terminal however went from 13.8(no load) down to as low as 12.1(with load) measured from the negative batt post to the post on the alternator.
Also as a note the alternator voltage output was not steady at 13.8 without the load. It would range between 12.8-14.0 at idle.

I'll try to add a little more information here that may help with diagnosing the problem. This pickup is a frame off restoration with about 1000 miles on it. As I stated in a previous post the frame is powder coated and I had a lot of issues with grounds from the start. However I cut down to bare metal in several parts of the frame to mount ground cables for the body, engine, battery, headlights etc. I still ended up connecting all of those frame grounds all the way back to the battery as well and everything works as it should.
I have a Dakota Digital dash cluster, also DD climate control and DD air ride control, the voltage drops I see (stated at the beginning of this thread) are what I see displayed on the Dakota Digital Voltmeter.


Still fishing for the answer...
 

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1950 Chevy Pickup custom!
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Discussion Starter #16
Just out of curiosity and also because I am grasping at straws here, I pulled out my redtop optima battery and put in a standard 700 cca 12v battery out of one of my other pickups. Changing nothing else, voltage off of the alternator never dropped below 13.8 with or without load. Without load it went as high as 14.9 again this was all measured at the alternator. Voltage drop across the ground circuit was the same as before. HMmmmmm. The optima still reads 12.25 across the posts as did the standard battery. What gives?
 

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1950 Chevy Pickup custom!
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Discussion Starter #17
Any ideas at all guys? Maybe the second battery wasn't fully charged? Or is it some sort of resistance issue with the redtop? Any other ideas or input would be very appreciated.
 

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There is a voltage drop in all wires carrying a current. Are you measuring the voltage at the battery posts, or the alternator?
AGM batteries also have slightly different voltages than wet lead acid batteries.
 

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Discussion Starter #19
I measured the voltage both at the alternator and at the battery. I did see about a 1 volt less when measuring at the battery as opposed to the alternator. The thing that is odd is that with the optima battery installed the alt would put out a lower voltage than with the standard lead acid battery installed. 2-3 Volts lower actually. Why would that be? Either way I think I have found the source of my problems, I just don't understand it.
 

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I have always had good luck with wet lead acid batteries, and have not had much experience with AGM (optimal) batteries. I used to work at a boat dealership, for a high end boat line, the type of boat that has lots of speakers on towers, with multi hundred watt amplifiers. These boats usually had AGM batteries in them, and if they tried to run one AGM battery, and one conventional lead acid wet battery, it would cause problems. The two types of batteries would "not play nice with each other"
I believe the charging systems on even the newest cars, if they are equipped with wet lead acid type batteries are "tuned" to provide the charging rates that work best for lead acid batteries. A lead acid battery likes to be recharged as soon as it can, after a discharge. It can tolerate a fairly initial charge rate, but it needs to be cut back fairly quickly. The charging currents, and voltages for a lead acid wet battery are not the same as what is best for an AGM battery

You stated you are losing about 1 volt from your alternator to your battery. I would consider a .500 volt loss as barely acceptable, in each cable. Is there more loss in the positive cable, or the negative?
If the loss is mainly in one cable, that cable needs attention.
 
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