What does OL mean on a multimeter?

OL means over limit — the resistance is higher than the meter can measure, so there is no path for current. It is an open circuit, not a short. A short shows a near-zero reading where you expected a high one.

What is the difference between 1 ohm and 1 megohm?

One ohm is effectively a joined wire and current passes freely. One megohm is a million ohms — for vehicle purposes that is not connected at all, and only leakage current can pass.

How much voltage drop is acceptable?

Under 50 mV across a connection is as good as no loss. Up to 200 mV is acceptable on a heavy cable, 0.2 to 0.5 V is suspect, and anything over 0.5 V is a fault to repair before testing further.

What fuse size do I need?

Work out the load current, add 25 to 50 percent headroom, then fit the next standard fuse up — provided the cable can safely carry that current. The fuse protects the cable, not the device.

How many milliamps is a normal parasitic drain?

A sleeping modern vehicle should draw under 30 mA, with up to 50 mA acceptable on a car full of modules. Above 80 mA is a drain worth chasing through the fuse box.

Reading the Meter — What 1 Ohm, 1 MΩ, mV and mA Actually Mean

Resistance — how big is big?

One ohm and one million ohms are both 'a reading', but they mean completely different things. Work down this ladder and you can judge any resistance figure in seconds.

The ohms ladder

Left to right: what the meter shows, what it means in plain words, and what it usually is on a vehicle.

Measurement	Plain meaning	On a vehicle	Verdict	What it means
0.00 – 0.50 Ω				Current can pass freely. On a high-current circuit even this much can matter — always confirm with a voltage drop test under load rather than trusting ohms alone.
0.5 – 5 Ω				Fine for a small coil or bulb filament. On a battery cable, earth strap or starter feed it is a fault — a few ohms will collapse the voltage as soon as current flows.
5 – 100 Ω				This is component territory, not wiring territory. Judge it against the manufacturer figure — a winding a few ohms low is shorted turns, a few ohms high is a corroded internal joint.
100 Ω – 10 kΩ				Most of these change with temperature or position, so a single reading proves nothing. Sweep the sensor or heat it and watch the number move smoothly — a jump or a dropout is the fault.
10 kΩ – 1 MΩ				If you read this between two wires that should be isolated, you have found a leak. Water and salt across a connector typically show tens to hundreds of kilohms and cause odd, intermittent faults.
Above 1 MΩ, or OL				OL means the resistance is higher than the meter can measure — effectively infinite. On a wire that should be continuous this is a break. On two wires that should be isolated from each other, OL is exactly what you want to see.

0.00 – 0.50 Ω
Plain meaning
On a vehicle
Verdict
Current can pass freely. On a high-current circuit even this much can matter — always confirm with a voltage drop test under load rather than trusting ohms alone.
0.5 – 5 Ω
Plain meaning
On a vehicle
Verdict
Fine for a small coil or bulb filament. On a battery cable, earth strap or starter feed it is a fault — a few ohms will collapse the voltage as soon as current flows.
5 – 100 Ω
Plain meaning
On a vehicle
Verdict
This is component territory, not wiring territory. Judge it against the manufacturer figure — a winding a few ohms low is shorted turns, a few ohms high is a corroded internal joint.
100 Ω – 10 kΩ
Plain meaning
On a vehicle
Verdict
Most of these change with temperature or position, so a single reading proves nothing. Sweep the sensor or heat it and watch the number move smoothly — a jump or a dropout is the fault.
10 kΩ – 1 MΩ
Plain meaning
On a vehicle
Verdict
If you read this between two wires that should be isolated, you have found a leak. Water and salt across a connector typically show tens to hundreds of kilohms and cause odd, intermittent faults.
Above 1 MΩ, or OL
Plain meaning
On a vehicle
Verdict
OL means the resistance is higher than the meter can measure — effectively infinite. On a wire that should be continuous this is a break. On two wires that should be isolated from each other, OL is exactly what you want to see.

Ohms are measured with the circuit dead and, ideally, the component unplugged from the rest of the loom — otherwise you are measuring everything else in parallel too.

The units, and what the meter shows

Same value, three different ways of writing it. Autorange switches for you — read the letter, not just the digits.

Measurement	Equals	Typical display	Where you see it	What it means
mΩ (milliohm)				Below the resolution of most hand-held meters. If you need this level, test voltage drop under load instead — that is the same measurement done properly.
Ω (ohm)				The working range for most components. Always subtract your lead resistance.
kΩ (kilohm)				A reading of 2.35 kΩ is 2 350 Ω — nearly two hundred times an injector. Mistaking kΩ for Ω is the classic misdiagnosis.
MΩ (megohm)				Anything reading in megohms is, for vehicle purposes, not connected. Treat it as an open unless you are deliberately testing insulation.

mΩ (milliohm)
Equals
Typical display
Where you see it
Below the resolution of most hand-held meters. If you need this level, test voltage drop under load instead — that is the same measurement done properly.
Ω (ohm)
Equals
Typical display
Where you see it
The working range for most components. Always subtract your lead resistance.
kΩ (kilohm)
Equals
Typical display
Where you see it
A reading of 2.35 kΩ is 2 350 Ω — nearly two hundred times an injector. Mistaking kΩ for Ω is the classic misdiagnosis.
MΩ (megohm)
Equals
Typical display
Where you see it
Anything reading in megohms is, for vehicle purposes, not connected. Treat it as an open unless you are deliberately testing insulation.

OL is not a short — the one to get right

These two get mixed up constantly, and they are opposites.

Measurement	What the meter says	What is physically happening	Typical cause	What it means
Open circuit (OL)				No continuity. The circuit is missing a piece. Nothing works, and there is usually no blown fuse because no current ever flowed.
Short circuit				Too much continuity. Current bypasses the load, so the fuse blows or the component runs constantly. Short to earth blows fuses; short to power makes things run on their own.
High resistance				The most common real-world fault and the hardest to spot. It passes a continuity beep and fails the moment load is applied — find it with a voltage drop test, not an ohmmeter.

Open circuit (OL)
What the meter says
What is physically happening
Typical cause
No continuity. The circuit is missing a piece. Nothing works, and there is usually no blown fuse because no current ever flowed.
Short circuit
What the meter says
What is physically happening
Typical cause
Too much continuity. Current bypasses the load, so the fuse blows or the component runs constantly. Short to earth blows fuses; short to power makes things run on their own.
High resistance
What the meter says
What is physically happening
Typical cause
The most common real-world fault and the hardest to spot. It passes a continuity beep and fails the moment load is applied — find it with a voltage drop test, not an ohmmeter.

Quick sanity check on any ohms reading: touch your two probes together first. Whatever that shows (usually 0.1–0.4 Ω) is your leads, and must be subtracted from every reading.

Voltage — how big is big?

Volts are pressure. A reading only means something once you know where in the circuit you took it and whether current was flowing at the time.

The volts ladder — voltage drop across a connection

Measured across a wire, joint or connection while the circuit is working. This is how you find bad connections.

Measurement	Plain meaning	Verdict	Action	What it means
0 – 50 mV (0.00 – 0.05 V)				The connection is doing its job. Any voltage lost here is negligible.
50 – 200 mV (0.05 – 0.2 V)				Normal across a long high-current cable or a starter feed. On a signal wire or a small earth it is already too much.
0.2 – 0.5 V				Something in that section is resisting. Half a volt lost on a 12 V circuit is 4% of your supply gone as heat.
Above 0.5 V				The connection is the fault. Expect dim lights, slow crank, odd module behaviour or a component that works cold and dies hot.

0 – 50 mV (0.00 – 0.05 V)
Plain meaning
Verdict
Action
The connection is doing its job. Any voltage lost here is negligible.
50 – 200 mV (0.05 – 0.2 V)
Plain meaning
Verdict
Action
Normal across a long high-current cable or a starter feed. On a signal wire or a small earth it is already too much.
0.2 – 0.5 V
Plain meaning
Verdict
Action
Something in that section is resisting. Half a volt lost on a 12 V circuit is 4% of your supply gone as heat.
Above 0.5 V
Plain meaning
Verdict
Action
The connection is the fault. Expect dim lights, slow crank, odd module behaviour or a component that works cold and dies hot.

Voltage drop only exists while current flows. Test with the circuit loaded — lights on, engine cranking, motor running — or you will read 0 V on a completely broken joint.

Landmark voltages to judge against

The numbers experienced technicians carry in their head.

Measurement	Expected	Meaning	If it is wrong	What it means
Battery rested				Every 0.2 V below 12.6 V is roughly a quarter of the charge gone.
Cranking floor				Measured at the battery posts while cranking, on a warm engine.
Charging				Smart-charge vehicles legitimately vary between 12.5 V and 15 V — read live data before condemning.
Sensor reference				Test at the sensor plug, not at the ECU, so the wiring is included.
Narrowband lambda				It is the switching that matters, not any single value.
Millivolts (mV)				If the meter shows mV, the number on screen is thousandths of a volt.

Battery rested
Expected
Meaning
If it is wrong
Every 0.2 V below 12.6 V is roughly a quarter of the charge gone.
Cranking floor
Expected
Meaning
If it is wrong
Measured at the battery posts while cranking, on a warm engine.
Charging
Expected
Meaning
If it is wrong
Smart-charge vehicles legitimately vary between 12.5 V and 15 V — read live data before condemning.
Sensor reference
Expected
Meaning
If it is wrong
Test at the sensor plug, not at the ECU, so the wiring is included.
Narrowband lambda
Expected
Meaning
If it is wrong
It is the switching that matters, not any single value.
Millivolts (mV)
Expected
Meaning
If it is wrong
If the meter shows mV, the number on screen is thousandths of a volt.

Ghost voltage: a wire lying next to a live one can show several volts on a high-impedance meter with nothing actually connected. If the reading collapses the moment you apply a small load, it was never real.

Current — how big is big?

Amps are flow. What counts as a lot depends entirely on the circuit — 50 mA overnight is a fault, 300 A for two seconds while cranking is normal.

The amps ladder

Measurement	Plain meaning	Typical source	Verdict	What it means
0 – 30 mA				A modern vehicle asleep should sit here. Allow up to 50 mA on a car full of modules, and always wait for it to go to sleep first — often 20 to 40 minutes.
30 – 80 mA				Will flatten an average battery in a week or two. Find it by pulling fuses one at a time and watching the clamp or meter.
Above 80 mA				Half an amp will flatten a healthy battery overnight. Work through the fuse box until the reading drops.
0.5 – 10 A				Compare against the fuse rating — a circuit sitting near its fuse value is running hot.
10 – 40 A				Double the expected figure here usually means a motor with worn brushes or a seizing bearing.
80 – 300 A				Roughly 80–150 A for a small petrol, 150–250 A for a large petrol or small diesel, up to 350 A on a big diesel. High current with slow cranking is a dragging starter or engine.

0 – 30 mA
Plain meaning
Typical source
Verdict
A modern vehicle asleep should sit here. Allow up to 50 mA on a car full of modules, and always wait for it to go to sleep first — often 20 to 40 minutes.
30 – 80 mA
Plain meaning
Typical source
Verdict
Will flatten an average battery in a week or two. Find it by pulling fuses one at a time and watching the clamp or meter.
Above 80 mA
Plain meaning
Typical source
Verdict
Half an amp will flatten a healthy battery overnight. Work through the fuse box until the reading drops.
0.5 – 10 A
Plain meaning
Typical source
Verdict
Compare against the fuse rating — a circuit sitting near its fuse value is running hot.
10 – 40 A
Plain meaning
Typical source
Verdict
Double the expected figure here usually means a motor with worn brushes or a seizing bearing.
80 – 300 A
Plain meaning
Typical source
Verdict
Roughly 80–150 A for a small petrol, 150–250 A for a large petrol or small diesel, up to 350 A on a big diesel. High current with slow cranking is a dragging starter or engine.

Never put a hand-held meter in series with a starter or any heavy circuit. The amps sockets are fused at 10 A (often 400 mA on the low range) — over that you blow the internal fuse or the meter. Use a clamp for anything above a few amps.

Reading the current units

Measurement	Equals	Where used	Watch out for	What it means
µA (microamp)				Only specialist meters resolve this. Rarely needed on a workshop job.
mA (milliamp)				This is the range for drain testing. Make sure the meter is in the mA socket, not the 10 A socket.
A (amp)				Above 10 A, switch to a clamp meter rather than breaking into the circuit.

µA (microamp)
Equals
Where used
Watch out for
Only specialist meters resolve this. Rarely needed on a workshop job.
mA (milliamp)
Equals
Where used
Watch out for
This is the range for drain testing. Make sure the meter is in the mA socket, not the 10 A socket.
A (amp)
Equals
Where used
Watch out for
Above 10 A, switch to a clamp meter rather than breaking into the circuit.

Continuity, diode and capacitance

The other positions on the dial, and what a good and bad reading looks like on each.

The rest of the dial

Measurement	Good reading	Bad reading	What it proves	What it means
Continuity (beep)				The beeper is deliberately generous. A joint hanging on by two strands still beeps happily and still fails under load. Use it to find a break, never to prove a connection is good.
Diode test				Used on alternator diodes, relay suppression diodes and injector protection diodes. A shorted diode is what puts AC ripple on the charging system.
Capacitance				nF is thousandths of a µF. Discharge the capacitor before testing, and expect the reading to take a couple of seconds to settle.
AC volts on a DC system				A DC system should have almost no AC on it. AC ripple upsets modules and produces faults that make no sense anywhere else.

Continuity (beep)
Good reading
Bad reading
What it proves
The beeper is deliberately generous. A joint hanging on by two strands still beeps happily and still fails under load. Use it to find a break, never to prove a connection is good.
Diode test
Good reading
Bad reading
What it proves
Used on alternator diodes, relay suppression diodes and injector protection diodes. A shorted diode is what puts AC ripple on the charging system.
Capacitance
Good reading
Bad reading
What it proves
nF is thousandths of a µF. Discharge the capacitor before testing, and expect the reading to take a couple of seconds to settle.
AC volts on a DC system
Good reading
Bad reading
What it proves
A DC system should have almost no AC on it. AC ripple upsets modules and produces faults that make no sense anywhere else.

Fuses and cable — sizing it properly

The fuse protects the cable, not the device. Size the cable for the load, then size the fuse to protect that cable.

Choosing a fuse for a known load

Work out the load current (watts ÷ volts), add 25–50% headroom, then take the next standard fuse up — provided the cable can carry it.

Measurement	Load current	Fuse to fit	Minimum cable	What it means
Up to 2 A (e.g. 20 W lamp)				Small signal and lighting circuits. Do not go below 3 A or vibration and inrush will nuisance-blow it.
3 – 5 A (e.g. 60 W)				Radio, interior lighting, small solenoids.
6 – 10 A (e.g. 120 W)				Headlamp pair, fuel pump, wiper motor.
11 – 15 A				Heated seats, small blower, auxiliary sockets.
16 – 25 A				Blower motor, heated rear screen, radiator fan on low.
26 – 40 A				Cooling fan on high, winch feeds, inverter supplies.
Above 60 A				Alternator output and main battery feeds. Fit the protection as close to the battery as possible.

Up to 2 A (e.g. 20 W lamp)
Load current
Fuse to fit
Minimum cable
Small signal and lighting circuits. Do not go below 3 A or vibration and inrush will nuisance-blow it.
3 – 5 A (e.g. 60 W)
Load current
Fuse to fit
Minimum cable
Radio, interior lighting, small solenoids.
6 – 10 A (e.g. 120 W)
Load current
Fuse to fit
Minimum cable
Headlamp pair, fuel pump, wiper motor.
11 – 15 A
Load current
Fuse to fit
Minimum cable
Heated seats, small blower, auxiliary sockets.
16 – 25 A
Load current
Fuse to fit
Minimum cable
Blower motor, heated rear screen, radiator fan on low.
26 – 40 A
Load current
Fuse to fit
Minimum cable
Cooling fan on high, winch feeds, inverter supplies.
Above 60 A
Load current
Fuse to fit
Minimum cable
Alternator output and main battery feeds. Fit the protection as close to the battery as possible.

Never fit a bigger fuse to stop it blowing. A fuse blows because the circuit is drawing more than it should — upsizing it just moves the failure point to the cable, and that is how looms burn.

Blade fuse colours and ratings

Standard colour coding across mini, standard (ATO) and maxi blade fuses.

Measurement	Rating	Typical use	Body	What it means
Grey				Lowest common blade rating.
Violet
Pink
Tan / beige
Brown
Red
Blue
Yellow
Clear / white
Green
Orange (maxi)
Red (maxi)
Blue (maxi)

Grey
Rating
Typical use
Body
Lowest common blade rating.
Violet
Rating
Typical use
Body
Pink
Rating
Typical use
Body
Tan / beige
Rating
Typical use
Body
Brown
Rating
Typical use
Body
Red
Rating
Typical use
Body
Blue
Rating
Typical use
Body
Yellow
Rating
Typical use
Body
Clear / white
Rating
Typical use
Body
Green
Rating
Typical use
Body
Orange (maxi)
Rating
Typical use
Body
Red (maxi)
Rating
Typical use
Body
Blue (maxi)
Rating
Typical use
Body

Colours are a convention, not a guarantee — always read the number stamped on the top before fitting.

Cable size against safe current

Continuous current for automotive single-core cable at ordinary underbonnet temperatures, on short runs.

Measurement	AWG	Safe continuous current	Typical use	What it means
0.5 mm²
0.75 mm²
1.0 mm²
1.5 mm²
2.0 mm²
3.0 mm²
6.0 mm²
16 mm²
25 mm² and up

0.5 mm²
AWG
Safe continuous current
Typical use
0.75 mm²
AWG
Safe continuous current
Typical use
1.0 mm²
AWG
Safe continuous current
Typical use
1.5 mm²
AWG
Safe continuous current
Typical use
2.0 mm²
AWG
Safe continuous current
Typical use
3.0 mm²
AWG
Safe continuous current
Typical use
6.0 mm²
AWG
Safe continuous current
Typical use
16 mm²
AWG
Safe continuous current
Typical use
25 mm² and up
AWG
Safe continuous current
Typical use

Long runs need a size up: keep total voltage drop under 3% of supply, which is about 0.36 V on a 12 V circuit.

Frequency, duty cycle and dwell

The modes people skip. Worth knowing, because a lot of modern actuators are controlled this way.

What the other modes read

Measurement	Typical range	Meaning	If it looks wrong	What it means
Digital MAF frequency				Snap the throttle and the figure should rise and fall cleanly.
Wheel speed sensor				Spin the wheel by hand and watch for a steady climb.
Fan or purge valve duty				Duty cycle is how modules control power smoothly with a simple on/off output.
Throttle actuator PWM				Read duty, not voltage — an average voltage reading on a PWM signal tells you very little.
Injector on-time (ms)				Measured with a scope or a meter with a pulse-width function.

Digital MAF frequency
Typical range
Meaning
If it looks wrong
Snap the throttle and the figure should rise and fall cleanly.
Wheel speed sensor
Typical range
Meaning
If it looks wrong
Spin the wheel by hand and watch for a steady climb.
Fan or purge valve duty
Typical range
Meaning
If it looks wrong
Duty cycle is how modules control power smoothly with a simple on/off output.
Throttle actuator PWM
Typical range
Meaning
If it looks wrong
Read duty, not voltage — an average voltage reading on a PWM signal tells you very little.
Injector on-time (ms)
Typical range
Meaning
If it looks wrong
Measured with a scope or a meter with a pulse-width function.

The meter says that. Is it good or bad?