Discussion:
Fuel tank sensor and gauge
(too old to reply)
Davey
2015-12-07 12:14:29 UTC
Permalink
I have to debug an old-style fuel gauge problem this winter, it never
moves. I will first check that the gauge is wired correctly, and has
power. As I see it, the gauge is really an ammeter, the current
depending on the resistance of the potentiometer in the sender unit.
Before tackling the tank sender unit, what sort of current
should I be able to measure passing through the gauge? If it is about
right, then the gauge would be indicated to be faulty. But if the
current flow is high or none, then any of the gauge, the sender, or the
wiring, could be at fault.
But knowing what sort of current should flow would be a help to start
with.
Any suggestions welcome.
--
Davey.
Dave Plowman (News)
2015-12-07 13:04:58 UTC
Permalink
Post by Davey
I have to debug an old-style fuel gauge problem this winter, it never
moves. I will first check that the gauge is wired correctly, and has
power. As I see it, the gauge is really an ammeter, the current
depending on the resistance of the potentiometer in the sender unit.
Before tackling the tank sender unit, what sort of current
should I be able to measure passing through the gauge? If it is about
right, then the gauge would be indicated to be faulty. But if the
current flow is high or none, then any of the gauge, the sender, or the
wiring, could be at fault.
But knowing what sort of current should flow would be a help to start
with.
Any suggestions welcome.
There is really no such thing as an ammeter.

They are volt meters, which measure the (tiny) voltage drop across a shunt
resistance, and are calibrated to read that as amps.

So the tank unit is basically a variable resistor which alters the voltage
to the gauge.

The gauge usually has 12v to it and is in series with the tank unit which
is grounded at one end. If this is the case, ground the other end and the
gauge should read full. It might well move slowly to maximum if a hot wire
type.

It's far more likely to be a faulty tank unit than the gauge or the wiring.
--
*IF A TURTLE DOESN'T HAVE A SHELL, IS HE HOMELESS OR NAKED?

Dave Plowman ***@davenoise.co.uk London SW
To e-mail, change noise into sound.
Davey
2015-12-07 15:02:31 UTC
Permalink
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
They are volt meters, which measure the (tiny) voltage drop across a
shunt resistance, and are calibrated to read that as amps.
True, with the desired final effect.
Post by Dave Plowman (News)
So the tank unit is basically a variable resistor which alters the
voltage to the gauge.
The gauge usually has 12v to it and is in series with the tank unit
which is grounded at one end. If this is the case, ground the other
end and the gauge should read full. It might well move slowly to
maximum if a hot wire type.
So the potentiometer of the sender acts as a voltage divider, the gauge
then reading the proportion of the available max. of 12 volts?
That makes more sense than having a continuous variable current drain,
which is what I thought, from looking at the schematic, although that
may still be the case, but at a lower level than what I was thinking of.

So if grounding the 'leaving' end of the gauge produces no effect, the
gauge is defective. If it gives a reading, then the fault is after the
gauge.
Post by Dave Plowman (News)
It's far more likely to be a faulty tank unit than the gauge or the wiring.
Agreed. But I can easily get at the gauge, the sender requires more
work!
Thanks.
--
Davey.
Indy Jess John
2015-12-07 19:39:59 UTC
Permalink
Post by Davey
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
They are volt meters, which measure the (tiny) voltage drop across a
shunt resistance, and are calibrated to read that as amps.
True, with the desired final effect.
Post by Dave Plowman (News)
So the tank unit is basically a variable resistor which alters the
voltage to the gauge.
The gauge usually has 12v to it and is in series with the tank unit
which is grounded at one end. If this is the case, ground the other
end and the gauge should read full. It might well move slowly to
maximum if a hot wire type.
So the potentiometer of the sender acts as a voltage divider, the gauge
then reading the proportion of the available max. of 12 volts?
That makes more sense than having a continuous variable current drain,
which is what I thought, from looking at the schematic, although that
may still be the case, but at a lower level than what I was thinking of.
So if grounding the 'leaving' end of the gauge produces no effect, the
gauge is defective. If it gives a reading, then the fault is after the
gauge.
Post by Dave Plowman (News)
It's far more likely to be a faulty tank unit than the gauge or the wiring.
Agreed. But I can easily get at the gauge, the sender requires more
work!
Thanks.
I have had a look at some old circuit diagrams. Many of them have two
connections on the fuel gauge, one side at battery voltage via the
ignition switch and the other side connected to the tank unit which goes
to the chassis earth. This suggests that the tank unit is a variable
resistor and perhaps a fixed resistor which together vary the current
available to the meter. Unfortunately, none of the diagrams show the
internal wiring of the "Tank Unit" sketch.

They are not all like that though. One circuit had a live feed an earth
feed and a third connection going to the tank unit, so this looks to be
a different arrangement electrically. Another circuit showed the live
side of the fuel gauge fed via the ignition warning light which appears
to adopt the role of dropper resistor.

It would help if you could tell us which car the gauge and tank unit are
in. There might be someone reading this with personal knowledge of that
vehicle.

I will go along with Dave - it is almost certainly the tank unit at
fault. I can't remember ever seeing a sound tank unit and faulty gauge.
Also don't forget that the tank unit is a glorified ball valve, and if
there is a rust hole in the ball it will sink to the bottom regardless
of the fuel level, so it might be electrically sound but mechanically
faulty, as opposed to the probable electrical fault.

I would also advise against putting full battery voltage across the
gauge. The tank unit typically gives some resistance regardless of
whether it is supposed to indicate full or empty, and you could ruin the
gauge by giving it full battery voltage.

My first port of call would be to put a resistance meter across the tank
unit, and if it shows zero ohms it is suspect.

Jim
Malcolm
2015-12-07 20:50:34 UTC
Permalink
I have had a look at some old circuit diagrams. Many of them have two
connections on the fuel gauge, one side at battery voltage via the
ignition switch and the other side connected to the tank unit which goes
to the chassis earth.

---------------------

There is actually a third connection on old gauges via the case gong to
earth. These gauges were used in the days before voltage regulators were
fitted to the instrument wiring. An unregulated voltage would cause the fuel
gauge to rise and fall as the current through the circuit changed. To
overcome this a "control" coil is connected from battery to earth (via the
case) and the combined effect of the magnetic fields of the control coil and
the coil connected to the tank sender controls the needle position.

As Jim says we need to know what age and type of car you are having the
problem with.

Malcolm
Davey
2015-12-07 23:28:12 UTC
Permalink
On Mon, 7 Dec 2015 20:50:34 -0000
Post by Indy Jess John
I have had a look at some old circuit diagrams. Many of them have two
connections on the fuel gauge, one side at battery voltage via the
ignition switch and the other side connected to the tank unit which
goes to the chassis earth.
---------------------
There is actually a third connection on old gauges via the case gong
to earth. These gauges were used in the days before voltage
regulators were fitted to the instrument wiring. An unregulated
voltage would cause the fuel gauge to rise and fall as the current
through the circuit changed. To overcome this a "control" coil is
connected from battery to earth (via the case) and the combined
effect of the magnetic fields of the control coil and the coil
connected to the tank sender controls the needle position.
As Jim says we need to know what age and type of car you are having
the problem with.
Malcolm
See reply to Malcolm. The car is from 1948, and has a standard Lucas
regulator. And Positive Earth, of course.
--
Davey.
Davey
2015-12-07 23:26:29 UTC
Permalink
On Mon, 07 Dec 2015 19:39:59 +0000
Post by Indy Jess John
Post by Davey
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
They are volt meters, which measure the (tiny) voltage drop across
a shunt resistance, and are calibrated to read that as amps.
True, with the desired final effect.
Post by Dave Plowman (News)
So the tank unit is basically a variable resistor which alters the
voltage to the gauge.
The gauge usually has 12v to it and is in series with the tank unit
which is grounded at one end. If this is the case, ground the other
end and the gauge should read full. It might well move slowly to
maximum if a hot wire type.
So the potentiometer of the sender acts as a voltage divider, the
gauge then reading the proportion of the available max. of 12 volts?
That makes more sense than having a continuous variable current
drain, which is what I thought, from looking at the schematic,
although that may still be the case, but at a lower level than what
I was thinking of.
So if grounding the 'leaving' end of the gauge produces no effect,
the gauge is defective. If it gives a reading, then the fault is
after the gauge.
Post by Dave Plowman (News)
It's far more likely to be a faulty tank unit than the gauge or the wiring.
Agreed. But I can easily get at the gauge, the sender requires more
work!
Thanks.
I have had a look at some old circuit diagrams. Many of them have
two connections on the fuel gauge, one side at battery voltage via
the ignition switch and the other side connected to the tank unit
which goes to the chassis earth. This suggests that the tank unit is
a variable resistor and perhaps a fixed resistor which together vary
the current available to the meter. Unfortunately, none of the
diagrams show the internal wiring of the "Tank Unit" sketch.
They are not all like that though. One circuit had a live feed an
earth feed and a third connection going to the tank unit, so this
looks to be a different arrangement electrically. Another circuit
showed the live side of the fuel gauge fed via the ignition warning
light which appears to adopt the role of dropper resistor.
It would help if you could tell us which car the gauge and tank unit
are in. There might be someone reading this with personal knowledge
of that vehicle.
I will go along with Dave - it is almost certainly the tank unit at
fault. I can't remember ever seeing a sound tank unit and faulty
gauge. Also don't forget that the tank unit is a glorified ball
valve, and if there is a rust hole in the ball it will sink to the
bottom regardless of the fuel level, so it might be electrically
sound but mechanically faulty, as opposed to the probable electrical
fault.
I would also advise against putting full battery voltage across the
gauge. The tank unit typically gives some resistance regardless of
whether it is supposed to indicate full or empty, and you could ruin
the gauge by giving it full battery voltage.
My first port of call would be to put a resistance meter across the
tank unit, and if it shows zero ohms it is suspect.
Jim
It is an HRG 1500, which probably doesn't help much. I have the circuit
diagram, which shows the wiring as described earlier, ie hot to one
connection on the gauge, then from the other connection to the
sender, and from there to earth. I am trying to do what I can with
access only to the dashboard, for now.
I will be removing the cover over the tank, and therefore getting to
the sender, at some later stage anyway, but I am trying to check as much
as possible before I get that far. If I identify that the gauge is
faulty, then I can send it away for repair. If it is not faulty, then I
won't need to do that.
I can ask the Association what a working sender unit resistance range
is likely to be, I'm attempting now to get a picture of what's what. I
have never had to fix a faulty fuel gauge.
--
Davey.
Indy Jess John
2015-12-08 09:14:57 UTC
Permalink
Post by Davey
It is an HRG 1500, which probably doesn't help much. I have the circuit
diagram, which shows the wiring as described earlier, ie hot to one
connection on the gauge, then from the other connection to the
sender, and from there to earth. I am trying to do what I can with
access only to the dashboard, for now.
The HRG 1500 had the Singer 12 mechanicals so it probably has the
similar gauges etc. The Singer wiring diagram shows 2 connectors on the
gauge, one to the ignition and one to the sender.

The obvious first test is to put a volt meter between the "battery"
terminal on the gauge and the chassis, and you should see battery
voltage indicated when the ignition is switched on. If not you have a
wiring or fuse problem.

If you disconnect the wire from the sender from the gauge and connect an
ohm meter to that ire with the other side of the meter to the chassis,
you should see some resistance, not an open circuit and not a dead short.

If you have fuel in the tank, then rocking the car with the meter still
connected should show a variation in the resistance.

If those tests succeed then there is probably a fault in the gauge. If
you don't get the expeted readings from the sender then you have a
wiring problem or a sender problem.

When you can get to the sender, try the ohm meter test again with the
meter connected directly to the sender terminal. If it looks OK then
the wire from the sender is faulty.

Jim
Malcolm
2015-12-08 09:30:43 UTC
Permalink
Post by Davey
It is an HRG 1500, which probably doesn't help much. I have the circuit
diagram, which shows the wiring as described earlier, ie hot to one
connection on the gauge, then from the other connection to the
sender, and from there to earth. I am trying to do what I can with
access only to the dashboard, for now.
The HRG 1500 had the Singer 12 mechanicals so it probably has the
similar gauges etc. The Singer wiring diagram shows 2 connectors on the
gauge, one to the ignition and one to the sender.

The obvious first test is to put a volt meter between the "battery"
terminal on the gauge and the chassis, and you should see battery
voltage indicated when the ignition is switched on. If not you have a
wiring or fuse problem.

If you disconnect the wire from the sender from the gauge and connect an
ohm meter to that ire with the other side of the meter to the chassis,
you should see some resistance, not an open circuit and not a dead short.

If you have fuel in the tank, then rocking the car with the meter still
connected should show a variation in the resistance.

If those tests succeed then there is probably a fault in the gauge. If
you don't get the expeted readings from the sender then you have a
wiring problem or a sender problem.

When you can get to the sender, try the ohm meter test again with the
meter connected directly to the sender terminal. If it looks OK then
the wire from the sender is faulty.

Jim

Beware of one thing, somewhat counter intuitively, if you remove the wire
that goes from the gauge to the tank the gauge should go to full.

There is all you want to know about these gauges at
http://www.mgaguru.com/mgtech/electric/fg_01.htm and following pages. It is
actually about the MGA ones but I am pretty sure yours will be similar if
not the same.

Malcolm
Davey
2015-12-08 11:58:43 UTC
Permalink
On Tue, 8 Dec 2015 09:30:43 -0000
Post by Indy Jess John
Post by Davey
It is an HRG 1500, which probably doesn't help much. I have the
circuit diagram, which shows the wiring as described earlier, ie
hot to one connection on the gauge, then from the other connection
to the sender, and from there to earth. I am trying to do what I
can with access only to the dashboard, for now.
The HRG 1500 had the Singer 12 mechanicals so it probably has the
similar gauges etc. The Singer wiring diagram shows 2 connectors on
the gauge, one to the ignition and one to the sender.
The obvious first test is to put a volt meter between the "battery"
terminal on the gauge and the chassis, and you should see battery
voltage indicated when the ignition is switched on. If not you have a
wiring or fuse problem.
If you disconnect the wire from the sender from the gauge and connect
an ohm meter to that ire with the other side of the meter to the
chassis, you should see some resistance, not an open circuit and not
a dead short.
If you have fuel in the tank, then rocking the car with the meter
still connected should show a variation in the resistance.
If those tests succeed then there is probably a fault in the gauge.
If you don't get the expeted readings from the sender then you have a
wiring problem or a sender problem.
When you can get to the sender, try the ohm meter test again with the
meter connected directly to the sender terminal. If it looks OK then
the wire from the sender is faulty.
Jim
Beware of one thing, somewhat counter intuitively, if you remove the
wire that goes from the gauge to the tank the gauge should go to full.
There is all you want to know about these gauges at
http://www.mgaguru.com/mgtech/electric/fg_01.htm and following
pages. It is actually about the MGA ones but I am pretty sure yours
will be similar if not the same.
Malcolm
Brilliant, just what I need. Thanks.
--
Davey.
Davey
2015-12-10 13:42:25 UTC
Permalink
On Tue, 8 Dec 2015 09:30:43 -0000
Post by Indy Jess John
Post by Davey
It is an HRG 1500, which probably doesn't help much. I have the
circuit diagram, which shows the wiring as described earlier, ie
hot to one connection on the gauge, then from the other connection
to the sender, and from there to earth. I am trying to do what I
can with access only to the dashboard, for now.
The HRG 1500 had the Singer 12 mechanicals so it probably has the
similar gauges etc. The Singer wiring diagram shows 2 connectors on
the gauge, one to the ignition and one to the sender.
The obvious first test is to put a volt meter between the "battery"
terminal on the gauge and the chassis, and you should see battery
voltage indicated when the ignition is switched on. If not you have a
wiring or fuse problem.
If you disconnect the wire from the sender from the gauge and connect
an ohm meter to that ire with the other side of the meter to the
chassis, you should see some resistance, not an open circuit and not
a dead short.
If you have fuel in the tank, then rocking the car with the meter
still connected should show a variation in the resistance.
If those tests succeed then there is probably a fault in the gauge.
If you don't get the expeted readings from the sender then you have a
wiring problem or a sender problem.
When you can get to the sender, try the ohm meter test again with the
meter connected directly to the sender terminal. If it looks OK then
the wire from the sender is faulty.
Jim
Beware of one thing, somewhat counter intuitively, if you remove the
wire that goes from the gauge to the tank the gauge should go to full.
There is all you want to know about these gauges at
http://www.mgaguru.com/mgtech/electric/fg_01.htm and following
pages. It is actually about the MGA ones but I am pretty sure yours
will be similar if not the same.
Malcolm
I just read the MG information, and then compared that with my Lucas
wiring diagram. The MG notes state clearly that the gauge should have
an earth connection, but my wiring diagram does not show that at all.

More fun!
--
Davey.
Dave Plowman (News)
2015-12-10 15:05:48 UTC
Permalink
Post by Davey
I just read the MG information, and then compared that with my Lucas
wiring diagram. The MG notes state clearly that the gauge should have
an earth connection, but my wiring diagram does not show that at all.
More fun!
You really can't go by one maker's wiring diagrams and expect it to be
correct for anothers.
You may get more of a guide by the wiring colours. But on a car that old,
I'm not sure how standard they were.
--
*The only difference between a rut and a grave is the depth.

Dave Plowman ***@davenoise.co.uk London SW
To e-mail, change noise into sound.
Davey
2015-12-10 16:56:18 UTC
Permalink
On Thu, 10 Dec 2015 15:05:48 +0000 (GMT)
Post by Dave Plowman (News)
You really can't go by one maker's wiring diagrams and expect it to be
correct for anothers.
You may get more of a guide by the wiring colours. But on a car that
old, I'm not sure how standard they were.
The implication in the document is that that is how the Lucas gauge
works, which would be true for many, many models, if it is fact.

The car has had a replacement loom, so I should at least be able to
follow wires from A to B easily enough. The might even match the Wiring
Diagram.
--
Davey.
Indy Jess John
2015-12-10 15:19:57 UTC
Permalink
Post by Davey
I just read the MG information, and then compared that with my Lucas
wiring diagram. The MG notes state clearly that the gauge should have
an earth connection, but my wiring diagram does not show that at all.
More fun!
If both terminals on your gauge show resistance to chassis along the
lines of the figures quoted for the MG, you might reasonably assume you
have something similar.

Don't forget you don't always have to have an earth wire. The mounting
clamp for the gauge might be doubling as a conductor.

Jim
Davey
2015-12-10 16:54:12 UTC
Permalink
On Thu, 10 Dec 2015 15:19:57 +0000
Post by Indy Jess John
Don't forget you don't always have to have an earth wire. The
mounting clamp for the gauge might be doubling as a conductor.
True, but the leather-covered wooden dashboard probably doesn't provide
much conduction back to the battery!
--
Davey.
Indy Jess John
2015-12-10 18:51:02 UTC
Permalink
Post by Davey
On Thu, 10 Dec 2015 15:19:57 +0000
Post by Indy Jess John
Don't forget you don't always have to have an earth wire. The
mounting clamp for the gauge might be doubling as a conductor.
True, but the leather-covered wooden dashboard probably doesn't provide
much conduction back to the battery!
You have the advantage over me in being able to view the dashboard. :-)
Davey
2015-12-08 12:01:51 UTC
Permalink
On Tue, 08 Dec 2015 09:14:57 +0000
Post by Indy Jess John
Post by Davey
It is an HRG 1500, which probably doesn't help much. I have the
circuit diagram, which shows the wiring as described earlier, ie
hot to one connection on the gauge, then from the other connection
to the sender, and from there to earth. I am trying to do what I
can with access only to the dashboard, for now.
The HRG 1500 had the Singer 12 mechanicals so it probably has the
similar gauges etc. The Singer wiring diagram shows 2 connectors on
the gauge, one to the ignition and one to the sender.
The obvious first test is to put a volt meter between the "battery"
terminal on the gauge and the chassis, and you should see battery
voltage indicated when the ignition is switched on. If not you have
a wiring or fuse problem.
If you disconnect the wire from the sender from the gauge and connect
an ohm meter to that ire with the other side of the meter to the
chassis, you should see some resistance, not an open circuit and not
a dead short.
If you have fuel in the tank, then rocking the car with the meter
still connected should show a variation in the resistance.
If those tests succeed then there is probably a fault in the gauge.
If you don't get the expeted readings from the sender then you have a
wiring problem or a sender problem.
When you can get to the sender, try the ohm meter test again with the
meter connected directly to the sender terminal. If it looks OK then
the wire from the sender is faulty.
Jim
Yes, I have that pretty much sorted out. I was just hoping to find out
what sort of resistance to expect from the sender unit, and identify
whether the gauge was faulty or not before getting that deep.

By the time I get down to the sender, I will have no problem removing
it if needed, then I will be able to exercise it at will. Having first
covered the hole in the tank, to prevent unwanted fume ignition!
--
Davey.
Dave Plowman (News)
2015-12-08 00:37:17 UTC
Permalink
Post by Indy Jess John
I have had a look at some old circuit diagrams. Many of them have two
connections on the fuel gauge, one side at battery voltage via the
ignition switch and the other side connected to the tank unit which goes
to the chassis earth. This suggests that the tank unit is a variable
resistor and perhaps a fixed resistor which together vary the current
available to the meter. Unfortunately, none of the diagrams show the
internal wiring of the "Tank Unit" sketch.
They are not all like that though. One circuit had a live feed an earth
feed and a third connection going to the tank unit, so this looks to be
a different arrangement electrically. Another circuit showed the live
side of the fuel gauge fed via the ignition warning light which appears
to adopt the role of dropper resistor.
I've a feeling there were quite a few ways of doing it before hotwire
gauges with voltage regulators arrived. Moving coil meters could have a
second winding to compensate for varying battery voltage etc.

I'd agree about not putting full battery volts across the gauge for a long
period. Just flicking it to ground would show if a moving coil type was
ok. It's the later hotwire type which moves slowly.
--
*Am I ambivalent? Well, yes and no.

Dave Plowman ***@davenoise.co.uk London SW
To e-mail, change noise into sound.
Davey
2016-02-18 19:17:02 UTC
Permalink
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
I give you the Lucas Type BM:
http://www.mgexp.com/phorum/read.php?46,3143910

which happens to be what I have for my non-functioning ammeter. It all
looks good, the needle moves freely, it passes current, but there is no
response.
Weird.

The fuel gauge system is now operable, after buying a replacement
sender, as the old one was dead, and supplying an earth connection to
the gauge body. I am in the process of transferring the float and rod
from the old unit to the new unit, not a straightforward process.

Gradually I'm getting there.......
--
Davey.
Dave Plowman (News)
2016-02-19 00:44:17 UTC
Permalink
Post by Davey
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
http://www.mgexp.com/phorum/read.php?46,3143910
Perhaps I should have explained. An ammeter is simply a volt meter which
measures the voltage drop across a series resistor - commonly known as a
shunt.
--
*Why is 'abbreviation' such a long word?

Dave Plowman ***@davenoise.co.uk London SW
To e-mail, change noise into sound.
Davey
2016-02-19 01:02:24 UTC
Permalink
On Fri, 19 Feb 2016 00:44:17 +0000 (GMT)
Post by Dave Plowman (News)
Post by Davey
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
http://www.mgexp.com/phorum/read.php?46,3143910
Perhaps I should have explained. An ammeter is simply a volt meter
which measures the voltage drop across a series resistor - commonly
known as a shunt.
I know what you meant.
But the Lucas BM uses the current flowing through the wire, or
coil, if it's coiled, acting on a magnetised vane to move the needle.
This is not a voltage drop across a shunt, it produces movement of the
needle depending on the magnetic force generated by the current. An
Ammeter by any reckoning.
--
Davey.
Dave Plowman (News)
2016-02-19 13:27:53 UTC
Permalink
Post by Davey
On Fri, 19 Feb 2016 00:44:17 +0000 (GMT)
Post by Dave Plowman (News)
Post by Davey
On Mon, 07 Dec 2015 13:04:58 +0000 (GMT)
Post by Dave Plowman (News)
There is really no such thing as an ammeter.
http://www.mgexp.com/phorum/read.php?46,3143910
Perhaps I should have explained. An ammeter is simply a volt meter
which measures the voltage drop across a series resistor - commonly
known as a shunt.
I know what you meant.
But the Lucas BM uses the current flowing through the wire, or
coil, if it's coiled, acting on a magnetised vane to move the needle.
This is not a voltage drop across a shunt, it produces movement of the
needle depending on the magnetic force generated by the current. An
Ammeter by any reckoning.
Yes - forgotten about those. Lucas trying to find the very cheapest way
of doing things. ;-)
--
*Don't squat with your spurs on *

Dave Plowman ***@davenoise.co.uk London SW
To e-mail, change noise into sound.
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