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How to use a MULTIMETER to diagnose electrical problems.
(By Fabio Dougie)


Here's some info that I've collated on how to use a Multimeter to help diagnose electrical problems with your scooter (Please be aware that I originally listed this information for my own use/reference and would not accept any liability caused by its use by others)

How to use a Multimeter
A multimeter can be used on a scooter to check for voltages, resistance or continuity of electrical components. It'll let you check to see if there is voltage present on a circuit, etc.



An Overview of a Multmeter
Here's an overview of an analogue meter but the principles are very similar for a digital meter
Display Dial.
This has arc shaped scales visible through the window. The pointer indicates values read from the scale.
Pointer
This is the thin black line at the left-most position in the dial face window in the image. The needle moves to the value measured.
Selector switch or knob.
This allows changing the function (volts, ohms, amps) and scale (x1, x10, etc.) of the meter. Many functions have multiple ranges. It is important to have both set correctly, otherwise serious damage to the meter or harm to the operator may result. Most analogue meters employ the knob type, but there are others. Regardless of the type, they work similarly. The meter should be set to off when stored.
Jacks/Leads
Most meters have several Jacks or openings in the case to insert test leads. One is usually labeled "COM" or (-) for common and negative. This is where the black test lead is connected. It will be used for nearly every measurement taken. The other jack(s) is labeled "V (+)" and (an upside down horseshoe) for Volts and Ohms respectively and positive. The + and - symbols represent the polarity of probes. Consult the meter manual if unsure which jacks should be used. There should be 2 test leads or probes. Generally, one is black and the other red.



Using the Meter


A] Use the Ohm Function to Measure Resistance
Firstly.... resistance/ohms should only be checked on a non powered (disconnected) circuit.

1)Set the multimeter to Ohms or Resistance. Understand that resistance and continuity are opposites. The multimeter measures resistance in ohms, it can not measure continuity. When there is little resistance there is a great deal of continuity. Conversely, when there is a great deal of resistance, there is little continuity. With this in mind, when we measure resistance we can make assumptions about continuity based on the resistance values measured. Observe the meter indication. If the test leads are not in contact with anything, the needle or pointer of an analog meter will be resting at the left most position. This is represents an infinite amount of resistance, or an "open circuit"; it is also safe to say there is the no continuity, or path between the black and red probes. Careful inspection of the dial should reveal the OHM scale. It is usually the top-most scale and has values that are highest on the left of the dial (a sideways "8" for infinity) and gradually reduce to 0 on the right. This is opposite of the other scales; they have the lowest values on the left and increase going right.
2)Connect the black test lead to the jack marked "Common" or "-"
3)Connect the red test lead to the jack marked with the Omega (Ohm symbol) or letter "R" near it.
4)Set the range (if provided).
5)Hold the probes at the end of the test leads together. The meter pointer should move fully to the right to prove continuity.
6)With the circuit you are trying to test, touch the black probe/lead to one point in the circuit/cable/connector and the red to the other, the reading you get from the meter will either indicate resistance or continuity.





B] Use the Volts Function to Measure Voltage
1)Set the meter for the highest range provided for DC Volts. Many times, the voltage to be measured has a value that is unknown. For this reason, the highest range possible is selected so that the meter circuitry and movement will not be damaged by voltage greater than expected. Start high, and work downward to the lowest range that can be safely displayed.
2)Insert the black probe in the "COM" or "-" jack.
3)Insert the red probe in the "V" or "+" jack.
4)Locate the Voltage scales. There may be several Volt scales with different maximum values. The range chosen the selector knob determines which voltage scale to read. The maximum value scale should coincide with selector knob ranges. The voltage scales, unlike the Ohm scales, are linear. The scale is accurate anywhere along its length.
5)Whenever possible, try to connect at least one probe in such a way that it will not be required to hold both while making tests. Some meters have accessories that include alligator clips or other types of clamps that will assist doing this. Minimizing your contact with electrical circuits drastically reduces that chances of sustaining burns or injury.
6) With the circuit you are testing touch the black and red to the connectors/unit to see the voltage in between the two points.

C]How to test Ampage using a Multimeter
1) Set the function selector to read amps, this function setting measures current in amps.
2) Set the range so the maximum is well above the highest reading expected. This will protect the meter from getting overloaded. If you overshoot the range and cannot get a reading, set it lower. Most meters have 2 ranges of measurement...a)mA which is normally good to 300mA, and b)A which is usually good for up to 10 amps.
3) Connect the leads to the proper terminals for measuring current. (Most meters will require the red lead to be moved to a different input socket when going from mA to A in addition to rotating the dial)
4) Break into the circuit to measure its current. Set the red probe on one open end and the black probe on the other. This may require cutting a wire to get the probes between the circulating current. The idea is to let the current flow through the digital multimeter to allow it to get a reading. Start with A settings, if the readings are too small then switch to mA.


The "Wiggle" Test
A useful test that may show up bad connections/bad earths. A wiggle test is carried out by attaching the meter to the circuit and obtaining a reading. Then wiggle the cables and connectors to see if the reading fluctuates, if it does then it indicates bad/loose connections and you can then investigate these further.





Testing a Battery Relay

Steps for checking the battery relay....
1) Find it by tracing the lead from the positive terminal of the battery (one will connect to the relay, the other to the ignition switch).
2) Disconnect the battery lead and starter motor lead connectors from the no. 30 and no. 87 terminals on the relay. With the ignition switch in the ON position, press the starter switch - the relay should click. if it doesn't click, take it out and bench-test it, as follows:

With multimeter on ohms x1 scale, connect it across the relay's 30 and 87 terminals (battery lead and starter motor). using a fully charged 12 volt battery and two insulated jumper wires, connect across the no. 85 and 86 terminals on the relay. at this point you should hear the relay click, and the multimeter should read 0 ohms. if this happens, then the relay is fine.... if it doesn't then the relay is faulty.


Sample Vespa Battery Readings

12.5v-----------Engine turned off
11-12v---------Engine turned off but with headlights (etc) on
12 - 13.5V ----Engine Idling
13.5 - 14.4V --Engine Revving at WOT with partially charged battery
14.4 - 15v-----Engine Revving at WOT with fully charged battery
13.2-13.6V----Battery being trickle-charged at C/20 (C being the capacity in Ah)
14.6V-----------Battery Fully Charged while on trickle-charger as above
13.2V-----------Battery fully charged at rest after a few hours




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Last Updated Sat Apr 02, 2011 3:06 pm
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