HOW TO TEST 7-PIN TRAILER PLUG WITH MULTIMETER

How to Test 7-Pin Trailer Plug with Multimeter

One of the most complicated things to test on a car often seems to be the 7-pin trailer plug as it hosts 7 different electrical connections in one plug/wire.

But with the use of a simple multimeter with a black/red probe you can test this easily at home to determine if there is an electrical break within the plug.

What will you need?

We recommend using Innova 3340 for this task

You’ll need the following items in order to test it effectively:

  • 7-Pin Trailer Plug
  • Running Vehicle
  • Multimeter with black/red probes that can test for volts 
  • 2 people – one to run the vehicle and one to run the multimeter
  • Replacement Light Bulbs (Optional)
  • Sandpaper (Optional)
  • Electrical Contact Cleaner (Optional)

The configuration of the 7-pin Trailer Plug

The 7-pin trailer plug is complicated because it hosts 7 different connections all in one (watch the video right below this paragraph).

You may have other types of plugs which have 3, 4, 5 or 6 different connections but for this one, I’ll focus on the most common 7-pin plug.

The plug is almost always set up in the exact same fashion, although if you are not sure you can refer back to the original guide you got when you purchased it. A typical 7-pin plug will be setup in the below fashion:

  • Top right = 12-volt hot lead
  • Middle right = right turn/brake light
  • Bottom right = brake controller output
  • Bottom left = ground
  • Middle left = left turn/brake light
  • Top left = tail and running lights
  • Center = reverse lights

If you are not sure which is which, I will also show you below how you can find out using your multimeter.

How to Test 7-Pin Plug with a Multimeter

To test if any of the wirings in the 7-pin plug is not working you will need to use your digital multimeter and ensure it is capable of testing for volts.

Turn your multimeter to the V symbol on a multimeter dial. 

Next, insert your black lead into the COM port and your red into the voltage port.

  1. When your multimeter is set to test volts, you’re ready to start. You can insert the black probe into the bottom left slot of the 7-pin plug as that is the grounding wire. Insert the red probe into the top right slot of the plug. If you are getting a reading near to 12 volts then it means that electricity is flowing in and out of the plug normally.  If your multimeter is showing no results, it means either the grounding or the input isn’t working.
  2. Now to test each of the lights to determine if any of the wirings for each light isn’t working you will leave the black probe into the grounding slot of the plug. You can then move the red probe into the first light slot – let’s pick the middle right slot in the plug for the right brake light.
  3. Ask the person assisting you to then press on the brake light. If the pin wiring for that is working correctly you should see 12 volts appear on your screen. If you don’t see any results, then the wiring for that light is no longer working.
  4. If it is working, then you can simply move the red probe to the next slot in the plug and test each of the brake lights, blinkers, and reverse lights until you’ve eliminated any of them as issues.

Additional Troubleshooting

If you have a specific light burnt out and you’re not sure what is causing it, testing for continuity is one thing you can do, but there are additional tests you can do that may help restore power to the circuit.

  • Clean the connector plug using an electrical contact cleaner to help ensure the plug is free from dirt and dust. You can use a brush to clean out the inside contact pings to ensure it’s clean which often can cause a disconnect in the circuit.
  • Check the ground wire to see that it’s connected tight enough to the metal frame of the vehicle. If it’s loose or not connected to the metal it may not be getting grounded which can cause your circuit to be broken.
  • Replace the bulbs which are not working as it may only be a faulty bulb and not an issue with the 7-pin plug. This is often the first thing you should do as it most commonly will be a burnt-out bulb.
  • Sand away corrosion on the sockets which can often occur from water getting into the socket. You can use sandpaper by rolling it around a metal rod and connecting it with tape or glue. Then insert it into the socket and grind out any corrosion.

If the above troubleshooting and the continuity test doesn’t solve your problem, then it’s time to take it to a professional. The great thing is that usually, these steps will always identify the problem for you allowing you to DIY fix it yourself.

Suggested Multimeters

There are many great multimeters on the market, but I have my own personal favorites which are perfect for automotive uses:

Innova 3340: This Innova multimeter is one of those all-around good multimeters but specifically it’s great for automotive uses as it can test both volts and amps, plus temperatures, continuity, and much more.

It is also a very affordable multimeter which makes it easy to get for home use (Check price on Amazon).

Fluke 87-V: 

This is one of my favorite all around multimeters which are great for automotive use but also for any HVAC use as it can also read temperatures. 

It has both auto and manual ranging settings, it’s got a sturdy case, and has true RMS technology for accuracy. It also has the Fluke brand name which assures you that it will be a long-lasting good multimeter (Check price on Amazon).

Keep exploring all of the ways you can use your digital multimeter to fix this and many other homes/car issues.

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