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Old 11-27-2019, 03:12 PM   #1
SuperKONR
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Default AC voltage at battery

I'm getting 31.8v AC at the battery, diodes check good in the alternator. Swapped to another alternator just in case and still getting the same AC voltage. What am I missing here

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Old 11-27-2019, 03:31 PM   #2
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Default Re: AC voltage at battery

What are you reading in DC at the battery?

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Old 11-27-2019, 03:51 PM   #3
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Default Re: AC voltage at battery

Quote:
Originally Posted by toggenburg View Post
What are you reading in DC at the battery?
14.8vDC

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Old 11-27-2019, 04:15 PM   #4
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Dizzy Re: AC voltage at battery

Battery charges fine, tranny is still acting up in a million different random ways. I thought when you got AC voltage at the battery it meant that the diodes are blown out and it will distort transmission sensor signals.

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Old 11-27-2019, 07:53 PM   #5
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Default Re: AC voltage at battery

Check the AC voltage of the battery with the car not running. Will verify if the meter is bad.

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Old 11-27-2019, 09:56 PM   #6
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Default Re: AC voltage at battery

^ Interesting suggestion. How ac voltage is generated with the engine off will amaze and astound me and anyone with any basic electricity knowledge.

As to observing 31.8v AC(?!) with engine running, it's amazing and should have wreaked havoc on every electrical and electronic circuit. Alternator output, usually regulated to a maximum of 15 volts dc has a very low ac voltage component. The ac voltage, if any, are usually measured in millivolts, thousandths of a volt. Known as ac ripple. Specs are usually around/less than 20 millivolts (ac), 0.020vac or twenty thousandths of one volt ac. Higher ac voltage usually points to a faulty diode where ac voltage isn't rectified. Six diodes are used, all blocking the negative voltage component of three phase ac voltage. The diodes rectify ac voltages to dc, blocking the negative component of ac voltage from getting thru to the voltage regulator. Very little ac voltage leaks out to cause problems with dc circuits and kept to a minimum by and large with electronics in the voltage regulator.

As suggested, try measuring for ac voltage with the engine off as you should never see any ac voltage displayed. If ac voltage is measured with engine idling, is this actually 31.8vac or an error? Another way to check on multimeter accuracy whether using a car battery or ac outlet is to understand some accepted values. It's generally accepted for car batteries to output around 12.5v (charged). Our home ac outlets are usually between 110-120 vac. A general purpose digital multimeter should display these two values when selecting DC voltage for DC voltages or AC voltage for AC voltages. With several ranges available, select the highest anticipated voltage for accuracy. Most DVMs will have a 0-20 VDC range suitable for car battery measurement. O-20 VAC isn't suitable for measuring 115 vac. Selecting 0-200 VAC will display correct ac voltage from home outlets. If measuring under one volt ac, selecting 0-20 VAC or 0-200 VAC will not measure less than one volt ac. Good DVMs will have a separate range to measure millivolts. Attempting to measure less than one volt ac on a 0-20 VAC selected range won't be accurate at all since expected ac voltage from alternators are usually well under one volt requiring sensitivity for any multimeter.

How are you measuring diode resistance? Some alternators are difficult by design and don't lend themselves to diode measurement.

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Old 11-30-2019, 02:43 AM   #7
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Default Re: AC voltage at battery

by FDryer:
> How are you measuring diode resistance?

Good question... since in many circuits need disconnect one side/pole/wire....

A decent test equipment set will allow view waveform output from alternator, which should be diagnostic.

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Old 11-30-2019, 04:52 PM   #8
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Default Re: AC voltage at battery

I'm guessing not everyone in repair shops and dealers pack an o'scope in their tool box. Even less in most homes with diyers. Other than using a dvm, the next best tool might be the local flaps with their equipment for in-car or bench tests.

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Old 12-06-2019, 03:54 PM   #9
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Default Re: AC voltage at battery

31,8 volts sounds very high. What does your meter read with the leads not connect to anything OR each other? Here at home I'm getting 00.1 on the 200 ACV scale on my freebe CEN_TECH, Longer leads should give higher voltage since they are acting like antennas picking up the signal your radio amplifies so you can listen to music. Are you near a radio station or large motor?

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Old 12-06-2019, 05:16 PM   #10
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Default Re: AC voltage at battery

^^ ?
In all my RF/EMI training and microwave/UHF/VHF/commercial, private and military radio antenna experience I doubt it.

I don't believe a longer set of DMV leads will induce/or amplify any AC signals, let alone an Frequency Modulated (FM) or Amplitude Modulated (AM) /hertz-to-kilohertz signal picked up from any car antenna; especially, as antenna feed to the radio receiver is shielded coaxial cable.

True, in the telcom industry, we have teflon-jacketed communication cable assemblies which if damaged could induce an interference signal from a co-located conduit (if AC power wires are also lying close in a tray), but not from a car radio setup. But even with that interference, it does not show up on a Time-delay-reflectometer (TDR, or OTDR)as AC current, just signal degrading and interference.

At best, expect static from the radio, but not alternating current induction or enhancement.

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Old 12-07-2019, 10:14 PM   #11
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Default Re: AC voltage at battery

Quote:
Originally Posted by toggenburg View Post
^^ ?
In all my RF/EMI training and microwave/UHF/VHF/commercial, private and military radio antenna experience I doubt it.

I don't believe a longer set of DMV leads will induce/or amplify any AC signals, let alone an Frequency Modulated (FM) or Amplitude Modulated (AM) /hertz-to-kilohertz signal picked up from any car antenna; especially, as antenna feed to the radio receiver is shielded coaxial cable.

True, in the telcom industry, we have teflon-jacketed communication cable assemblies which if damaged could induce an interference signal from a co-located conduit (if AC power wires are also lying close in a tray), but not from a car radio setup. But even with that interference, it does not show up on a Time-delay-reflectometer (TDR, or OTDR)as AC current, just signal degrading and interference.



At best, expect static from the radio, but not alternating current induction or enhancement.
My experience is simple. I noticed voltage readings on my meter when the leads were not touching anything. I bought a more expensive meter - same thing. Did a little research and came across this answer to this question (Weird Multimeter Behavior???

Hello, Is it normal for a multimeter when it's set to measure voltage and it's probes not touching anything to show a floating reading of fractions of volts? This happens with Agilent U1253B. Any idea?) on EEWeb:

Answer:

6 years ago by Frank Bushnell (edited)
There is a vast amount of electrical energy (electromagnetic radiation) in the air, from radio station and mobile phone transmissions, and radiated energy from all the mains cables and electrical devices in your house. The multimeter leads act like a radio aerial, picking up this energy and producing a reading on the meter. You will see this effect even more clearly with an oscilloscope.

Rarely seen now, there are simple radio receivers, called "crystal radios", that are powered purely by the radio signal transmitted from the station But they only work on AM signals, which are all being switched off in this digital age.

As Steve says, this effect should normally only be seen on low AC voltage ranges, and touching the probes together shorts out this energy by putting the two terminals at the same electrical potential, so an absolute zero reading would then be normal.


Having built a crystal radio, I know I was not supplying the power that produced the music coming from that ear plug.

I find it hard to believe one would see 31.8 v but I though I would throw this out there. I found it interesting. Just wish I had more time to fully investigate everything I find interesting.

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Old 12-07-2019, 11:39 PM   #12
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Default Re: AC voltage at battery

General speaking when you measure the AC voltage to ground (Block) or between the positive and negative you will read the AC component of the output of the alternator and that should be insignificantly low value as the rectifier is a 3 phase bridge rectifier with built in filtering by design. If you are actually seeing voltages of those magnitudes, the alternator rectifier has failed

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