Author Topic: Fluctuating Voltmeter  (Read 5372 times)

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #15 on: January 17, 2019, 09:39:39 AM »
I think I’m with you. Forgive my ignorance, but I probe the #2 on the alternator with the NEG lead on the multimeter and the POS lead of the multimeter on the NEG battery terminal?  Also, I’m assuming to test the #2 I can just insert the lead into the connector so that it is next to the wire?

This morning I did connect the NEG meter lead to the red wire terminal end (where the red wire from the two wire connector attaches to the B+ on the alternator. Those numbers were the same as when testing at the battery.

Offline claude

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Re: Fluctuating Voltmeter
« Reply #16 on: January 17, 2019, 12:43:13 PM »
I had the same problem with my 1967 Chevelle SS. I ran a ground wire to voltage regulator and solved the problem. Good luck with your high voltage problem.
1985 K20, manual T, 373 gears, 208 transfer case
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421 ft*lbs torque at 4000RPM

Online bd

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Re: Fluctuating Voltmeter
« Reply #17 on: January 17, 2019, 01:29:35 PM »
The 67 Chevelle used an external regulator.  DO NOT run a ground wire to the regulator of an internally regulated alternator!

An abbreviated orientation to the 1987 R10 charging system:

The alternator is a self-regulating, "dynamic" power source that functions only while the engine is running.  The purpose of the alternator is to charge and compliment the battery, which is a "static" power source that remains somewhat constant through chemical processes (assuming no external influences are present).  Together, the alternator and battery form an interdependent system.  The alternator maintains battery full-state-of-charge by regulating alternator output at a predetermined fixed voltage that lies within the range of 14.2 - 14.7 volts.  The regulator calibration determines the exact voltage within that limited range.  A fully charged "12-volt" lead-acid automotive battery actually produces 12.6 volts.  Therefore, the difference in voltage (dV) between the battery when fully charged and the alternator output while the engine is running is approximately 2 volts.  That's not very much.  Due to a fixed direct relationship between voltage and current, dV determines the current demand imposed on the alternator.  In other words, as the battery discharges its voltage drops and dV increases.  Consequently, alternator current output to recharge the battery increases in direct proportion.

Why is this relevant?  The key is that the regulator controls the voltage output of the alternator by monitoring the voltage at the B+ output post on the back of the alternator via the short external jumper previously pictured.  If there is a poor connection or open in the jumper wire running between the B+ post and the regulator #2 terminal, the regulator cannot "see" the alternator output voltage and the alternator will charge uncontrollably (otherwise known as "full-fielding").  Since the alternator is a power source, the regulator is concerned only with the output from the alternatorTherefore, the relevant voltage measurements are between the alternator case (as ground) and the B+ post, and between the alternator case (as ground) and the #2 regulator terminal.  If the voltage measurements are identical, either the regulator or an internal connection has failed and the alternator should be replaced.


...I probe the #2 on the alternator with the NEG lead on the multimeter and the POS lead of the multimeter on the NEG battery terminal?...

No.  The negative probe of the meter connects to the alternator case (ground) and the positive lead is used to probe for voltage at the #2 terminal and the B+ output post.  By using the alternator case as the ground connection point, you eliminate any variables that may be introduced by faulty ground connections that could exist between the alternator and the battery.


...I’m assuming to test the #2, I can just insert the lead into the connector so that it is next to the wire....

Correct.


Did this help clarify the circumstances?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #18 on: January 17, 2019, 02:04:50 PM »
Gotcha. Thanks for your patience. I’ll look at it again this evening.

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #19 on: January 17, 2019, 02:58:26 PM »
Ok, just got done again. Probing the #2 red wire with the POS lead of the meter and the NEG lead of the meter on the alternator housing resulted in a 15.40 or so reading. Same results when on the B+ post on the alternator.

Online bd

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Re: Fluctuating Voltmeter
« Reply #20 on: January 17, 2019, 03:43:54 PM »
There ya go.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #21 on: January 17, 2019, 06:48:13 PM »
Thanks for all of your help.  Now I gotta figure out what size alternator it is and the size I need.

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #22 on: January 17, 2019, 08:38:47 PM »
Disregard. K22 RPO code is a 94 amp alternator.

Online bd

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Re: Fluctuating Voltmeter
« Reply #23 on: January 17, 2019, 10:29:58 PM »
Not to be a killjoy, but referring back to your original post, once you resolve the charging system overvoltage don't be surprised if you still have a voltmeter issue.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #24 on: January 18, 2019, 09:42:46 AM »
All right. You have my attention....

Online bd

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Re: Fluctuating Voltmeter
« Reply #25 on: January 18, 2019, 12:06:06 PM »
Only that once the charging system is under control you may discover that the voltmeter is still registering a slightly high voltage.  It may be within acceptable limits and tolerable, considering that the OEM gauges are not precision instruments.  So not to worry about it before that bridge is in sight.  Just be aware to avert disappointment.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #26 on: January 18, 2019, 07:32:59 PM »
Gotcha. Thanks again for your patience and help. I can honestly say I learned something.