ECU Grounding – the DOs and DONTs

ECU Grounding – the DOs and DONTs

If you follow any online forum where vehicle wiring is discussed it won’t take long before you come across a debate about the best way to ground your ECU. So today we are going to get our nerd on and look at the theory of electronic circuits and help you figure out the optimal grounding solution for your particular wiring application.

That isn't a red warning lamp!

First let’s look at some of the problems we’re trying to avoid, analyse them and figure out how to optimise the ground layout for each of the cases.

Circuit = Closed Loop

The fundamental rule that applies to any electrical circuit is just that – it is a circuit. And by “circuit” we mean it’s a complete loop.

We have one side of the battery that has a power supply, there is a device that we are controlling or a sensor we are reading a value from, and a ground. The ground completes the circuit loop back to the battery.

In the case of an automotive electrical system, the whole vehicle is made of metal and therefore conducts electricity so rather than running 2 wires to every device, we put a wire between the battery negative and the vehicle chassis.

Now the whole vehicle is effectively the return loop that allows current to flow back to the battery.

So Chassis = Ground right?

This is where things can start to get confusing for some people because in our ECU wiring harness we also have “Ground” wires. We have battery ground wires and also separate signal ground wires.

It’s all too easy to look at these wires and say: “OK, the ground is ground, it's just a loop back to the battery” but it’s not that simple. 

While it’s very tempting and quite possible and to connect all these black wires to the chassis just about anywhere it’s definitely not advisable.

The Voltage Drop

Remember that every wire in the system can carry current and every wire has a different resistance. This means you’re going to get a voltage drop across each wire and that voltage drop will vary depending on:
• the length of the wire
• the wire gauge
• the amount of resistance in the wire
• the number of joins in the wire
• the amount of current flowing through the wire

Since we’re talking about this in the context of ECU grounding, let’s look at voltage from the ECU’s perspective and then predict what can go wrong due to different voltage drops.

Bad Example No.1

In this first example, we have grounded the ECU to the engine and also to the battery. 

One of the reasons this happens a lot is because when you see ground straps on both the battery and between the block and the chassis you could be forgiven for thinking: “if one ground is good then two is better!” That, however is not the case.

Let’s take a look a the problems this can cause:

During cranking, a lot of current flows through the ground strap between the engine and the battery, so there’s a voltage drop between the engine and the battery. 

When you have multiple ground wires that connect between the same 2 points, the current is shared between the two alternate ground paths. In other words, the ECU shares some of the starter motor current. 

Exactly how much of that current is shared depends on the relative resistances between the ECU ground and the ground strap. If the ground strap is in good condition then not a lot of the current is shared, but when this ground strap is not in good condition or you forgot to tighten that bolt or painted the block underneath reducing the contact efficiency then all that starter motor current now gets carried by these other wires. Wires which were never designed to run a starter motor!

Bad Example No.2

Another similar but different example we often see is where the sensor ground is externally grounded. 

Because the ECU is already grounded through the power ground wire we are creating parallel ground paths again and a similar thing occurs to the previous situation with the starter motor – but in a less extreme way. 

When injector duty cycle increases, the average ground current of the ECU will also increase, and therefore so will the voltage drop between the ECU and the battery.

Creating an alternate ground path for the sensors will have a different voltage drop and offset the output reading of any sensor that connected to this alternate ground path. The result is erratic sensor readings which can play havoc with the way your ECU adjusts your engine tune.

Bad Example No.3

In this last example, the car has coil-on-plug ignition, the coils are grounded to the engine and the ECU is grounded to the battery.

As the engine speed increases, the alternator charge current increases and the voltage drop between the engine and the battery increases.

Let’s assume the grounding to the ECU from the battery is also substandard, which means that as injector duty cycle increases, the voltage drop between the ECU and the battery also increases. 

We now have a double effect causing the ECU ground to sit higher than the engine ground. The coils however are grounded to the engine which means when the ECU is outputting zero Volts on its ignition output, the coil sees a positive voltage (equal to the voltage drop described earlier) on its input. 

Some coils with built-in ignitors only need 0.7V to trigger, which means that in extreme cases you could even get the coils to trigger by themselves and that’s really bad! No engine likes random ignition coil firing!

In all the examples above the problem is common impedance paths. The solution for this is star point grounding.

Star Point Grounding

How does it work? You pick a single point for your ground and reference all grounds to there. It doesn’t really matter whether this point is the engine or the chassis or the battery negative, but there are other factors which you need to be aware of.

For your sensors you should only use a signal ground that is supplied from the ECU. That’s why this wire exists so don’t be lazy and just run a wire from the sensor looped around to the engine.

There are circumstances where a sensor’s ground is not isolated from the body of the sensor and it has to screw onto the engine. For example some cam angle sensors, narrowband oxygen sensors and many knock sensors ground through the body of the sensor.

In this case you have a choice – you could replace the sensor with one that has a separate ground wire (away from the sensor body) or you could choose the star point grounding location to be at the engine. 

What’s the best location for star point grounding?

We usually recommend the cylinder head because it minimises the likelyhood of a voltage drop causing random ignition events. It also allows for those sensors that ground through the body of the sensor to be used in their native form.

If you have any questions shoot us an email on

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