Looking to learn how to test a tachometer with a multimeter?
Fantastic, you are in the right place!
In this ToolsGaloreHQ.com guide, we will show you:
- Testing procedures
- Correct multimeter settings
- Internal tachometer wiring requirements
- And so much more!
Read the table of contents below before we continue with the rest of the guide on how to test a tachometer with a multimeter.
What you need to know about how to test a tachometer with a multimeter
As you know, the tachometer is basically the rev gauge on your car. It typically is a standard feature of most manual and automatic transmission vehicles.
It’s purpose is to provide the driver a feel for when to change gears. These tachometers are also very common on motorbikes as well.
In an automatic transmission vehicle, technically it is not really required.
However because most people are so used to having it there, it serves more as a cosmetic guide rather than anything else.
Tachometers as you can imagine come in so many different shapes and sizes. Some of them include resistors/capacitors. Other have push buttons on the rear to control cylinder modes etc.
Specifying a tachometer is not that hard – but testing one with a multimeter requires a bit of knowledge. That is why this guide exists.
There are a couple of things you need to know first before we delve into testing your tachometer with a multimeter.
How your tachometer is wired to the cylinders
Typically cars have what is referred to as a 4 stroke engine. See the image below.
What this means is that the engine’s pistons have 4 sequential moves that ultimately produce power and revs. These are:
- Intake stroke - This takes in the correct combination of air and fuel.
- Compression stroke - This involves the compression of the fuel/air mixture.
- Power stroke - This stroke is responsible for producing the torque of the engine to get the vehicle moving.
- Exhaust stroke - This stroke is responsible for expelling the exhaust fumes though your exhaust manifold and out via your exhaust pipe.
Typically the tachometer connects to your four cylinders via a control box sensor unit.
Some vehicle manufacturer use a central box to connect the tachometer to all of the cylinders at once.
Be sure to check this to understand how your tachometer connect to cylinders.
Understanding how engine rpm is measured using a tachometer
For a 4 stroke engine you have 1 spark that happen per two strokes. The spark also tends to happen in between the compression and the power stroke.
Therefore you get 2 Revolutions per spark. Now using a multimeter you can measure the Frequency between ground and the tachometer wire (car starter wire).
These two values above can then be combined mathematically to create the RPM value you see on your tachometer.
So basically what is going to happen is that you will get a frequency measurement from your multimeter probe.
This will then be combined with your type of engine to provide you with an estimate for your RPM as seen below.
When working with live automotive parts, it is important to make sure that you operate under very strict safety precautions.
You cannot believe how often people put their hands and fingers close to spinning/hot/live electrical parts.
Whenever measuring your tachometer with a multimeter please exercise caution. If you are not comfortable do not be afraid to contact your local mechanic.
Equipment You’ll need to test purge valve with multimeter
- Multimeter (That can measure frequency) with test leads
Testing Tachometer With A Multimeter ( 4 Step Guide)
Step 1: Setting up the Multimeter
In this first step, switch on your multimeter.
This is followed up by ensuring that you put your multimeter on frequency mode. This is typically shown as a “Hz” value on your multimeters turn dial.
Then what you will do is connect the multimeters’ “black lead” to ground (GND). This is is basically any metallic part that is connected to the cars body.
If you are struggling with this, then your best bet is to connect to the negative lead of the battery.
Once this has been setup correctly, take the “Red Lead” and connect this to one of your tachometer wires on the engine cylinders.
Step 2: Measuring Frequency (Hz)
Turn on your engine and let it idle.
At this stage If your tachometer is operating correctly you should see a Hz rating on your multimeter of between 5 to 30 Hz whilst the engine is idling.
It is great if your multimeter also has functionality to provide min and max values.
I have seen that this is a great feature to be able to measure continuously for different revs. And eventually get a maximum and a minimum frequency value that you can use to compare with your rpm measurement.
Step 3: Converting Frequency to RPM
This is the last step. Once you have a frequency there are a couple of things you need to do before converting this into RPM.
Determine if engine is 4 stroke/2 stroke
This is important because if you have a 4 stroke engine then the formula below would apply.
When using a motorcycle it would typically have a two stroke engine
Compare measured frequency value with tachometer reading
This is typically a visual confirmation. Do not worry to much about getting an exact match, however your RPM should be within range +/- 200 revs.
From what I have seen, if you measurement is out of this range, you probably have a faulty tachometer that might need to be fixed or completely replaced.
Step 4: Test tachometer without running engine
This step is a bonus test. You will need some form of rectifier to provide the signal.
Remove the tachometer completely from the engine housing and detach the electrical wires.
Connect these wires up to your rectifier to create a DC voltage with very high ripples.
Once this is done you can perform the same frequency measurement as you had done in Step 2 above.
Final thoughts on how to test tachometer with a multimeter
As you can see testing a tachometer with a multimeter is not that difficult. All you need is to make sure that you have a good enough multimeter with frequency settings and a min/max hold feature.
Hope this article has helped equip you with the necessary knowledge you would need to test tachometer with a multimeter correctly.