Meters

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Submitted by T on Thu, 26/01/2006 - 12:43.
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Click to Enlargen. Image by T

==Dwell, Tacho, RPM, Amps, Volts Test Meters:==

===Cautions:===Here is some info on dial settings and lead placement. Note that an incorrect setting or lead position can blow you meter. Always start with the range set to the highest and only go smaller later.
That is, set the Voltage range to the maximum to take the first reading, only adjust it down to the next lowest range when you are sure the meter can handle it.
e.g You want to measure the battery voltage on a 12 volt car. You use the DC Volts range. Your meter has 60 volt, 30 volt and 10 volt ranges. Set the meter to 60 volts. Connect the meter up, red lead to battery positive, black lead to battery negative. The reading comes up as about 11 volts.
Change the range to 30 volts and you can now see 11.57 volts more clearly.
Don't use the 10 volt range because the 11.57 volts are likely to blow the meter.

 

Volt, Ohm, Dwell, Tach, Ammeter. Photo by HRAmbo. Click to Enlargen.

Volt, Ohm, Dwell, Tach, Ammeter. Photo by HRAmbo. Click to Enlargen.

 

==Lead Connections and Meter Settings:==

===Tacho:===The tachometer counts the number of pulses that appear on the coil's negative terminal and displays it as RPM.
It's important to match the number of cylinders in the engine to the setting of your meter.
====Tacho Connection:====Black lead to Ground (the car's chassis or clean metal on the engine), Red lead to Coil Negative.
Set the switch to the largest RPM you think you'll see and with the proper number of cylinders selected.
i.e 6,000 RPM max for a V8, select V8 and a range that can display 6000 rpm or more.
===Dwell:===Dwell is a sophisticated way of setting the time that the Distributor's Breaker Points are open and closed. For a standard Breaker Point Ignition system, the open/closed time should be equal. In other words the points should be closed for the same amount of time they are open.
This is called a 50% duty cycle because the points are only on for half the time.
====Dwell Connections:====Black to Ground, Red to Coil Negative. Set the switch to the number of cylinders of the engine.
i.e V8, select 8, 6 select 6.
Find the dwell angle in the engine's repair manual. As a rough rule of thumb it's one half the cam firing angle.
====Four Cylinder Dwell Angle:====360 degrees divided by 4 equals 90 degress. 90 divided by 2 equals 45 degrees. The ideal dwell angle for a 4 will be 45 degrees.
This only a guide. Each engine will have specific numbers.
====Six Cylinder Dwell Angle:====360 degrees divided by 6 equals 60 degress. 60 divided by 2 equals 30 degree. The ideal dwell angle for a 6 will be 30 degrees.
This only a guide. Each engine will have specific numbers.
====Eight Cylinder Dwell Angle:====For a V8, 360 divided by 8 equals 45 degrees. 45 divided by 2 equals 22.5 degrees.
The ideal dwell angle for an 8 will be 22.5 degrees.
This is only a guide. Each engine will have specific numbers.
===Points Resistance:===There is only one setting for this. Red to coil negative, black to ground.
===Volts:===You want to measure the battery voltage on a 12 volt car.
You must use the DC Volts settings.
Your meter has 60 volt, 30 volt and 10 volt ranges. Set the meter to 60 volts.
Connect the meter up, red lead to battery positive, black lead to battery negative.
The reading comes up as about 11 volts.
Change the range to 30 volts and you can now see 11.57 volts more clearly.
Don't use the 10 volt range because the 11.57 volts are likely to blow the meter.

You connect the Meter red lead to Battery +ve and black Lead to Battery -ve.
===Amps:===You must use the DC Amps settings.
Be careful with this one. This is for measuring the current flowing through a wire. Do not connect the Meter directly across the Battery when Amps mode is selected or the Meter will be destroyed. The Amps Range is used to measure how much Current flows through an active device like a Wiper Motor or a Headlight. In Amps Range the Meter is connected in series with the device under test. 

Once again set the switch to a scale that can accept the amount of current you expect to see. If 10 amps, select a range that will read 10 amps or more.
Be aware that these meters don't handle starter current which will more than likely blow the meter apart if you try to measure it.
The amps range is OK for wiper motors and headlights but nothing big.
===Ammeters and Shunts:===There is more than likely a *shunt* that came with the meter. A shunt is a metal bar with electrical connections on it. The shunt takes the bulk of the current and only passes a small amount to the meter.
e.g. Wiper Motor current. To connect the shunt, connect the positive lead of the shunt to the wiper motor's supply wire, and the negative of the shunt to the wiper motor. There will be two small connections on the shunt. A small positive connection and a small negative connection. The small positive wire goes to the meter's positive terminal. The meter must be set to amps on the range high enough to display the output without being destroyed. The small negative connects to the meter's negative side.

===Ohms:===When the Meter is set to Ohms it will tell you the resistance of an item under test.

When the Meter reads 0 Ohms, the item has no resistance.

When the Meter reads Infinity Ohms, the item has infinite resistance.

===Terms:===

Terms
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