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MG TD TF 1500 - Ammeter
|I just read an article that essentially said that ammeters are dangerous - the entire electric load of the vehicle is passing through two not very large wires. I do recall this with my motorbike (Royal enfield, with ammeter; and some people replaced them with voltmeters, citing safety).|
What's the general opinion on this?
Please understand - I do not have a concourse car, it never will be a concourse car, and while I don't want to turn it into some chromium harlot, I have no problems with modifications done appropriate to the look and style of the car, including such modern ideas as LED lights, roller cams, and other technology invented since the closure of the Abingdon factory...
That being said, just wanted to get a take on what people thought ... and may have done...
|Geoffrey M Baker|
|There is nothing wrong with the OEM ammeter wiring. The standard wires are quite substantial. Those who don't know what they're doing can create a hazardous condition. Bud|
|I'll agree with the above . But working under the dash with the wiring ,a lesson I learned the hard way when I was young. Unleash your watch and your wedding using wedding cars sevenoaks ring . Amperage go's the way of least resistance and give you bad burns.|
|As the MG TD/TF is wired,yes, there are substantial wires going to and coming from the ammeter. Tsngeling with either of these wires or the ammeter terminals could cause a substantial short. These wires are sized to at least carry the total current load. Not the starter load, however. Such a short would almost instantly raise the temperature of the copper to a point where severe burns could result. However even in systems without an ammeter there still must be an equally large feed. Usually this goes only as far as the main fuse panel . This panned can still be inside the vehicle. (Many new cars have the fuse box external). The same danger exists with or without an ammeter|
Before working on the electrical circuit disconnect the battery.
There are some ammeters which do not require large cables. These are really millivolt or microvolt meters that read the voltage drop across an external resistor called a "shunt". The wires going to the millivolt meter are much smaller in diameter so with a short the current is much less. BUT unless the millivolt meters are fused, during a short these smaller sides will still heat up
Likely to the same or similar temperatures as the larger wires. Serious burns could again result.
Even a system without an ammeter, using a voltmeter still needs a feed wire (and a ground). A feed wire short can cause a burn.
Again before working on ANY electrical circuit REMOVE ALL THE POWER. This is true independent of the voltage, whether 12V or 120V or 240V.
|The wires to the Ammeter don't normally carry the full load. They carry the load between generator and battery which can sometimes be the full load (except for the starter which always goes direct from the battery). |
The generator is the source of the power, and all power directly goes to the users that are connected to it (via switches and sometimes fuses). Excess power goes to the battery via the ammeter -which can still be a lot (30 amps) but normally is much less. That's why it shows zero most of the time. When the load is higher than what the gen can deliver, the excess comes from the battery. Only when the generator is broken, the full load would come from the battery via the ammeter.
All the given advices still hold true ;-)
Not to worry from a design perspective, though. The wires are designed strong enough to carry the load if in good working order. The main light wiring carries compareable loads and for much longer periods- for me much more a source of concern in that case.
And concerns about the cables are perfectly valid- I still have the original harness with brittle isolation....
On having a fire start in the engine room first vs. Passenger compartment, I'd probably vote for the latter where I can smell it earlier, no fuel is right next to it and the extinguisher is at hand (if it is there at all..). But that is probably another discussion...
|It's been said, however, thata a voldmeter tells you if something is going south whereas an ammeter tells you that something has already gone south. I agree with all of the precautionary statements above and, if reasonably maintained, see no significant safety concern to having and using the ammeter but I like to hook up an after market voltmeter, especially if I've just replaced or repair something electrical, to monitor what's going on. Not wanting to drill a big hole in an otherwise good dash, I mount the voltmeter under the dash where it can be easily removed after I'm reasonably confident whatever I replaced or repaired is working satisfactorily.|
|J K Chapin|
I try to limit my responses to electrical issues where I feel some competence. Most of us can remember that cars from the 1930s 40s 50s and 60 until replaced by the idiot light had an ammeter. They functioned safely for the life of the vehicle and many of that time are still functioning today. I treat my TD as one would an aircraft scanning the instruments at regular intervals. If you become familiar with the behavior of the ammeter it can forewarn of failure as well as a voltmeter. Never the less having an after market volt meter is no sin. A $9.95 meter from Radio Shack or equal can plug into the seldom used outlets in the dash for those with short term curiosity.
Jim Haskins 1953 TD
|J. M. Haskins|
|Now dang it! That's a great idea. I've been trying to figure out a meaningful use for those outlets AND a way to monitor voltage when I want to. BINGO!!|
|J K Chapin|
|Before doing any kind of car work, not just electrical, disconnect the battery.|
|This was also my excuse for never getting a wedding ring. I explained to her she did not want to risk me loosing my finger in a short. (still works after 35 years) :-)|
|There is no such thing as an ammeter.|
An ammeter is simply a voltmeter with a big slug of metal between the posts, and the voltage is measured as the potential between the posts of the slug.
You're still measuring volts, but the face of the dial says "amps". the reading of which is always directly proportional to the load between the battery and the system.
|Gordon A Clark|
This thread was discussed between 19/04/2014 and 01/05/2014
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