Technical Voltmeter readings Electrical experts

My Classic Instruments voltmeter is acting erratically. When the car(Hudson) is running it shows around 13 volts and with a voltmeter of the battery it is 13.3V so gauge a little out. When fans come on either as the cooling system controller activates or if it is below temp and the air con is switched on and activates it drops below 13V but comes back up just below 13V. Once again voltmeter on the battery shows over 13V producing.
The main culprit is sitching the headlights on and gauge drops to 11V but voltmeter still show 13V. It would sound logical that it in the gauge but most of the time it is close to the
13V as per the voltmeter reading on the battery. I am thinking it is in the wiring somewhere. Can a faulty earth on the gauge cause this intermittent problem that mainly is when the headlights are on. I realise that the voltmeter is showing correct charging but would like to pinpoint problem so the gauge is showing correctly.

 

Let me ask a few questions... One, are you using a one-wire alternator? Two, what size is the wire supplying your fuse panel? Three, where is the dash voltmeter connected?

I'm pretty sure it's not a problem with the gauge.

 

Yes one wire alternator. not sure on 2 and the third question not sure what you mean? I have been having discussions with my auto electrician and just trying to have plenty of info for him. This was all professionally wired back when built but since then there have been some retrofits but not headlights.

At the same time we are chasing a slow battery discharge that may or may not be associated with this.
I have to do some heavy lifting to get to the back of the voltmeter and fuse panel and relays etc. Then take it to the expert.

 

I suspect it's more than just one thing, and it's not the gauge or how it's wired in. First, one-wire alternators don't have a 'sense' circuit like an actual automotive alternator. The one-wire regulators measure voltage at the alternator output only, the three wire measure it at the fuse panel or ignition switch and if there's any voltage drop between those points will boost output to bring voltage at the fuse panel/switch up to spec. What you're seeing is the voltage drop at the fuse panel under higher loads without that compensation. There's likely more than one solution. Now, replacing the alternator with a three-wire unit and it's additional wiring will address this at least partially but may or may not be needed or fully solve it. I suspect the wire feeding the fuse panel is probably a #10, bumping that up at least two sizes, preferably three, should eliminate the problem by reducing/eliminating the drop at the panel. Lastly, removing the lighting power from the fuse panel and operating the lights through a relay off a separate dedicated circuit would reduce the drop seen at the gauge. This is the solution typically recommended by the harness suppliers to 'fix' the inadequate wire size feeding the panel that they use that causes this. Your gauge is telling you what's wrong, it's your option if you want to fix it or how you fix it.

One last comment. If this problem is new, you may have a connection starting to fail from heat/overload, I'd do a thorough check of all connections at and around the fuse panel/ignition switch to verify that all are still good.

 

If it is a 1 wire GM alternator it should be putting out about 14.2 volts. If not you may have a bad voltage regulator. Not uncommon on the one wire alternators. This would also explain a draw. To check it out disconnect the negative battery cable from the battery. Hook up a test light between the negative battery terminal and cable. If it lights up. Disconnect the alternator. If the light goes out you found your problem. If it stays on keep going.

 

That may be part of the problem but he's still seeing about a 15% drop from the battery to the gauge. That's 3X what it should be... and that has everything to do with wire size.

 

What voltage do you get straight on the alternator? The voltages you say are a bit too low to charge the battery fast enough, good for backup batteries being charged 24/7 but not for starter batteries that need to be refilled quickly after the big drain a start means.

 

Have to agree with both comments. Sounds like the wire to the fuse panel is too small, and if there is a draw, the voltage regulator is suspect. I wired my 53 with a #8 to the fuse panel, and a # 10 through the ammeter as a shunt to the fuse panel. Having a relay on the lights would also help.

 

This is another time when every reply to the OP's question is spot on. If you do what has been said here it should fix your problem.

 

Its amazing to me that some folks view 1-wire alternators as the "devil incarnate" while literally thousands of them are & have been being used for a very long time with no problem what so ever !

 

I would bet the cooling fans are wired through relays, not the panel like the lights. That's why you get different readings when turned on and off. Great advise in the above posts, and here some more not often followed,
" You can never have to big of wires "

 

Or to many grounds

 

There must be resistance between the volt meter and the head lights I would check for a voltage drop in the dimer switch. Any thing over .5 volts is to much on a high amp circuit.

 

A one-wire alternator will work fine if the vehicles electrical system is minimal, has short circuit paths and/or has adequately-sized wire feeding the panels to keep voltage drop to a minimum. But they were never intended for or installed by GM in cars, being designed for use on industrial and agricultural equipment. If you have a forklift, tractor, big generator or compressor, etc etc, that's what they're designed for... not cars.

 

What I think you are saying is that the voltage at the battery remains at 13 volts, but the voltmeter in the dash drops with the headlights on. First, the fact that the battery voltage remains stable tells me there is nothing wrong with the alternator. I have a similar problem with my 64 Chevy C20. When I turn on high current accessories (lights, blower motor, wipers) the voltmeter in the dash drops, but the voltage measured at the battery remains pretty constant. Your problem is a voltage drop to the main power feed to the inside of the car.

When I first got the truck about 12 years ago, there was a lot of extra wiring and I mean a lot. I pulled out a ton of extra wire that ran something that was no longer in the truck. I also cleaned up the wiring under the hood especially the connection between the alternator and battery. Like many extra wires I had removed, there was one going directly to the positive terminal on the battery. It was connected to the horn circuit so I left it. What really bothered me was that the voltmeter in the dash would drop to about 11 volts when I had to run the lights, defroster, and wipers (a common occurrence in Oregon). I finally found a schematic for my truck online and discovered that that extra horn circuit was actually powering the fuse panel. As a quick fix I ran a larger gauge wire on this circuit. My dash voltmeter still drops with all the accessories on, but now stays above 12 volts.

Some day I'm going to dig into the wiring and find the actual feed wire to the fuse panel and repair it. However, the old truck starts easily, drives pretty good, and all the accessories work like they have for the past 12 years. It isn't high on my list of priorities.

 

My friend had a one wire in his 61 Chevy and his high dollar radio went out the volt meter was stuck at 30 v.

 

Steve may be [probably is ] correct. Although the one in my Buick has served me well for 10-11 years and 20,000 plus miles. I did run an 8ga wire to the starter/battery via the "out" post on the amp meter. Meter is thus bypassed. All wiring is at least one size larger than original.
I agree with what I THINK Steve is saying . Size counts. Wire size that is.

Ben

 

The alternator is relatively new so maybe it relates to some of the other suggestions that have ben made.
Thanks for all the input.

 

They are a good compromise. And honestly they work well.....as long as the wire going from them to the battery is big enough. A 3 wire alternator will actually compensate for undersized wire and voltage drop with the sense wire. A one wire alt just has the sense wire connected to the output terminal. Therefore it cannot see the actual battery voltage.

I use Bosch 2 wire alternators which are essentially a 1 wire with an idiot light. But I use battery cable (4awg) and soldered ring terminals going from alt output to the starter relay. So a minimum of 4awg all the way to the battery. Also a very short run.

Can't get away with the old days of 8 awg from the alternator. With a 1 wire alt.

 

LOL. You and Steve have convinced me!! Mine must not be working. Seriously, it does. AC, OT fuel, OT ignition. I don't know how many amps it is putting out. Maybe I should check it for V drop. I never have.

Ben

 

Guys don't understand how automotive electrical systems have evolved. Back when all those power-consuming extras were optional, the OEMs usually had a 'basic' harness for a given model car that took care of all the standard equipment. By the mid-60s they had finally adopted a unified fuse panel, no more scattered fuse locations. But when you ordered most of these extra options with their added electrical load, rather than having a specialized 'main' harness they used an 'overlay' harness that was ran next to the regular harness, generally picking up circuit power at the same point as the main harness. Relays were used for control, and relay power was installed in the main fuse panel. Worked fine, but you no longer had all the fuses in one place, these were mounted where convenient. There were a few exceptions like Caddys and Lincolns, but even there you may find it done this way.

Enter the aftermarket harness suppliers. Somebody had the bright idea to include all of the circuits in a truly unified fuse panel which wasn't a bad idea but they fell down on the job by not increasing the feed wire size to the panel. This was done for various reasons, mainly because wire terminals and crimp tools for wire larger than #10 wasn't as commonly available and tended to be expensive when you did find it. For whatever reasons they don't advertise this fact, leading guys to overload the panel feeds. This shows up as dim lights usually, but there's other less obvious issues. It's also very hard on ignition switches, switching currents they were never designed for. Their solution when asked? Adding overlay circuits, just like the OEMs did. Relays to correct dim lights, separate power feeds to things like AC, electric fan, etc. Some aftermarket suppliers like Vintage Air recommended this, recognizing the flaw in these harnesses. Now you're back to having fuses or circuit breakers located away from the panel.

There are solutions to this problem, but none of the aftermarket harness suppliers have addressed it to my knowledge and whether or not their panels are suitable for this is an open question. If you want a full discussion of all this and solutions, go here: https://www.jalopyjournal.com/forum/threads/wiring-101.843579/

The late models have solved this issue by clustering all the main electrical fuses and relays and mounting them as close to the battery as possible and keeping circuit paths as short as possible. Won't work if you're trying to clean up the engine compartment and/or hiding the components.

 

Automotive electrical is fun! There are great sources of learning if you are so inclined.

A 1, 2, or 3 wire alternator is covered well on many YouTube videos and the pros and cons of each.

Honestly with many "modern" electronics things have become simpler with things like built-in regulators etc.

As Steve said, a nice fuse panel close to the battery as the MDP goes a long way to simplify things. I like to run engine stuff pit of there, as well as headlights, rad fans, and a couple of 30A feeds to the passenger compartment. One hot all the time, the other with a relay to make it hot in run. Both #10 wires. I use XHHW fine stranded wire as it is good automotive heat and is very tough insulation.

That keeps the only big wires going to the alternator, the MDP box, and the starter. Everything else is smaller and everything is fused within a foot of the battery.

Unfused wire, no matter the size is what burns cars down.....an no one wants that.

 

Will keep you up to date on what will happen after further discussions with my Auto electrician.
Yesterday I did the charge test on my battery and it is still only in the mid 13's and then I tested my Riviera and it is 14.3V. This wasn't a problem initially when the car was built but I think adding stuff with different people has contributed. Even though the battery is in the trunk and fuse panel at the front it wasn't a problem. We just need to do a thorough check of everything. As long as I keep it trickle charges it is OK.

 

Update: I managed to get in behind the dash and found the earth to the Voltmeter a little loose and repositioned it in the dash(a bit off centre). Well now it reads on the gauge more accurately at around
14 approx give or take. Now at the battery which is in the trunk with a long run of cable on charge it gives 13.7-13.8V. It doesn't change much with headligts on either.
I did a parasitic draw measurement using the old method of disconnecting the earth and doing the measurement off the cable to the earth post and got a reading of .06 amps which is 60 milliamps. I am now not charging the battery and just letting it sit and measuring each day to see what voltage I have on the battery and so far after 2 days it hasn't really changed at 12.4V. I don't use the car very much at the moment so that no doubts contributes to the battery slowly going down even with only a 60milliamp draw(which when I look it up is "acceptable". Maybe look at keeping trickle charge on.

To measure at the alternator I need to get access under the car so needs to go on the hoist(after I move the 63 Riviera off). Can someone school me on how to measure using voltmeter. Just put positve on the post for the lead from the alternator and earth somewhre on the chassis? When the car is running and measuring at the battery there is also 2 fuel pumps running(a solid state that draws from the tank and a high pressure that send the fuel up to the engine) and I also have a transmission cooler with a fan that comes on with ignition. I may measure with fan connected and disconnected.
So maybe the best solution is drive it more!!!

 

Yes, Positive of the voltmeter on the main (Large) terminal on the alternator, and earth on a clean piece of the chassis or the battery negative. You can isolate the 60 mA draw by keeping an eye on the ammeter and pulling the fuses until the meter reads zero. If it is still there after pulling each fuse out, it is in the alternator, or before the fusebox.

 

A battery cut-off switch will eliminate the problem of battery drain when the car is not being used, and will lengthen the life of the battery.

 

I have to add, interesting discussion about alternators. But I'm a pretty simple guy, and I like my cars being pretty simple too. A 1 wire alternator should be more than capable of maintaining the battery of a traditional hot rod. If the electrical system in a car is too complex for a 1 wire alternator, maybe it isnt a traditional hot rod or custom...

 

I put one of those on my riding mower because the battery(s) kept going dead. Tried a battery maintainer which did nothing. Had to charge it every time I used it. Tried a fancy charger that "reconditions" batteries. I mention this because I know a lot of old cars have long periods of setting.
One thing I don't get is why a 12V battery shows something like a 40% charge when its say 12.4 volts. Why isn't a 40% charge something like 5 volts. Admittedly I'm an electrically challenged individual...............along with my other shortcomings

 

I have one but mainly for when the power needs to be turned off while working on the car. It is not easily accessible and the radio loses presets and has to be reprogrammed every time.

 

Those radio presets may be your small draw in the battery.