Author Topic: Fluctuating Voltmeter  (Read 407 times)

Offline 502Silverado

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Fluctuating Voltmeter
« on: January 09, 2019, 08:56:35 PM »
I apologize for the long post, just trying to be thorough.  I did a search for my specific issue and couldn't find an answer. 
I have an 87 R10 350 TBI that I have recently replaced the battery, the inner passenger fender, bushings under the core support and cab, radiator, headlights, shocks front and rear, rear brakes and had the original core support sandblasted and then I rattle can painted it myself.  I finally drove it to work today after working on it for about 5 months and noticed that the voltmeter is nearly pegged at the 18 mark (original gauges, stock engine) whenever the throttle is applied.  When at idle it reads just past the 13 on the gauge.  When I accelerate or traveling at constant speed is when the gauge nearly pegs.  I have not noticed any fluctuations in the headlights/tail lights, interior or dash lights.  The left turn signal does seem to blink a little slow and the oil pressure gauge slowly drops to zero but I believe that is a sending unit issue.
I'm thinking it could be a grounding issue?  I have not used a ohm meter at all because I know more about the gross national product of Zimbabwe than electricity. 
Any help or advice is greatly appreciated.

Offline VileZambonie

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Re: Fluctuating Voltmeter
« Reply #1 on: January 10, 2019, 04:26:34 AM »
Check the actual voltage with a voltage meter to verify the accuracy of the gauge.
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Offline 75gmck25

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Re: Fluctuating Voltmeter
« Reply #2 on: January 10, 2019, 12:15:40 PM »
Just buy a cheap VOM at Walmart or the auto parts store. It should not be more than about $10-$12, or even cheaper if you have a Harbor Freight nearby and can buy one like this https://www.harborfreight.com/7-function-digital-multimeter-63604.html .

Check the voltage at idle with the negative meter lead grounded to the engine or alternator bracket, and the positive meter lead on the BAT post of the alternator (the threaded terminal on  the back of the alternator), or on the battery positive.  It should read about 12.5-13.0 volts while idling, and then go up to about 13.8-14.0 volts when you raise up the RPM.   

If you get a reading of anything more than about 14.5 volts, the voltage regulator is not working right.  You can buy a new regulator, but it might be simpler to just get a rebuilt alternator.

Bruce

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #3 on: January 10, 2019, 05:42:33 PM »
Thanks for the replies.  I cleaned all the ground connections today and plan on picking up a multimeter tomorrow.  Thanks for letting me know where to connect everything, electrical systems are definitely not my thing.

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #4 on: January 12, 2019, 11:41:20 AM »
Ok, I connected a multimeter to the battery while it was off and got around 12.5 or so.  Once it was started it went to about 14.  When I gave it a little gas (I don't have tach) it jumped to above 15.  I also noticed that whenever I turned on the heater it dropped off quite a bit.  I'm assuming that my regulator for the alternator is bad due to how high the multimeter jumped when I gave it a little gas?  Thanks for the help.

Offline bd

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Re: Fluctuating Voltmeter
« Reply #5 on: January 12, 2019, 12:05:00 PM »
Ground the negative lead of the voltmeter to the alternator case, set the parking brake, start the engine and probe the negative terminal of the battery with the positive voltmeter lead, both at idle and at very fast idle (~2,000 RPM).  If the alternator is properly grounded, the measurement should be zero or very nearly zero.  Next, probe the #2 (or "F") regulator lead that plugs into the rearward side of the alternator case (red wire in the two-wire connector imaged below).  Post the results.
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 #6 on: January 12, 2019, 06:09:34 PM »
Thanks, bd.  It will be a few days before I get a chance to do that.  I actually borrowed the multimeter from an electrician that was building the house next door to me.  Gonna try and pick up one of my own.

Offline bd

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Re: Fluctuating Voltmeter
« Reply #7 on: January 12, 2019, 06:20:24 PM »
Purchase a digital meter as opposed to analog.  For occasional use, figure on spending between $20 and $30.
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 75gmck25

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Re: Fluctuating Voltmeter
« Reply #8 on: January 13, 2019, 06:39:41 AM »
If this is your first multimeter, consider both price and whether you plan to use it for something other than your car. 

I think most digitals are high impedence, (but I'm not sure) which works well if you also want to use it for more sensitive electronics (stereos, HVAC controls, etc.).  However, the downside of a high impedence meter is that on basic tasks (checking for voltage before replacing a light switch, etc.) it may pick up stray voltage in house wiring when wires are bundled together.  I've had this issue with house wiring, where I measured voltage at a light switch and it never dropped completely to zero because it was picking up ghost voltage from wiring next to it.

When you use a digital meter, also make sure its set to measure DC voltage, not AC.  My Klein meter will default to AC when you turn it on, which gets confusing when you are working with DC.  I spent several minutes trying to troubleshoot a DC circuit in a radio before realizing the meter was set to AC volts.

Bruce

Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #9 on: January 16, 2019, 06:24:40 PM »
Ok, thanks for being patient with me.  I started looking at multimeters and quickly got over whelmed with specs and brands and prices and decided to borrow one from my father.
With the truck off the battery is reading 12.51
While cranking it drops to 10.40 or so
Once started and at idle it was reading 15.9
With lights, radio and heater on it would fluctuate between 15.00 and 15.20
All of the above were read at the battery.
I took a reading with the negative lead of the meter on the alternator case and positive lead on the negative battery terminal and got .01
I next took readings with the negative lead of the meter on the grounds (frame, firewall, and both on the core support) and the positive lead on the negative battery cable.  None of those were above .01 and some were all zeros.  When reading the grounds and at the alternator case it was slightly above idle speed.
I attempted to read from the #2 on the alternator with the negative lead from the meter and the positive on the negative battery terminal but I don't believe I was making contact because the meter never registered anything. 
While doing all this and letting the truck go back to idle the meter read 15.12 at the battery with no lights, radio or heater on.  I also took a reading with the negative lead of the meter on the screw terminal on the back of the alternator and the positive meter lead on the negative battery terminal.  It was also 15.12.
I'm thinking the regulator may be bad from everything I've read so far.  I have not checked the actual gauge in the dash.
Hope this helps and thanks to you both!

Offline Rapid Roy

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Re: Fluctuating Voltmeter
« Reply #10 on: January 16, 2019, 07:33:41 PM »
I think regulator is bad too. Usual voltage while running, I have known about, is 13.5-14.5. Does your alternator have internal regulator? I am not up to date with 87 but it may, then new alternator is needed.
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Offline 502Silverado

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Re: Fluctuating Voltmeter
« Reply #11 on: January 16, 2019, 07:38:39 PM »
Yes, its an internal regulator.

Offline bd

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Re: Fluctuating Voltmeter
« Reply #12 on: January 16, 2019, 07:43:37 PM »
Before you condemn the alternator you need to verify the running voltage on the #2 regulator terminal as previously described.  The #2 regulator terminal is the voltage sensing input to the regulator.  If the running voltage on that terminal is "zero" as you described the alternator will charge uncontrollably.  The voltage on the #2 terminal should match the output voltage at the B+ post on the back of the alternator.  If the #2 terminal measures 15.9+ volts with the engine running, replace the alternator.
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 #13 on: January 17, 2019, 06:12:12 AM »
Sorry, Im an idiot. I was trying to get a reading from where the red wire where they plugged into the alternator instead of just following that wire to the screw post on the back of the alternator where access is easier. Ill try there.

Offline bd

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Re: Fluctuating Voltmeter
« Reply #14 on: January 17, 2019, 08:07:11 AM »
No, you were on the right track.  With the engine running, measure the voltage on the #2 regulator terminal and compare that to the voltage on the B+ output post at the back of the alternator.  The two measurements should match.  Then unplug the two-wire connector and make sure the wire terminals are clean, intact and fit snug on the regulator pins.  If the voltage measurements are identical and there are good wire connections to the regulator, replace the alternator.  On the other hand, if the voltage measurements are not the same, repair the wire or its connections between the B+ post and the #2 regulator terminal.  Capiche?
« Last Edit: January 17, 2019, 08:12:47 AM by bd »
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)