The Stovebolt Site Forums The Shop Area The Electrical Bay Voltmeter ticks down w/heater motor speeds

I`ve pulled the negative off the battery while the truck was running.
No change in the motor or headlight brightness. ALT is good?
Its having a hard time cranking,like the battery is low. I am trying to diagnosis
this so I don`t get stranded. Strange the voltmeter drops as I change heater motor speeds. Is that a sign
of something? The battery seems fine. 327sbc

1 --- Its good as in outputting enough current without the battery. Its not a full test but most likely not your issue since you are referring to cranking as the issue. --- Don't leave that battery disconnected for long period of time.

2 --- Easiest way is swap it with a known good battery. If it cranks better then most likely other battery is not holding enough amps. If no change and still hard to crank could be other issue like starter or wiring with to much corrosion or bad contact to locations etc.

3 --- No that would be normal you are adding more windings and amp draw all affecting volts. volts = amps x resistance

Well, the post was being responded inside quotes, without a color change, so i missed it at first. Don't see Jerry. So I hesitate to enter the "Twilight Zone" BUT.......

1. Don't test the alternator that way! Put a multimeter on battery to see static at 12.6V and running at 13V-14V.
2. Slow crank troubleshooting is cable clean, then have battery load tested, then starter check, bypass ignition switch and on down the line.
3. Voltmeter will drop for a few seconds when a new load is presented usually.
4. The best approach on Stovebolt is to tell us the issue you have before doing anything, just to get our questions so we can suggest ideas and tests.


I ask that comments and answers NOT be inside quotes, unless with a color change. Tuff on an old guy.

My fault on that
forgot to move it outside the quotes. hazards of phone calls in middle of typing.

Disconnecting either battery cable with the engine running is risky at best and could be catastrophic. With an oscilloscope waveform monitoring voltage instead of a voltmeter, I've seen voltage spikes in excess of 100 VDC as the charging system attempts to keep current going into the battery as the connection is broken. Back in the dark ages when cars ran DC generators, disconnecting the hot (+) cable (not the ground) was done as a shade tree method of checking to see if the generator was charging. Even back then, people who understood electricity didn't do it. Now, with solid state components such as diodes and integrated circuits, disconnecting a battery cable with the charging system in operation is stupid squared! Use a voltmeter!
Jerry

One possibility is that you might have an open circuited rectifier diode in the alternator. That reduces the amperage (not the voltage) to less than one third of the rated capacity. I've seen a 65 amp alternator with one bad diode drop to less than 9 amps max output- - - -and the regulated voltage stayed normal. The car would run normally in the daytime, but it would drain the battery as soon as the headlights were turned on. The reduced capacity alternator couldn't meet the extra current demand of the headlights. Testing amperage output requires loading the battery down with a carbon pile load tester with the engine at a fast idle while monitoring the current output of the alternator. It can't be done with just a voltmeter.
Jerry

Is it the ALT or the battery ,more than likely with the voltmeter dropping down with
each speed of the heater motor? My other truck with the same setup does not.

The answer is both and the answer is also neither because of host of things could be causing the voltmeter to be dropping as you change the fan.
What’s the real issue you’re trying to solve first. the slow hard cranking, or the fact that the voltmeter moves? Let’s work the issues in order of priority. if you want to completely rule out the alternator do the test Jerry posted above if you wanna rule out the battery, put a new one /different one in. And see if it changes anything.

If the engine is running, the alternator is the primary source of electric current. The battery is just there to get the process started. Once the engine is running, the alternator takes over, supplies the current to run lights, ignition, heater blower, and any other accessories. Extra amperage capacity (if any) goes to recharge the battery. The battery also serves as a "shock absorber" to dampen out momentary voltage fluctuations as accessories are switched on and off.
Jerry

"Is it the ALT or the battery?" You are basically asking the same question each post.

We have NO WAY of knowing if it's either or neither. That's why you need to take battery in as suggested. Testing beats guessing.
You need to spring into action. Do not dwell on the heater motor drawing amps or the alternator or the voltmeter. Concentrate on the slow crank. That is your ONLY issue. FIRST THING is get the battery load tested by someone with a carbon pile tester. If good, Check cables, if good check alternator output by placing multimeter probes across battery posts while motor at about 1200 RPM to get 13V-14V. If all that is good.............may be starter.

Does the motor always finally start? Define "hard time cranking". How long before starting? Is battery at 12.6V before you begin cranking?

Thanks Bartamos , I will check the battery. Motor does start , sometimes w/o any effort.

Just a note for everyone that I split up hcb3200's original quote with replies inside the quote to make it easier to read and follow. Have also cleaned up no longer need posts on that so the topic discussion can continue.

Guitplayer....your last info kind of sounds like connection issues. Power AND ground. Not sure yet.
That list is: Battery post cleaning, inside of battery clamp cleaning, battery cable ring tongue cleaning and it's landing spot. Solenoid wire check for tight and good connection. Are you points, Stock HEI or Pertronix?? Does battery show static 12.6V in the morning? Motors/batterys do crank slow in the winter.

Gdads51...that is excellent work and helpful. thank you for that.

Signs of a battery going bad. Its a 2 1/2 year old Super Start from Oreielys.
That won`t hold a charge very long.
The battery did not show 12.6 V. I have stock HEI. I did check all wires at the starter and made sure all was tight and clean
at the battery etc.
I had thought maybe the clamp was split. All looks good and running.