Battery Measures 15.5 vdc

I have a 1968 Dodge Dart 270, SL-6. Very nice 2 DR H/T and totally restored.

Here is my question. My battery voltage while the car is idling is 15.5 Vdc. This seems high. VR is the "new type" solid state like the old VR-101 style w/ IGN and FLD markings on the black cover. Alternator squeals a bit so will be replaced next week. Alternator is also original

What voltage should I measure at the battery???. ALT meter is slightly to the right when driving. With lights ON, ALT dips just below mid point on meter.

I could replace ALT w/ new rebuilt ALT, eliminating squeal and whine. Belts are new and good tension.

Any thoughts, recommendations? words of wisdom?

Thx...great forum, great people, great experience!!!!

You are being tricked the same way many of us were. LOL.
So here's the deal. The voltage you are reading is the the voltage available in the wire. It's at 15.5 V in the wires because the alternator output is at 15.5 Volts.
As you probably know, current flows from higher voltage to lower voltage.
A automotive battery will typically 'float' up over its normal 12.5ish Volt to 13.5 ish Volts when the battery is fully charged.
The ammeter is probably showing a slight current to battery because the battery is around 14 Volts and won't go higher. Current flows but instead of charging, its just cooking the acid.

Most likely the voltage regulator is the problem. Either the regulator is
a) getting the low system voltage because there is resistance in the circuits.
b) needs adjustment.

For (a) measure the voltage on the wires leading to the voltage regulator, and if different than at the alternator output (and battery positive), then also check at the ballast resistor.
With new solid state type, if its (b) then you need a new one.

As far as the alternator noise goes, try new belts and also feel for bearing play. I've also had bearings seize on these. Find a decent rebuilder if it needs a new bearing.
When the headlights are on, the ammeter should be centered. If the needle is toward discharge, it means the alternator can't provide enough power and the battery is filling in. At idle speed its a little marginal so increase the rpms a little and see what happens.
One of the other misnomers is that many ammeters are labled 'alt' for alternator.' This also has misled many of us. The ammeter in most cars that have 'em show whether the battery is charging or discharging. That's all.

Voltage at battery,14.2-14.6 is optimum.
Key off-battery should be no less than 12 volts (13.2 new in factory)
Any more its the regulator,bad wiring or dirty ground. But im thinking dirty ground would cause lower voltage.

Here's a diagram of the charging circuits for '68 Barracuda Valiant. Dart will be similar but you may find small differences.
The Voltage Regulator (blue Ign wire) determines whether to pass current to the field based on the voltage in the blue wire from the ignition.
It should be the same as at the Alternator output (BAT stud) and the Battery's positive.
If not, look for the voltage drop.
If it is, check for poor grounds.

tips:
Field is shorthand for electromagnetic field. The windings inside the alternator create an electromagnetic field. When they are spun, the magnetic field induced electric flow in the non-moving windings. That's the power the alternator can provide.
Also notice power from the alternator does not go through the ammeter except for charging the battery. So this is what the ammeter tells us.

Imho. Take it to your local autoparts store and have them put their tester on it. They can tell if the alt has a bad diode etc. Dont take their diagnosis as verbatum, do the other testing sugested. But a whining alt doesnt sound good.

Mattax said:

Y

Most likely the voltage regulator is the problem. Either the regulator is
a) getting the low system voltage because there is resistance in the circuits.
b) needs adjustment.

Click to expand...

I don't get you. You are sort of on the right track BUT IT IS ALMOST NEVER THE ACTUAL VOLTAGE REGULATOR that is the problem

IT IS ALMOST ALWAYS voltage drop in the system

OP: HERE IS HOW to measure for voltage drop "and why."

With the engine not running and with the key in "run" Hook your voltmeter to the VR ign terminal. This is same as the "key" side of the ballast resistor. Hook the other probe to the battery POSITIVE post. This will give you a VERY low reading, the lower the better. You very likely will read somewhere around 1 1/2V which is a HIGH reading. What you are reading, there IS the voltage drop FROM the battery, through all the harness, switch, connections, and bulkhead connector...........to the VR. That reading is ADDED to the VR setpoint. That is, IF the VR is "correctly" working at a nominal 14V, and you measure 1.5V in your test, then you can EXPECT the battery to "run" at 15.5

Review the "MAD" article, even if you do not "do" the modification, this article gives a good overview of part of the problem with the harness, as well as a simplified diagram:

Catalog



The diagram gives a good simplified view, although it does not show the ignition switch and "run" (IGN1) wiring.

The functional path is from BATTERY, through FUSE LINK, through BULKHEAD CONNECTOR, AMMETER, WELDED SPLICE, IGNITION SWITCH CONNECTOR, THE SWITCH, OUT THE SWITCH CONNECTOR (on dark blue, "run" IGN1)---------back out through BULKHEAD CONNECOR to the underhood loads and VR

All the capitolized words are points for concern for voltage drop........bad connections..........You must chase them through that path

One way "out" of this is to install a relay to run your underhood loads. Electrically cut the "ignition run" wire coming out of the bulkhead, use the bulkhead end to trigger a relay, fed, say, off the starter relay, and then the relay feeds the underhood end of the cut wire. I say again "that is just one way"

ANOTHER test you should make is for voltage drop on the GROUND side of the system. The VR MUST be grounded to the battery to be accurate. To do this, measure first with engine running "fast" to simulate cruise RPM, with all accessories off, and again with heater, lights, etc all powered on.

Stab one probe into the mounting flange of the VR. Stab the other probe into the top of the battery NEG post. As in the first test, the lower the reading the better, zero is perfect.

One way to achieve a good battery--to engine--to body ground is to buy a short "starter" cable..........eye to eye.........and bolt to the rear of the drivers side head. Bolt the other end to a good bolt and nut through the firewall, or to the master cylinder mounting stud.

Remove VR, scrape around bolt holes, and remount tight with star lock washers. BOTH the VR and the Mopar ignition box MUST be grounded to operate

67Dart273 said:

I don't get you. You are sort of on the right track BUT IT IS ALMOST NEVER THE ACTUAL VOLTAGE REGULATOR that is the problem

Click to expand...

Well that's been your experience. Mine has been I've had at least a half dozen regulators fail in various ways in just my own cars. The mid-80s P-part ones were the worst. To my disappointment I had a NAPA one flake out last year.

Not sure why you get worked up about this. We're both telling folks how to trace the problem and not just go and start swapping parts. But I don't like MAD's site, some misleading info amongst the good. and their drawing doesn't show the VR leads. So clearly we have different perspectives and experiences. That's OK. In fact its probably good. Sometimes the same info delivered a different way connects where the other didn't. As far as I'm concerned if we can help people understand how this stuff works, they'll become better at solving problems when we're not around.

Most regulators now days are crap. Read what I found: Voltage regulator

Not sure why you get worked up about this. We're both telling folks how to trace the problem

That is EXACTLY why I get "worked up." You made the statement that "this is almost always the regulator" and that is NOT TRUE. This suggests that maybe the OP ought'a go right out and buy a new one. The fact is that regulators rarely fail "a little bit off" voltage. They almost always fail either "full field" or no charge at all.

I won't dispute that modern quality is crap. In this case "I'd bet" the regulator itself is just fine

IN FACT you can actually check that the VR is OK by measuring as another member does on here. Run the car, warmed up, battery charge at "cruise" speed. Measure voltage from the "key" side of the ballast to the battery NEG. If it measure "normal" AKA 13.8--14.2, the regulator is just fine. ..................The high battery voltage is due to harness drop.

Oh. LOL. Now I get it. Sorry. I can see how you read it that way. That sentence was in context of the OP's concern the high voltage related to the alternator and was meant to be read with the (a) and (b) underneath; (a) being the first thing to check, and only if that checks out then to look at (b).

67Dart273 said:

Measure voltage from the "key" side of the ballast to the battery NEG. If it measure "normal" AKA 13.8--14.2, the regulator is just fine. ..................The high battery voltage is due to harness drop.

Click to expand...

That's basically the same I wrote. I suggested a comparison rather than providing a normal. Because even a normal voltage of 14.2 at that location is a harness drop if the voltage at the alternator (or battery) is 15.5.

Mattax said:

For (a) measure the voltage on the wires leading to the voltage regulator, and if different than at the alternator output (and battery positive), then also check at the ballast resistor.
With new solid state type, if its (b) then you need a new one.

Click to expand...

mydart270 said:

Alternator is also original

Click to expand...

By the way if the alt is in fact the OE 1968 you might want to keep the core for future if you ever want to go full OE date coded parts etc.