[OT] Inductive clamp

[Skeleton Man]

Hi,

I have a wiring loom with at least 25 wires, and I need to check voltages on
each of them. They all terminiate into a couple of plugs, in which the pin
sockets are far too small to get a mulimeter probe into. It's simply not
practical to cut each one, test, and re-solder (not to mention messy!).

Can you get some kind of inductive clamp that you can clip onto an
individual wire to check voltage ? (even better if it just attaches to a
dmm)

Regards,
Chris

 

[Ken]

Hi,

I have a wiring loom with at least 25 wires, and I need to check voltages on
each of them. They all terminiate into a couple of plugs, in which the pin
sockets are far too small to get a mulimeter probe into. It's simply not
practical to cut each one, test, and re-solder (not to mention messy!).

Can you get some kind of inductive clamp that you can clip onto an
individual wire to check voltage ? (even better if it just attaches to a
dmm)

Regards,
Chris

Chris,

Without knowing what the voltages on these wires might be, I will
suggest the following: Insert a sewing needle into the insulation of
the wires you wish to test and attach your DVM to the needle. Once the
needle is removed, it is very unlikely that any shorting will take place
unless a very high voltage exists on these wires. The insulation simply
closes the hole made by the needle.

 

[kony]

Chris,

Without knowing what the voltages on these wires might be, I will
suggest the following: Insert a sewing needle into the insulation of
the wires you wish to test and attach your DVM to the needle. Once the
needle is removed, it is very unlikely that any shorting will take place
unless a very high voltage exists on these wires. The insulation simply
closes the hole made by the needle.

Agreed, that's a good way to do it, or use the needles to
make contact with the pin sockets that are too small for the
DMM probes, perhaps insulating the needles by slipping a
piece of wire insulation over each up to the last millimeter
or so.

The lazy way of hooking needles up to probes:
http://69.36.189.159/usr_1034/probe_needles.jpg

 

[General Schvantzkoph]

Hi,

I have a wiring loom with at least 25 wires, and I need to check voltages on
each of them. They all terminiate into a couple of plugs, in which the pin
sockets are far too small to get a mulimeter probe into. It's simply not
practical to cut each one, test, and re-solder (not to mention messy!).

Can you get some kind of inductive clamp that you can clip onto an
individual wire to check voltage ? (even better if it just attaches to a
dmm)

Regards,
Chris

You can measure current that way but not voltage.

 

[VWWall]

Agreed, that's a good way to do it, or use the needles to
make contact with the pin sockets that are too small for the
DMM probes, perhaps insulating the needles by slipping a
piece of wire insulation over each up to the last millimeter
or so.

The lazy way of hooking needles up to probes:
http://69.36.189.159/usr_1034/probe_needles.jpg

An even easier way is to get a couple of alligator clips to fit the
probe ends. Just grip the needle with the clip, and you're all set!
They're insulated, so measuring higher voltages is safer.

They're also handy for securing one probe to a "ground" or common when
you're making readings.

Virg Wall

 

[kony]

An even easier way is to get a couple of alligator clips to fit the
probe ends. Just grip the needle with the clip, and you're all set!
They're insulated, so measuring higher voltages is safer.

They're also handy for securing one probe to a "ground" or common when
you're making readings.

You must have better alligator clips than I do, mine allow
the needle to slide around too much if there's any pressure
against it. I don't find I need needles that often though,
last time I had that pair of probes out was to burn a bridge
on an Athlon Thorton, they work pretty well for those
laquer-covered bridge points.

 

[ric]

I have a wiring loom with at least 25 wires, and I need to check voltages on
each of them. They all terminiate into a couple of plugs, in which the pin
sockets are far too small to get a mulimeter probe into. It's simply not
practical to cut each one, test, and re-solder (not to mention messy!).

Can you get some kind of inductive clamp that you can clip onto an
individual wire to check voltage ? (even better if it just attaches to a
dmm)

I have one pair of DVM leads with needle sharp points which will easily
go through wire insulation. These leads come in handy in situations such
as the above.

 

[VWWall]

You can measure current that way but not voltage.

I don't know of any "clamp on" method for measuring DC current. The
ones that measure AC current do so by using the current carrying wire as
the one turn primary of a transformer whose core is clamped around the
wire. These are fairly accurate and inexpensive, but are limited to
power line frequencies. Some DC devices use the Hall effect or the
un-balanced hysteresis of a magnetic core, but all I have seen require
the unbroken current carrying wire to be threaded through the measuring
device. These are expensive and not very accurate. If the air gap of a
clamp around magnetic core could be made small enough, these might work.
Anybody have a reference to such a device? How accurate and what do
they cost?

Virg Wall

 

[Skeleton Man]

Thanks for the suggestions, I was forgetting inductance only works with AC.

The wiring loom in question goes to the speedo/instrument panel, and I'm
trying to work out which wire is for the speedo itself.
(my theory being the voltage varies with the speed). I wanted to avoid
buying a $170 workshop manual just for one diagram.

I'll try the suggestion with the needles.


Regards,
Chris

 

[CBFalconer]

You must have better alligator clips than I do, mine allow
the needle to slide around too much if there's any pressure
against it. I don't find I need needles that often though,
last time I had that pair of probes out was to burn a bridge
on an Athlon Thorton, they work pretty well for those
laquer-covered bridge points.

Or look in your great-aunts attic for an old mechanical 78rpm
wind-up record player, the type with an acoustical horn. Use the
solid steel needles from them - they are relatively thick.
Actually, probes used to come with something quite similar.

 

[CBFalconer]

General Schvantzkoph wrote:
.... snip ...

I don't know of any "clamp on" method for measuring DC current.
The ones that measure AC current do so by using the current
carrying wire as the one turn primary of a transformer whose core
is clamped around the wire. These are fairly accurate and
inexpensive, but are limited to power line frequencies. Some DC
devices use the Hall effect or the un-balanced hysteresis of a
magnetic core, but all I have seen require the unbroken current
carrying wire to be threaded through the measuring device. These
are expensive and not very accurate. If the air gap of a clamp
around magnetic core could be made small enough, these might work.

Anybody have a reference to such a device? How accurate and what
do they cost?

Tektronix used to make a probe for current measuring. Its end
looked roughly like an old fashioned clothes pin, with a closed up
hole of about 1/16 inch (1.5 mm) diameter. IIRC it cost in the
order of USD 200 twenty years or more ago. It could be used to
measure such things as core memory drive pulses.

 

[JS]

I'm trying to work out which wire is for the speedo itself.
(my theory being the voltage varies with the speed).

If you are talking about a late model computer controlled car your theory
is most likely not right. It is more likely that the speed signal is a
digital signal of some kind.

 

[Skeleton Man]

I'm trying to work out which wire is for the speedo itself.

If you are talking about a late model computer controlled car your theory
is most likely not right. It is more likely that the speed signal is a
digital signal of some kind.

Any suggestions on how to tell which wire then ?


Regards,
Chris

 

[JS]

Any suggestions on how to tell which wire then ?

Without a shop manual it may be very difficult. Can you get to the wire
that feeds the speedo itself? The wire inside the instrument cluster
that connect to the speedometer meter? That siganl is probably what your
looking for. The signal that goes directly into the speedometer meter,
the actual movement, should be an analog signal proportional to speed.
If you find it, don't load it down very much or your speedo may read
low.

P.S. Seems like chilton or Haynes should have a $20 electrical repair
manual for your car/truck.

 

[Noozer]

Anything you do with this wire will screw up the signal. What are you trying
to do?

Without a shop manual it may be very difficult. Can you get to the wire
that feeds the speedo itself? The wire inside the instrument cluster
that connect to the speedometer meter? That siganl is probably what your
looking for. The signal that goes directly into the speedometer meter,
the actual movement, should be an analog signal proportional to speed.
If you find it, don't load it down very much or your speedo may read
low.

P.S. Seems like chilton or Haynes should have a $20 electrical repair
manual for your car/truck.

BAHAHA.. Have you seen one of these in the last 10 years that had any useful
information? If a fix requires more than two tools it's deemed "to difficult
for the owner - take it to the shop"... not to mention that it's pretty much
all generic advice now.

 

[Skeleton Man]

Anything you do with this wire will screw up the signal. What are you
trying

to do?

Well I'm actually after the odometer.. but in 90% of cases the same signal
wire drives both odo and speedo..
I'm fitting out a taxi actually, there's a wire that goes from the meter to
the odo signal (so it can increment the fair with every X KM, etc).

Looks like I'll just have to get a $170 manual and be done with it..

Regards,
Chris

 

[Noozer]

Well I'm actually after the odometer.. but in 90% of cases the same signal
wire drives both odo and speedo..
I'm fitting out a taxi actually, there's a wire that goes from the meter to
the odo signal (so it can increment the fair with every X KM, etc).

Many of these use pulses and not analog signals at all. A timer and pulse
counter will get you the speed... Counting just the pulses will get you
distance.