Voltage drop when I hit the brakes |
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Voltage drop when I hit the brakes |
PinetreePorsche |
Jan 10 2007, 08:03 AM
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#1
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Member Group: Members Posts: 235 Joined: 14-November 05 From: Falls Church, VA Member No.: 5,124 |
What's wrong here? The voltage on my meter (stock- center console) falls from near 13 to 11-and-a-bit, daytime, and has a corresponding drop from a slightly lower starting (mid-12 area) with the lights on. Turning on the lights, without other loads, barely deflects the needle down, maybe .3 volts--that's for 2 fr. corner, two taillights and two markers. Why would the brake bulbs drain so much more? -is there a short somewhere--and no, I haven't been blowing any fuses. Just wondered--don't want to get stuck in an after-dark traffic jam, sit with the brakes on and drain the battery. -C
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Dr. Roger |
Jan 10 2007, 08:10 AM
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#2
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A bat out of hell. Group: Members Posts: 3,944 Joined: 31-January 05 From: Hercules, California Member No.: 3,533 Region Association: Northern California |
if all of your connectors are clean and wiring is healthy, then i'd load test your battery to verify it's health.
everyone says, "ohhhh, my battery is only X years old..." whatever. load testers are cheap easy insurance for your battery. and you'll drive with a bit more confidence and probably save one AAA use. =-) Roger (IMG:style_emoticons/default/beerchug.gif) Good luck. |
PinetreePorsche |
Jan 10 2007, 08:24 AM
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#3
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Member Group: Members Posts: 235 Joined: 14-November 05 From: Falls Church, VA Member No.: 5,124 |
I'll check the battery--but that doesn't answer why so little voltage drop with the first position of the lights in the on position --and I might add, not much more drop when the fog/driving lamps are on as well, or enen (a bit more drop, but not much) when the headlights are on. Just the brakes draw it down a lot. -C
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VaccaRabite |
Jan 10 2007, 08:30 AM
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#4
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En Garde! Group: Admin Posts: 13,584 Joined: 15-December 03 From: Dallastown, PA Member No.: 1,435 Region Association: MidAtlantic Region |
Check your wiring first. Make sure that the connectors arn't all rusted, etc. More resistance = more load = more heat and could eventually lead to burned wires.
Zach |
pfierb |
Jan 10 2007, 08:48 AM
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#5
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oldest member Group: Members Posts: 606 Joined: 1-May 05 From: The sign of good government in Connecticut is to keep raising taxes Member No.: 4,008 |
What's wrong here? The voltage on my meter (stock- center console) falls from near 13 to 11-and-a-bit, daytime, and has a corresponding drop from a slightly lower starting (mid-12 area) with the lights on. Turning on the lights, without other loads, barely deflects the needle down, maybe .3 volts--that's for 2 fr. corner, two taillights and two markers. Why would the brake bulbs drain so much more? -is there a short somewhere--and no, I haven't been blowing any fuses. Just wondered--don't want to get stuck in an after-dark traffic jam, sit with the brakes on and drain the battery. -C Mine seems to do the same thing and the battery is new. |
dr914@autoatlanta.com |
Jan 10 2007, 09:20 AM
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#6
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914 Guru Group: Members Posts: 8,092 Joined: 3-January 07 From: atlanta georgia Member No.: 7,418 Region Association: None |
No reason to check ANYTHING! This is typical for 914! The volt meter is wired in the same circuit as the brake light switch and we all know how much power two brake light bulbs take to operate! This is one of the tech tips in the Tech Tips 700!! Typical 914 (we love their little quirks!)
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Dave_Darling |
Jan 10 2007, 09:52 AM
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#7
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914 Idiot Group: Members Posts: 15,051 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
As I always tell people, the voltmeter is there "strictly for entertainment". It even jumps around when you have your turn signals on, which makes me snicker.
If you want an actual indication of the health of your charging system, you need to measure with a decent voltmeter directly across the battery posts. Not the clamps, the posts themselves. Once you get to know what your stock VM does under different conditions, you may be able to use it to know when a light bulb has burned out. Apart from that, it's pretty useless... --DD |
robby750 |
Jan 10 2007, 10:08 AM
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#8
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Senior Member Group: Members Posts: 646 Joined: 6-April 03 From: Lexington, Ky Member No.: 521 Region Association: South East States |
No reason to check ANYTHING! This is typical for 914! The volt meter is wired in the same circuit as the brake light switch and we all know how much power two brake light bulbs take to operate! This is one of the tech tips in the Tech Tips 700!! Typical 914 (we love their little quirks!) I have to agree here. Mine does the same thing and I used to worry about it all the time. After never having a charging problem, I just learned to ignore it. |
PinetreePorsche |
Jan 10 2007, 11:36 AM
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#9
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Member Group: Members Posts: 235 Joined: 14-November 05 From: Falls Church, VA Member No.: 5,124 |
No reason to check ANYTHING! This is typical for 914! The volt meter is wired in the same circuit as the brake light switch and we all know how much power two brake light bulbs take to operate! This is one of the tech tips in the Tech Tips 700!! Typical 914 (we love their little quirks!) Thanks, George. I read the book, cover to cover, in two straight stints while I was waiting to see if they would call me in as a substitute for jury duty (they never did). Guess I should go back and take another look at the ones I flagged or earmarked as important--I'd probably retain more w/a second reading. So, good doctor, is there any modification to the hook-up that would render the gauge more accurate/useful? |
Brando |
Jan 10 2007, 11:52 AM
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#10
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BUY MY SPARE KIDNEY!!! Group: Members Posts: 3,935 Joined: 29-August 04 From: Santa Ana, CA Member No.: 2,648 Region Association: Southern California |
Yeah... if it were run between the voltage regulator positive and battery negative (or engine ground). That would give an accurate reading of your voltage output.
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PinetreePorsche |
Jan 10 2007, 09:46 PM
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#11
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Member Group: Members Posts: 235 Joined: 14-November 05 From: Falls Church, VA Member No.: 5,124 |
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ClayPerrine |
Jan 10 2007, 09:51 PM
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#12
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Life's been good to me so far..... Group: Admin Posts: 15,879 Joined: 11-September 03 From: Hurst, TX. Member No.: 1,143 Region Association: NineFourteenerVille |
I always looked at the voltmeter as a "mechanical turn signal indicator".
The damn thing is fairly worthless for actual voltage readings. |
davesprinkle |
Jul 10 2007, 12:49 AM
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#13
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Senior Member Group: Members Posts: 720 Joined: 13-October 04 From: Berkeley, CA Member No.: 2,943 Region Association: None |
Just for fun, I took my 914 voltmeter to work and verified its calibration with a laboratory power supply. Within its typical operating range (say, 10V to 14V), the meter's error was significantly less than 0.25V. Outside of that range, the error was less than 0.5V.
OK. You won't be making any precision measurements with this kind of accuracy, but it's good enough for use on the dash of a 914. So if the meter is so decent on the bench, why is it so darned awful when hooked into the 914 wiring? The answer is this: the problem isn't with the meter -- it's with the wiring. The meter is accurately reporting the voltage at the FUSEBOX, not at the BATTERY. So if the voltmeter twitches by 0.5V in synch with the turnsignals, it's because the voltage at the fusebox is twitching by 0.5V. So why does this happen? OK, just some rough numbers: 2 21W turnsignals will pull roughly 3.4 Amps. Given this current, a 0.5V twitch requires a wire resistance of only 0.15 Ohm. The circuits probably had that kind of resistance when they were fresh out of the factory -- 30 years of oxidation on the terminals has only made the resistance greater. The fundamental problem is one of design -- the engineers neglected to ensure that the voltage sense lead carries little current. So what's the solution? Run a new voltage sense lead all the way from the battery positive terminal. Connect it to the voltmeter by passing it through the contacts of a relay. Switch the coil of the relay with the previous voltmeter lead. There you go. The resistance of our new sense lead isn't significantly less, but because none of our lighting currents flow across that resistance, we'll no longer have the voltage error. The key here is to use the sense lead ONLY for sensing -- if you have any other load being fed by the lead, you'll corrupt the accuracy of the meter. Or you could just tolerate the twitching needle... |
Bartlett 914 |
Jul 10 2007, 10:02 AM
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#14
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Advanced Member Group: Members Posts: 2,216 Joined: 30-August 05 From: South Elgin IL Member No.: 4,707 Region Association: Upper MidWest |
Just for fun, I took my 914 voltmeter to work and verified its calibration with a laboratory power supply. Within its typical operating range (say, 10V to 14V), the meter's error was significantly less than 0.25V. Outside of that range, the error was less than 0.5V. OK. You won't be making any precision measurements with this kind of accuracy, but it's good enough for use on the dash of a 914. So if the meter is so decent on the bench, why is it so darned awful when hooked into the 914 wiring? The answer is this: the problem isn't with the meter -- it's with the wiring. The meter is accurately reporting the voltage at the FUSEBOX, not at the BATTERY. So if the voltmeter twitches by 0.5V in synch with the turnsignals, it's because the voltage at the fusebox is twitching by 0.5V. So why does this happen? OK, just some rough numbers: 2 21W turnsignals will pull roughly 3.4 Amps. Given this current, a 0.5V twitch requires a wire resistance of only 0.15 Ohm. The circuits probably had that kind of resistance when they were fresh out of the factory -- 30 years of oxidation on the terminals has only made the resistance greater. The fundamental problem is one of design -- the engineers neglected to ensure that the voltage sense lead carries little current. So what's the solution? Run a new voltage sense lead all the way from the battery positive terminal. Connect it to the voltmeter by passing it through the contacts of a relay. Switch the coil of the relay with the previous voltmeter lead. There you go. The resistance of our new sense lead isn't significantly less, but because none of our lighting currents flow across that resistance, we'll no longer have the voltage error. The key here is to use the sense lead ONLY for sensing -- if you have any other load being fed by the lead, you'll corrupt the accuracy of the meter. Or you could just tolerate the twitching needle... (IMG:style_emoticons/default/agree.gif) Good writeup Dave. You are spot on here. I like the idea of using the old volt meter connection to energize a small relay. This also will save on draining the battery over long periods (although that is a very small drain). |
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