"Pulsing" voltage, RPM drop with load.

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JoePole1

A dude in a B body
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Hey guys. I have a 72 SBM (b-body) with electronic ignition an I am trying to track down the source of a nagging electrical nuisance. The car is charging properly (14+ volts) however I can see the ammeter gauge pulsing as I use turn signals, wipers, lights, etc. and the voltage drops as I add more. This is ONLY when I am stopped and gets down to about 11v. Headlights get fairly dim at stop.
Been going on for a few years and I am trying to eliminate. Today I did a pseudo mad bypass by running a wire and fuse link to the battery relay from the alt output. Ammeter is still hooked up but obviously not functioning fully however I have a voltmeter and still have the pulsing. I also cleaned my engine, core support, regulator, ign module grounds.
Over the years I have upgraded my ign module, volt regulator and alternator (stock output) to update and none have had any effect. The other thing I notice is that I can see and hear a slight (100 rpm) drop when I turn my headlights on at high idle.
Admittedly I am no electrical expert so any advice is appreciated.
 
Semi normal at idle. My bet from personal experience is the alt is not being driven fast enough at idle. Ether the alt pulley is to large dia or crank pulley is too small dia or both. My bet is on the alt pulley unless the engine has been Frankensteined at some point in its life, as the oe alt has most likely been changed a time or two.

What measurements do you get for pulley dia?
Also have you had the charging system checked at the auto parts store, might show a bad diode, or the like.
 
Have not had the system checked but can do. The alt pulley is 2 3/4 inch. Not sure what old one was. Crank pulley is stock for 78 motor.
 
2 3/4 not bad if crank is in the 6 to 6.5 range iirc. My 67 dart did this at idle. I found the alt pulley to be 3, swapped to another alt with 2.5 to 2.75 pull and problem mostly went away. There is a minimum speed the alt can produce power and if you are under that it can't keep up with demand.

With key in run position but engine off press on brake, look at amp meter, should be left of center. Turn on turn signal it will pulsate to the left.
 
When your car is running, the alternator is the main source of power, until it's output voltage drops below the voltage of the battery.
When the engine is at idle, the rotational speed of the alternator may not be sufficient to create a higher output voltage than the battery. This causes a discharge condition, which you have noted on the ammeter as well as the dim headlights.
There are two common causes for the conditions you described. First, the alternator is not capable of properly charging the battery at idle speeds. Usually, this means you need a better alternator. Second, the condition of the battery and all related cabling also comes into play. If the battery is weak, it adds additional load on the charging system at all times, but it's particularly evident at idle. The fact that you can load the engine to drop 100 rpm simply by turning on the lights makes me suspect the battery. Also, you mentioned it drops to around 11 volts... another sign that the battery is junk.
Still, I'd make 100% sure the cables that connect the battery to the car, and the output of the alternator to the battery are clean, tight, and of adequate gauge for the current needed. Lot's of times a charging system looks suspect when the problem is a corroded or loose cable.

If this was my car, I would load test the charging system, and the battery. If one or both were not 100%, I'd replace them. If your battery is is top condition, you won't likely need a high performance alternator, unless you do a lot of low speed driving, and have a lot of idle time with moderate to heavy electrical loads.
 
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I checked an cleaned all battery and alt connections. The charging wiring is stock with the exception of the 10g wire I added from alt to starter relay. My battery was replaced a few years ago and I keep it on a maintainer in the winter. I have had this issue prior to the battery replacement. I'll go ahead and have a load test performed.

Dana...My volt meter does as you said. Ignition on shows about 11.5, drops to 11 with brake and alternating to 10.4v with turn signal. Key on run.
 
Those voltages tell me your battery is bad. Ign on should be over 12 volts. The resting voltage of a good battery is 12.6V.
Also, make sure the case of your alternator is grounded well. Sometimes paint or grime can cause the case to be poorly grounded. But my bet is on a bad battery.
 
I've also added ground wires at vreg and ignition module as well as dielectric grease, star washers and cleaned grounding points
 
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When I get home I'll do the same test and report. My battery is less than 2 years old and hasn't failed to start the car once.
Questions...
  • Points or electronic ignition
  • Stock headlights
  • Added loads like radio amp,
  • Have you looked at the bulkhead connectors
  • What size is crank pulley
 
Make sure the ground on your regulator is good. Scrape the paint on the firewall where the bolts go through. Also you can also get a adjustable regulator on eBay for around $15.00 and this will allow you to set your charging voltage. I have mine set to run at a max voltage output of 14.5 volts at speed and it sets at about 13.4 volts at idle.
 
Please do not confuse my results with OPs (JoePloe1) results, mine are only for comparison.

Did some testing on my 67 Dart w/273, stock 52 year old wiring, point ignition, key in the off position

Measurements at battery terminals, engine off, last started 1-2 weeks ago
Everything off 12.5V
Headlights on 12.1V
Brake lights on 12.2V
Brake lights and head lights on 12.0V

Measurements at Cigarette lighter , engine off, last started 1-2 weeks ago
Everything off 12.4V
Headlights on 11.5V
Brake lights on 12.0V
Brake lights and head lights on 11.1V

Measurements at Cigarette lighter , engine cranking, last started 1-2 weeks ago
Everything off engine cranking 10.6V
Everything off engine running at idle 14.3V
Engine running at idle headlights on 13.3
Engine running at idle brake lights on 13.9
Engine running at idle headlights and brake lights on 12.8
 
Nice Job Dana.
JoePole, I'd say Dana's test results are a good representation.
Those numbers were in car guage. Voltage at battery is 12.6 volts.
See Dana's post. ' In car' is too vague. The location of the probe connections and the load at time of measurement will make a difference.

Poor output at low rpm can happen when one of the windings open or one of the diodes is out. In that situation, 2/3 of the alternator is still working normally so at higher rpm it can supply moderate demands, but really struggles to do so at low rpm. A diode out is more likely to show ripple voltage and look like pulsing than an open winding connection. Only point I'm making is that it could be alternator.
 
The first thing you need to do in these cases is determine the voltage drop in the ground path from the voltage regulator case to the battery negative, AND the voltage drop from the pattery POS to the VR power terminal. That is important.

This can in rare cases be a bad battery. If the battery is low, I would not pay too much attention to "key on" readings, but voltage drop. IE put one probe on battery positive, the other probe on an "ignition run" terminal such as alternator blue field wire. You want as low a reading as possible. Most drop is in the ignition switch, the switch connector, and the bulkhead connector.

Consider dutting loose the underhood "ignition run" line, us the bulkhead connector side to fire a relay, pull power off your starter relay through a breaker to the relay, and then feed the "run" buss you just cut off the relay
 
Hey guys. I have a 72 SBM (b-body) with electronic ignition an I am trying to track down the source of a nagging electrical nuisance. The car is charging properly (14+ volts) however I can see the ammeter gauge pulsing as I use turn signals, wipers, lights, etc. and the voltage drops as I add more. This is ONLY when I am stopped and gets down to about 11v. Headlights get fairly dim at stop.
Been going on for a few years and I am trying to eliminate. Today I did a pseudo mad bypass by running a wire and fuse link to the battery relay from the alt output. Ammeter is still hooked up but obviously not functioning fully however I have a voltmeter and still have the pulsing. I also cleaned my engine, core support, regulator, ign module grounds.
Over the years I have upgraded my ign module, volt regulator and alternator (stock output) to update and none have had any effect. The other thing I notice is that I can see and hear a slight (100 rpm) drop when I turn my headlights on at high idle.
Admittedly I am no electrical expert so any advice is appreciated.
Just a shot but could be that your battery is undersized.
I never did have a Mopar alternator that would charge at idle speed.
And yes pulley size does make a difference.
 
Thanks to all. I'll get some readings with my fluke today.
Just to be clear. The cabin numbers I stated were from my volt gauge which IIRC is connected to my cigg lighter.
I have scraped paint off and used dielectric grease and star washer on firewall mounted VR and Ign module plus have added
ground wires to each. I do run a small amp but have a heavy gauge feed wire.
I hesitate to mess with the bulkhead connector......yet. The car is originally from AZ so everything is brittle. I've disconnected it once and broke a tab off. My plan is to clean connectors somehow and pack with grease but only want to do once and I am planning an engine swap this winter. Maybe I'm being paranoid.
 
The car is originally from AZ so everything is brittle. I've disconnected it once and broke a tab off. My plan is to clean connectors somehow and pack with grease but only want to do once and I am planning an engine swap this winter. Maybe I'm being paranoid.
I agree if it ain't broke don' fix it.... BUT I believe the bulkhead connector plastic parts are reproduced so just a matter of switching over the metal connectors with wires still attached.
 
my volt gauge which IIRC is connected to my cigg lighter.
Lighter is connected to the hot side of the fuse box.
and Ign module plus
That shouldn't matter, although it draws a little more than points.
I've disconnected it once and broke a tab off
Black ABS cement may work as a temporary repair.
Do your tracing. Voltage drops due to poor connections will be most apparent when the current flowing through that circuit are higher.
 
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This is a crude schematic of your charging system.
It doesn't show the steering column connector, and your '72 B-body may have a second fusible link and other differences.
Everything connected to the battery positive and the alternator output (black wire) should be the same voltage under any given condition.
Voltage only drops if current is flowing through resistance. For example voltage on the coil side of the ballast resistor will be lower when the ignition is on.
upload_2019-5-14_14-30-8.png


IF the amp power feed is attached to the battery or starter relay and the amp is cranked up, the ammeter will show power flowing to it when the engine is running. The amp has to be turned off to see if current is flowing to the battery or not. (ie to see if its getting charged by the alternator)
 
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Using Dana's car as an example.
With no current flowing. Voltage was 12.5 V at the battery, and should be the same at the alternator, key switch feed, etc.
upload_2019-5-14_14-46-48.png


The battery voltage drops when loads are applied. The headlights, parking and brake lights is roughly a 12-15 amp draw.

If the connections were perfect, the inside measurements should have been the same as the battery voltage during those tests.
Lets look at the heaviest loads.
With the headlights on, engine off, there was a 0.9 Volt drop with the lights on and brake applied.
Because we know the path the current is flowing, we can deduce the voltage drop was between the battery and the fusebox. This assumes the ground connection is not contributing to the drop. But it is not surprising there would be some resistance in 52 year old wiring connections.
upload_2019-5-14_14-44-53.png


Engine running, current flows the other way.
The voltage at the alternator and battery should have been same as measured inside. But knowing there is some resistance in the wiring, there probably was a drop in voltage between the alternator and the battery.
upload_2019-5-14_14-57-23.png


All this is an aside from the fact that alternator's capacity increases with rpm.
When Dana measured 13.3 volts with the headlights on, that is typical of alternators getting maxed out at idle.
A check at the alternator output terminal would confirm this.

When the engine speed is at 1250 rpm, the alternator should easily supply lights including brake lights, plus heater fan or wipers.
 
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lighter not switched in 67 at least
Thanks! I was wondering about that. I thought it wasn't, but then recently was working on something that was..

edit: yes and should be a red wire in '67. My car was a little hacked with an aftermarket a/c probably done in the 60s, and then what I did in 80s. Hope to clean it up this year.
Just checked a '73 Dodge FSM and cigar lighter is not switched then either. I'll correct my post. Thanks.
 
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You can take a lot of the load off of the bulkhead connector by simply running a 10 gauge wire from the alternator battery stud (the one with the nut on it) to the starter relay battery stud (the one that has the battery cable connected to it). This may help with your problem.
 
Ok....here's what I got using Dana's method:

Measurements at battery terminals, engine off.
Everything off 12.6V
Headlights on 11.9V
Brake lights on 12.0V
Brake lights and head lights on 11.8V

Measurements at Cigarette lighter , engine off.
Everything off 12.4V
Headlights on 11.2V
Brake lights on 11.9V
Brake lights and head lights on 10.9V

Measurements at Cigarette lighter , engine cranking.
Everything off engine cranking 10.6V
Everything off engine running at idle 14.7V 850 RPM otherwise car was last running yesterday.
Engine running at idle headlights on 14.0V
Engine running at idle brake lights on 14.5V
Engine running at idle headlights and brake lights on 13.0V

Voltage at alt is 12.75V but I measured this after warming up the car. It was also 12.75 at battery. Remember that I ran a 10ga wire from alt output to starter relay with fuse link.
 
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