Voltage discrepancy, WTF? |
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Voltage discrepancy, WTF? |
saigon71 |
Dec 11 2018, 06:17 PM
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#1
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Senior Member Group: Members Posts: 1,999 Joined: 1-June 09 From: Dillsburg, PA Member No.: 10,428 Region Association: MidAtlantic Region |
As long as I can remember, when driving at night in the 914, my center console voltmeter hovers right around 12V when cruising with the lights on. When I hit the brakes, it drops well below 12V...anything else drops it even lower.
I'm running an Optima Red Top near the end of it's service life and want to make sure my charging system is healthy before popping on a new AGM battery. I had been considering switching to LED headlights and brake lights to lower the current draw. I hooked up the VOM at the battery and did a load test with the car running, adding an accessory at a time. With headlights, brake lights and aux fan on the voltmeter in the center console read just over as 10 volts, while the VOM hooked to the battery showed 12.58V. Is this discrepancy normal? |
toolguy |
Dec 11 2018, 06:23 PM
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#2
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Senior Member Group: Members Posts: 1,268 Joined: 2-April 11 From: San Diego / El Cajon Member No.: 12,889 Region Association: Southern California |
perfectly normal and as expected. . old wire with poor connections, as well as bad grounding lugs are the most probable cause. . each connection sucks out a few 1/10 of a volt till they add up. . .
Check the main battery ground and positive leads 1st, then work your way vack thru the wiring. . don't forget the trans ground strap |
TheCabinetmaker |
Dec 11 2018, 06:56 PM
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#3
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I drive my car everyday Group: Members Posts: 8,300 Joined: 8-May 03 From: Tulsa, Ok. Member No.: 666 |
Nothing to do with old wires! They worked like that when they were brand new!
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adolimpio |
Dec 11 2018, 07:09 PM
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#4
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Art Group: Members Posts: 181 Joined: 10-March 10 From: Greenwood SC Member No.: 11,449 Region Association: South East States |
Ohm's law. Voltage drops as current passes through a conductor, increasing with resistance of conductor. Measuring at the battery, very little resistance between source and measuring point, so no voltage drop.
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North Coast Jim |
Dec 11 2018, 07:21 PM
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#5
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Member Group: Members Posts: 323 Joined: 11-December 15 From: Northern Ohio Member No.: 19,450 Region Association: MidAtlantic Region |
You also could have lost the ground connection within the appearance group wiring. Had the same problem. All is well after I reconnected the ground wire
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PanelBilly |
Dec 11 2018, 08:08 PM
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#6
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914 Guru Group: Members Posts: 5,808 Joined: 23-July 06 From: Kent, Wa Member No.: 6,488 Region Association: Pacific Northwest |
I added a new ground wire to mine and that fixed it
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GregAmy |
Dec 11 2018, 08:21 PM
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#7
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Advanced Member Group: Members Posts: 2,307 Joined: 22-February 13 From: Middletown CT Member No.: 15,565 Region Association: North East States |
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campbellcj |
Dec 12 2018, 08:56 AM
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#8
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I can't Re Member Group: Members Posts: 4,545 Joined: 26-December 02 From: Agoura, CA Member No.: 21 Region Association: Southern California |
I'm fighting the same issue now too, and it was at the point where I thought my alternator was failing because I was only seeing ~10V at the dash even with a brand-new battery. It could be the gauge but way more likely the wiring harness and/or a ground. I was super happy it wasn't the alternator...
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ClayPerrine |
Dec 12 2018, 09:00 AM
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#9
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Life's been good to me so far..... Group: Admin Posts: 15,474 Joined: 11-September 03 From: Hurst, TX. Member No.: 1,143 Region Association: NineFourteenerVille |
The volt meter in the center console is nowhere near accurate. You can use it to see if your brake lights are working (voltage drop when you step on the brakes) and it will twitch with the turn signals, but it being an accurate measurement of voltage, well forget it.
The gauge is connected to the same power source as all the other instruments. So it has lots of connections and other stuff in the circuit to affect its performance. If you want it to be more accurate, run two heavy wires, one red, and one brown all the way back to the battery. Use a relay to switch the brown one off when the car is not running. After doing that, it still may not read correctly, as the calibration is questionable. Or just ignore the gauge. (IMG:style_emoticons/default/biggrin.gif) |
dr914@autoatlanta.com |
Dec 12 2018, 09:09 AM
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#10
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914 Guru Group: Members Posts: 7,872 Joined: 3-January 07 From: atlanta georgia Member No.: 7,418 Region Association: None |
You also could have lost the ground connection within the appearance group wiring. Had the same problem. All is well after I reconnected the ground wire I agree check the aux instrument wiring harness ground connection under the console shift base plate, remove clean and plug back in. Volt meter should read 13.6 at idle with no load |
Spoke |
Dec 12 2018, 08:49 PM
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#11
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Jerry Group: Members Posts: 6,985 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
With headlights, brake lights and aux fan on the voltmeter in the center console read just over as 10 volts, while the VOM hooked to the battery showed 12.58V. Is this discrepancy normal? As others have noted the difference is expected in the 914 because of small gauge wire causing IxR voltage drops as loads like headlights, turnsignals are active. I've measured my gauge separately and found the gauge to be accurate to about 0.1V. Unfortunately where it resides in the 914 wire loom causes the gauge to read very low at times. Although not an accurate measurement of battery voltage, the gauge does give an indication of the voltage going to loads in the 914. For instance the headlights may only be getting 10V instead of 14V which the alternator puts out at elevated RPM. |
Dave_Darling |
Dec 12 2018, 10:48 PM
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#12
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914 Idiot Group: Members Posts: 14,990 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
I like to say that the voltmeter is there "for entertainment purposes". It's not a great indication of the health of the charging system...
--DD |
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