In this blog post, we'll discuss some basic ways to utilize a multimeter to test a few common issues on your Subaru. We carry a very inexpensive but useful multimeter that you can purchase to help with your troubleshooting. We use this in all of our examples, so it would make it very easy to follow along if you have one. Our kit includes leads with small tips to help work with tight connectors, which makes things much easier while working on your Subaru. You can purchase it by clicking the link below.
DISCLAIMER! YOU ARE WORKING WITH ELECTRICITY, IT CAN HARM YOU IF YOU ARE NOT CAREFUL.
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What is a multimeter?
A multimeter is a tool for testing wiring. It can be used in many ways, but the most common in an automotive application is to test for power, ground, or contact between wires.
How do I use it?
The multimeter meter will come with two leads called "test leads." One is red and intended to touch the positive or power wire. The other is black and used to touch the negative or ground wire. There are several settings to switch to, but we will focus on just two for this guide.
Key Terms
- DC or Direct Current - The type of electrical output supplied by a vehicle's battery.
- Test Leads - These wires come out of the multimer. They have small metal probes on the end to touch the surface you are looking to measure.
- Volts or Voltage - Ranges from 0-14 volts (or so) for Subaru and described by the letter "V."
- OHM - units used to measure the resistance on a wire.
- Resistance - a force that counters the flow of electricity.
- Resistance = bad for the most part. Sometimes, resistors are added for specific purposes, but we won't worry about that for now.
- the longer the wire, the more likely resistance will increase
- Continuity - a way to talk about points touching.
- Good continuity = low resistance = good
- Bad continuity = high resistance = bad
Important Note About Test Lead Probes!
Whenever possible, you want to insert the probes into the back side of the connector. If the connector is sealed, you will need to go through the front side, but do not jam the probe into the open end of the terminal because you can damage the terminal or bend it so that it no longer makes contact with the mating terminal. Just touch it enough to see it make contact; do not force it.
This leads to something called "terminal tightness." When there isn't terminal tightness, the mating terminals are connected via the housing they are inserted into but do not make enough contact to pass current between them. A very common connector where this happens is on the OBD2 port because scan tools get connected and disconnected, often wearing them out and deforming them. We carry this connector below if you are experiencing this issue.
OHMS Test for Resistance
This is a great way to tell if two points are making contact. You may start here to ensure there isn't a break in a wire. You could also use this to determine which wire touches a certain point or if a ground is good.
- Switch the unit to OHMS by turning the large gray knob from off to the Q looking symbol. Our unit is auto-ranging, so it will adjust the range to find the correct output for the item you are testing. Other meters may not be, so you would have to switch them to a specific setting within the OHMS group to find the correct range.
- Perform a "calibration" by touching the two leads to the same metal spot (bolts are great for this) to determine what good continuity on your multimeter looks like. Ideally, this will be something close to zero, but due to the leads, wire, terminals, etc. You likely will get a number higher than zero, but generally, we are looking for something .05 or less.
- Put one lead (red or black) on the part you want to test and the other lead on the other point you are looking to test
- See what number comes up to determine if and how well they are touching.
Helpful tips!
Many times, wires touch through computer boards, relays, gauge clusters, etc. You want to test for resistance with as many modules unplugged as possible to eliminate seeing continuity through these devices. If you see something with continuity but it's very high resistance, and you don't expect to see it, this may be why.
Finding Continuity Issues
It's worth noting here that wire breaks are extremely rare. We find that most often when a wire is not making continuity, the failure is at a connection point. Always inspect the connectors for damage, loose terminals, terminal tightness, wires pulling out of the terminal, or connector gaskets folding over terminals.
Grounding!
We have a lot of information about grounding in the post below. You would confirm grounding by testing the wire that you want to be grounded to the chassis, the manifold, or both, depending on which wire you are testing.
Measuring Voltage
When measuring voltage, we are looking at the power and ground of the circuit. Remember that the voltage with the key is in the "on" position, but with the engine off, it will not get higher than 12V because that is the maximum voltage of a standard car battery. With the engine running and the alternator doing its job, you may see voltage in the 14-15 volt range. It's also important to note that some interior electronics are regulated due to the fusebox, so with the engine on, you may only see 12 volts, and that's normal.
- Switch the multimeter to "DC Volts." Our meter is auto-ranging so that it will adjust to the conditions. You may need to set this manually if you use a different meter.
- "Calibrate" your multimeter by testing the battery first. Put the red lead on the positive terminal and the black lead on the negative terminal. With the car off, this should read around 12V, but it may be as low as 10V, depending on how charged the battery is. This will give you a sense of what "good" voltage is for your application and meter. You can test this again with the car running to see what that voltage looks like.
- Test the item you want to see voltage for by putting the red lead on the wire you expect to see power on and the black lead on a good, clean ground location. You can also probe a wire that you know to be ground.
Helpful Voltage Testing Tips!
When measuring voltage, you are actually measuring power and ground, so keep that in mind. If you are probing wires for both power and ground (like a fuel pump) and you don't see voltage when doing this, switch over the ground point to a known ground spot like an unpainted bolt and see if that changes the voltage reading.
You can also measure ground this way. If you have access to a power source, you can see if voltage pops up using the ground you are looking to test to see if it's well grounded.
Sidenote on Power Probes
Although it can be a powerful tool, it's fairly expensive and not usually needed for most basic automotive diagnostics. If you have access to one, great, but we also find that using a multimeter is helpful to ensure you have a basic understanding of how things work.
Hopefully, this is a great introduction to using a multimeter, which should be in every tool kit if you are working on a car. Once you get the basics, they aren't scary at all!
