Voltmeter installation

[EMGEBE]

Giday boys

Since ive just had my alternator replaced i think its a good idea to go grab myself a voltmeter. (going to get one, litterally right now)

i know the negative wire goes straight to ground, but the +ve wire.... whats the story?
ppl have told me different stories, some say directly off the alternator, others off the ignition?
ne one?
Cheers
also, if ne one can give me ne idea as to how to wire up the lights for it to the dash. CHeers

Thanks fellas /ubbthreads/images/graemlins/thumbsup.gif
J - newbie electrician

 

[adam90009]

Hook it up to a green wire which is hot when the car is on and it is fused and then depending on where it is just slpice in to another dash light.

 

[Keoke]

EMGEBE, Just what do you expect the Voltmeter to tell you? It would seem if you want to monitor the alternators status an Ampmeter would be more fitting.Just curious ---Keoke

 

[EMGEBE]

i thought the whole idea of having a voltmeter was to measure the voltage going through your car AFTER the alternator isnt it? to ensure it is actually doing its job

 

[piman]

Hello Emgebe,
yes a voltmeter measures the system voltage, and should indicate about 14 volts if the alternator is charging. It is not quite as instantaneous as an ammeter which tells you the charge or discharge rate of the generator. (By the way voltage doesn't "go through the car", current does, the voltage is the 'push' that gives current flow.)
It is easier to connect a voltmeter (as Adam describes) and probably safer.

Alec

 

[adam90009]

For an ammeter you will need to run all of your wires through it as I understand and it can be a big fire hazard.

 

[John S Farrington]

I agree with Adams comment. The voltmeter is a simpler and better solution to monitor your charging circuit. You can get +ve anywhere including the battery but it should go through the ignition switch to turn the voltmeter off with the key. Use a green or white color coded wire. Your dash panel lights are red

 

[John S Farrington]

The message got sent before itd was complete. Your dash panel lights are red and white and should be attached to a red wire that is usually the running lights of the outside lighting. There is no need to power the dash panel lights tdhrough the rheostat in the dash since all it does is dim the dash panel. MGBs and most British car pnel lights are sufficiently dim at full power and do not need a rheostat in line. Hope this helps.

 

[SteveL]

[ QUOTE ]
For an ammeter you will need to run all of your wires through it as I understand and it can be a big fire hazard.

[/ QUOTE ]

That would be incorrect, an ammeter only sees a
small portion of the current the alternator is putting
out, most of the current is running through a shunt, which
is wired in parallel with the ammeter...

SteveL

 

[adam90009]

So how would the ammeter work if only a small portion of the electricity actually goes through it? I would think all of it has to go through it for it to measure all of it?

 

[Dave Russell]

An ammeter consists of a low resistance high current resistor "shunt", connected to a sensitive low current meter movement. On some ammeters, the shunt is inside of the meter case & all of the current to be measured must be run through the meter case & shunt.

Other ammeters have the shunt mounted externally. With this type, the shunt can be connected outside of the passenger compartment inline with the high current carrying, heavy, wires & two small low current wires run from the shunt to the meter movement inside. Obviously, this is the preferred type as it keeps the high current wires outside of the passenger compartment. In either case, if the shunt becomes defective, all electrical functions will stop.

A volt meter only requires two low current carrying wires & can easily be connected behind the dash. Since it has no shunt & no high current circuits it is much less likely to cause reliability/safety problems. As far as giving information about the charging system, the volt meter will rise from about 12.5 volts, no charge, to around 14.5 volts when things are charging or fully charged.
D

 

[Guinn]

Take a look at this site: https://www.houlihane.co.uk/amps.htm
It has a short discussion about ammeters and voltmeters and also a general wiring diagram

Guinn

 

[Keoke]

Gee thanks Guinn, I had just begun to wonder why all the old Jags had not burned to the ground because they contain Ampmeters. /ubbthreads/images/graemlins/rolleyes.gif---Keoke

 

[piman]

Hello Steve\Adam,
UK automotive ammeters I know of have the full current cables connect to them so the hazard of a short in an ammeter cable is quite high due to the large cross sectional area. If they are shunted, then it must be internal, but I doubt it. The usual 30-0-30 ammeter is too small for an alternator anyway. Adam, shunting is normal practice for large current D.C ammeters. The shunt resistance is carefully designed to be a large proportion of the ammeter internal resistance. The ammeter true reading may be say 5 amps full scale, but with a shunt resistance one ninth of the ammeter resistance will flow 90% of the current so applying 50amps will give a full scale delection with 5 amps through the meter and 45 amps through the shunt. The ammeter would be marked as 50 amps not 5. Heavy current shunts are very large physically to get an extremely low resistance.

Alec

 

[Paul Slice]

EMGEBE,
I put a voltmeter in place of the clock when I did the 78 B. Any 2 inch gauge should fit without modifications, I used a Sunpro sourced from e-bay for $8.00. I have attached a picture, actually does not look to out of place with the Smith's gauges.

 

[Keoke]

OF course they are shunted and require only 1 amp for full scale deflection--Keoke /ubbthreads/images/graemlins/rolleyes.gif

 

[Guest]

An amp meter alone only takes a small amount of current to make it go full scale. Say 1 amp. (Some meters are only in the micro amp range like meters on multi meters or volt meters. A good sensitive meter will have a meter movement that will go full scale at 10 micro amps.) In order to make this meter read say 10 amps you put a shunt resistor across it to take all but the 1 amp it takes to send the meter full scale. If you number the scale on it's face to read 0 to 10 amps, it will read the true amperage from 0 to 10 amps accurately. You can take that same meter and shunt it for 100 amps if you want and it will read that accurately. Even though the meter is only seeing 0 to 1 amp.
The shunt resistor might be a piece of wire that has more resistance than copper wire. I had some one time I coiled up around a dowel then removed the dowel and used it for a ballast resistor for a home made ignition.
On a volt/ohm/amp meter, the switch on the front is just introducing resistors into the circuit to make the meter read which scale you want. In the case of the voltmeter, it just has resistors in series with the small amp meter to read the voltage. So a volt meter is an amp meter just hooked up differently. You figure the resistance value using ohm's law formula E=IR
Bob

 

[piman]

Hello all,
just a point of interest, car ammeters tend to be much simpler (cruder) and, on British cars anyway, are not shunted. They are a moving iron rather than moving coil movement.

Alec

 

[Dave Russell]

Hi Alec,
Whether the ammeter is moving iron or moving coil, you can be assured that there is a shunt. It may be just a metal bar or short piece of heavy wire inside of the meter case, but it is still there.
D

 

[piman]

Hello Dave,
no, I'm sorry but I dug out my car electrical manual and the description given is that the meter has just a few turns of heavy gauge wire, no mention of a shunt. I can see I'll have to get a gauge and have a look inside. As I said in an earlier post, shunts are bulky, the quoted 1 amp movement will need a shunt 1\30th approximately the resistance of the meter movement.

Alec

Alec