How to use an automotive test light (tespen)

Clip the lead to a good ground, touch the probe to the circuit, and look at the bulb. Light on means power is there, light off means it isn't. In Indonesia this tool is called a tespen, and it's the cheapest way to answer the first question in any electrical fault: is power getting this far?

A 12V test light won't give you a number. It gives you a fast yes or no, and for fuses, bulbs and dead accessories that's often all you need.

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What a 12V test light is

It's a probe with a lamp in the handle and a lead with an alligator clip. When the clip and the probe sit on opposite sides of a voltage, current flows through the lamp and it lights.

Traditional automotive test lamps use a small incandescent bulb designed to run on about 12 volts (Wikipedia). A trained eye can guess roughly how much voltage is present from the brightness, but Wikipedia calls that a crude indicator. They're built for low-voltage DC. Wikipedia warns that touching one to household mains will destroy the lamp and can short the tester.

Incandescent vs LED and computer-safe lights

The type you buy decides what you can safely probe. The difference is how much current the light pulls through the circuit.

An incandescent light puts a real load on the circuit. Members of HP Academy's forum like that, because a bulb that glows dim under load can expose a bad connection that a meter or LED would miss. Garage Journal owners say the same thing, preferring a bulb to "load the circuit." The downside is that some electronic circuits can be damaged by that extra current (HP Academy forum).

An LED light reacts faster. One HP Academy member says a bulb is too slow to show a roughly 2 millisecond injector or ignition pulse, so he prefers LEDs for quick signal checks on fuel injection wiring. That doesn't automatically make it safe for computers. Innova's 3410 uses an LED, and its instruction sheet still says "DO NOT USE THIS TOOL ON COMPUTER CIRCUITRY."

A computer-safe light is one built and labeled for low draw. Lisle's 24550 is sold as safe for automotive computers, with a listed draw of under 21 milliamps at 12 volts (Home Depot). A Garage Journal owner uses the OTC 3633 mini for computer circuits and puts its draw at about 0.25 amps. That's an owner's figure, not a spec.

Powered probes like the Power Probe III are a different tool again. They can send battery power into a circuit to run a motor or light. A Garage Journal owner says one can dump around 10 amps into a circuit before its breaker trips, and another warns that one wrong touch on a module can ruin it.

How to use a test light, step by step

Check the light first, then test power, ground and fuses.

Prove the light works. Clip it to one battery terminal and touch the probe to the other (Rick's Free Auto Repair Advice). Wikipedia notes it's customary to check a tester on a known live circuit before and after testing an unknown one. A dead bulb looks exactly like a dead circuit.

Test for power. Clip the lead to the battery negative or a clean chassis ground. Touch the probe to the wire or terminal. The light comes on when voltage is present (Innova 3410 instructions).

Test a ground. Reverse it. Clip the lead to the battery positive and touch the probe to the ground point. Rick's Free Auto Repair Advice says a good ground lights the bulb as brightly as it does across the two battery terminals. A dim glow points to a poor ground.

Test a fuse. Turn the key to RUN, because most fuses have no power with the key out. Clip to ground and touch the probe to the two small test points on top of the fuse. Lit on both sides means the fuse is good. Lit on only one side means it's blown (Rick's Free Auto Repair Advice).

A fuse that's dark on both sides isn't proof of anything except that no power reaches that fuse. Check the key position, then the circuit feeding the fuse box.

When to use a multimeter instead

Use a meter when you need a number or when you're near electronics. A test light tells you power is present. It can't tell you if it's 12.6 volts or 10.

That matters on sensor circuits, charging problems and voltage drop tests. A digital multimeter with 10 megohm input impedance barely loads the circuit. Fluke says its 88V's 10 megohm input "won't damage computer circuits," and Innova sells its 3320 meter as safe for the ECU for the same reason.

The test light still earns a place next to the meter. A Garage Journal owner points out that a light shows whether a circuit can actually carry current, which a voltage reading alone won't show. A corroded wire can read full voltage on a meter and still fail to light a bulb.

Safety around airbags and ECUs

Never put a test light on airbag wiring. Innova's instruction sheet says never to use the tool on airbag or safety restraint systems. An ASE study guide warns that a test light or ohmmeter can send enough energy through the squib wires to deploy an airbag.

Airbag connectors are almost always yellow, according to Rick's Free Auto Repair Advice. If you have to work near them, disconnect the battery and wait. The ASE guide says the wait is commonly 1 to 3 minutes and up to 10 on some vehicles. Rick's suggests at least 15 to 30 minutes. Your service manual has the right number for your car, and waiting longer costs nothing.

For engine computers and body modules, use a computer-safe light or a 10 megohm meter. Don't back-probe airbag connectors at all (ASE study guide). Innova also says not to test or puncture spark plug wires with a test light, and to watch for hot engine parts.

What a test light costs

A basic light costs under $20. Powered probes and pro meters cost far more.

For most home mechanics, one cheap bulb light plus a computer-safe light or a basic 10 megohm meter covers nearly everything.

Tools worth a look

• GearWrench 129D circuit tester: a plain bulb tester for fuses, bulbs and loaded ground checks. It isn't sold as computer safe, so keep it off ECU wiring.
• Lisle 24550 computer safe circuit tester: low draw for module and sensor feeds. Low draw also means it won't load a circuit enough to show a weak connection.
• Innova 3320 multimeter: an affordable 10 megohm meter for real voltage readings. Its 10 amp current range needs a 15-minute rest after each 15-second test (Innova).

How to do it

1. Check the light on the battery before every job, and again after any test that showed nothing.
2. Clip to a clean ground, turn the key to RUN, and probe both sides of each suspect fuse.
3. Test the ground at the part that doesn't work by clipping to battery positive and comparing brightness with a direct battery connection.
4. Switch to a computer-safe light or a 10 megohm meter before touching anything that runs to a module or sensor.
5. Leave yellow airbag connectors alone, and follow the service manual if you have to disconnect anything near them.

Sources

• Wikipedia: Test light
• Rick's Free Auto Repair Advice: How to use a test light
• Rick's Free Auto Repair Advice: How to diagnose electrical issues with an automotive test light
• Rick's Free Auto Repair Advice: How to test an airbag safely after a trouble code
• Innova 3410 Test Light instruction sheet (PDF)
• Open Exam Prep: ASE G1 body electrical accessories and SRS precautions
• HP Academy forum: Test light LED
• Garage Journal forum: 12V circuit tester
• Fluke: 88V Deluxe Automotive Multimeter
• Innova: 3320 Auto-Ranging Digital Multimeter
• Home Depot: Innova Combination Test Light EPI3410
• Home Depot: Lisle Computer Safe Circuit Tester
• AM Industrial: GearWrench 129D Low Voltage Circuit Tester
• Lordco: GearWrench 6 and 12 Volt Low Voltage Circuit Tester
• Walmart: Power Probe III clamshell, red

Featured image: Close view of a car fuse box showing various fuses and wires in a vehicle's engine compartment by Shixart1985, CC BY 2.0. Source