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Old 09-29-2011, 06:48 AM
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What's going on with my amp gauge?

I noticed last week that my amp gauage will twitch to the left when ever the turn signal will blink. In addition to that, it also shows a large drop off whenever the lights are turned on.

It's a 48 Merc street rod with a 350 chevy engine. I bought this car about a month ago, so I did not do the wiring job. The battery is located in the trunk so is it possibble the battery is too small voltage wise?

The car starts up fine and runs good but I can't help but think there is something wrong somewhere and I have no idea what to look for.

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Old 09-29-2011, 06:59 AM
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Get rid of it, here´s why.
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Old 09-29-2011, 08:33 AM
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Quote:
Originally Posted by maddog48merc
I noticed last week that my amp gauage will twitch to the left when ever the turn signal will blink. In addition to that, it also shows a large drop off whenever the lights are turned on.

It's a 48 Merc street rod with a 350 chevy engine. I bought this car about a month ago, so I did not do the wiring job. The battery is located in the trunk so is it possibble the battery is too small voltage wise?

The car starts up fine and runs good but I can't help but think there is something wrong somewhere and I have no idea what to look for.
An AMP gauge needle will show a negative reading whenever the directionals are being turned on and will return to '0' when they are off. You will also see negative movement on the gauge when any lights (headlights, brake lights, etc.) or any accessories are turned on. The gauge is showing you that something is pulling current from the battery.

That being said, IF your charging system is working correctly, the reading should return to the (+) side of the AMP gauge if the engine is running. If it does not, then you are drawing from the battery and your charging system is not working. If all is well the AMP gauge will show just a small reading in the (+) direction. If a larger reading is constant (Not just immediately after starting the engine), then your charging system may be overcharging the battery. This could be the regulator and/or the generator. Check the water level of the battery.

The general thinking today is to replace the AMP gauge and use a VOLTMETER to monitor your charging system. This is primarily because of the high current draw (amperage) that is present in the wiring to and from the AMP gauge. Some consider this to be a safety measure. If wired with the correct size wire and insulated well so there is relatively no chance to short out, it should be quite safe. A VOLTMETER does not normally draw a high current and therefor is considered to be safer.

The battery will not be too small 'voltage' wise as you ask. However, the battery cables (both positive and negative) must be sized accordingly for the extra length needed with the battery installed in the trunk. I would also suggest that the ground cable, in your case the (-) negative battery cable, be run from the battery all the way to and connected to the engine block or the bellhousing. Running a short ground cable from the battery to the frame will not always provide a good ground connection for the distance traveled. Additional ground straps from the engine to the frame and firewall are often needed to insure proper grounding.
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Old 09-29-2011, 10:51 AM
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Disconnect and jump out that amp meter , it's like a fuse and will burn eventually
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Old 09-29-2011, 04:33 PM
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amp gauge

I've ran a amp gauge in my street rod for at least 25 years. What gave them a bad rap was when Ron Francis loaded up on volt meters and could not sell them. You remember when he ran the ads in Street Rodder mag that all amp meters were burning up cars and you should switch to volt meters. The thing that people don't understand is if you run a altenater with more amps than your amp gauge will make the gauge fail like a fuse. How many times have you heard of a fuse blowing burning up a car. You will lose all power from the altenater. No matter what system you use there is a power wire from the altenater to the starter that charges the battery. You should use the proper size wire no matter what gauge you have. This just my opinion and by all means do what you like, do not take this as advice. I've heard of people that know of a car that burned from a amp meter going bad. I've been into cars for 40 years and have never met anyone that has had this happened to them. Please don't come and jump all over me for stating a opinion. Opinions are like belly buttons, everyone has one, some are a little different.
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Old 09-29-2011, 05:56 PM
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Thanks to everyone for there advice and opinions on this subject. I had never owned a vehicle up until now that had an amp gauge in it. So I was concerned about what I saw happening. Frisco's explanation of why it was happening and that it was not an indication of a bad ground; or too small of a battery has put my mind at ease; because the meter in my street rod behaves just as he describes.
I may or may not change out the amp meter at this time but I do think when the day comes to re-do the interior my new instrument cluster will have a volt meter.
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Old 09-29-2011, 06:27 PM
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Since you've only had the car a month, you are probably seeing a normal condition. When properly installed and maintained an ammeter is as safe as a voltmeter.

Voltmeters indicate the basic status of your electrical system, a steady 12V reading shows that the alternator and battery are producing enough energy to operate your car's electrical equipment. The reading is steady because the voltage regulator adjust's the current from the alternator to match demand (turn signals, lights, AC, etc).

Ammeters indicate the status of your charging circuit, a slight positive reading shows current is flowing from the alternator to charge the battery. A negative reading indicates that current is flowing from the battery to operate your car's electrical equipment. The reading twitches because (especially with older design regulators and/or generators) the sensitivity of the regulator circuit does not react to the small change in demand. A sustained negative reading could indicate a problem with the alternator (generator), especially if you are having slow starts, dim lights, or other electrical issues.

It's good that you are paying attention to your guages. The ammeter will show if you have a charging system problem.

Watch for extreme positive or negative readings, these mean you have an immediate problem. Find a safe place to stop and check. Look for broken/abraded/loose wiring, belt on the alternator/generator, water in the battery kind of stuff.

BTW, since the car is new (to you), a good visual inspection of all the electrical equipment, connections, wiring is advised.
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Old 09-30-2011, 06:54 AM
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The safety of amp gauge wiring depends on whether it is a full current flow gauge, or uses a shunt.

For a full flow gauge you need to have all the current from the battery flowing through the gauge. The wiring to the gauge must be big enough to allow the total current flow to go from the battery to the gauge and back to the power junction. If you have any shorts in that wiring, you have a lot of current shorted to ground and it gets hot very fast. These gauges are the ones that are described as "dangerous."

Most newer amp gauges use a shunt design, where the two legs of the amp gauge connect to two ends of a shunt in the wiring under the hood. Based on the voltage drop between these two points the gauge "calculates" a representative amperage to display. This type of gauge is much safer because only a small current flows through the wiring to the gauge. However, it appears that the mechanism is subject to a fairly high failure rate, since most of them I've seen in older cars do not work correctly after a few years.

This article tries to explain how the shunt works. http://www.bracketracer.com/nova/shunt/shunt.htm

Bruce

Bruce
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Old 09-30-2011, 09:08 AM
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I believe that a '48 Merc would have an inductive type of ammeter that has no terminals to corrode and cause problems. Should just be a tin loop on the back of the gauge that the main battery feed wire passes through. It senses the current passing through the feed wire similarly to how a modern clamp-style amp probe does.



Hope this helps.....
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Old 10-02-2011, 01:50 PM
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Well everyone I need to apologize to you all.
I hopped into my car this morning and look over at what I though was an amp gauge and am shocked to see it is a volt meter; boy do I feel like an idiot.

So now here is the question; should my VOLT gauage be showing a 10amp drop every time the blinker blinks? OR show a 15 amp draw whenever I turn the lights on for the car?

After reading all your replies here for when I mistakenly refered to it as a amp gauge I am beginng to think I may have a major electrical fault here.
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Old 10-02-2011, 04:15 PM
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Maddog48merc your explanation is still very confusing. If you have a voltmeter, how are you measuring a 10 amp and 15 amp draw?

15 amps for your headlights is fairly high, but not that unusual with older wiring where you have poor connections due to corrosion.

Bruce
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Old 10-02-2011, 07:34 PM
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My use of the word amp is incorrect; what I meant to say is volts.

What I'm trying to say and not doing a very good job of it is that when the turn signal is on and working the volt meter in the car will show a drop of 3-4 volts everytime that the indicator blinks; this is a condition I have never seen before. Am I wrong in thinking that there is a problem somewhere in the wiring harness?
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Old 10-03-2011, 04:06 AM
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Above is good advice, well most of it, In these older cars the OEM wiring harness was just barely good enough to handle the system........ If you have put in any modern equipment it really taxes the older system. One solution many, including me, have found VERY helpful is to replace the old alt and external voltage regulator with a modern alt with a built in reg. Super easy to do and it fixed my dimming headlights, flickering lights with the heater on, blinkers etc..... It was a great inprovement on my car.
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Old 10-03-2011, 05:04 AM
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Quote:
Originally Posted by maddog48merc
My use of the word amp is incorrect; what I meant to say is volts.

What I'm trying to say and not doing a very good job of it is that when the turn signal is on and working the volt meter in the car will show a drop of 3-4 volts everytime that the indicator blinks; this is a condition I have never seen before. Am I wrong in thinking that there is a problem somewhere in the wiring harness?
Some gauges are very sensitive and will show a dip from the turn signals even w/everything as it should be. It is important to note the value of each line or division on the gauge face, i.e. is each line or division indicating full volts or 0.1 volts, for example?

The draw from a bulb like a turn signal should be easily handled by the battery (W/V=A. Bulb draw is miniscule compared to the battery capacity in amps). BUT if the gauge is showing you a dip of 10-15 VDC- and it's accurate- there is a problem. I'd suspect the battery except it's starting the engine OK (I presume). Inadequate grounds, inadequate wire gauge, corroded, loose or damaged connections or sockets, etc. can all contribute to excessive current draw seen at the volt gauge.

I'd suggest you do a voltage drop test to assure yourself there is no major problem that could cause melted wires or, god forbid, a fire. If you have a generator or external regulated alternator, I'd suggest swapping over to an internally regulated alternator like the SI or newer series alternator. Often the charge wire off the back of the alternator is undersized. This is easily fixed in most cases and will assure you that there's no bottleneck there.

Last edited by cobalt327; 10-03-2011 at 05:13 AM.
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Old 10-05-2011, 11:44 AM
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Cobalt is on the right track, but note that the loose/corroded connections increase resistance along the current path and that results in less current not more (Ohms law: I=E/R, current equals voltage divided by resistance).

The drop in voltage you see on the meter is because the added resistance in the circuit path (loose/corroded wires) drops more voltage (across the resistance) as current flows through the wiring, the voltmeter and the turn lamp(s); thus the reading at the voltmeter is less.
If you would like me to draw you a diagram, let me know.

So checking your wiring for loose or corroded connections, correct wire size, etc is a good idea. If you don't know what your looking for get someone who does.

Also, if you have an older charging system (alternator with external regulator or a generator) then what you are seeing is the systems (lack of) response to change in load.
Newer systems react more quickly and maintain a steady voltage so you don't see the guage move much.

An upgrade is not a bad idea, but may not be necessary.
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