Why are multiple grounds in the engine bay/engine important?

Question

On certain cars (perhaps even most? all?), such as the old Toyota MR2 turbo, there are multiple ground points. Some of them are described with pictures here: http://www.mr2oc.com/59-general-mk2-faq-newbie-center/408099-engine-bay-ground-pics.html

I naively thought that these are for redundancy, but after failing to connect one of them after performing some work, I found that the car would not start.

Why are there multiple ground points, and what could cause the car to fail to start? In the case of the failure to start, the missing ground was for the ground on the intake manifold.

I've even heard in a video giving an overview of the E153 transmission that failure to connect the ground on the transmission (why does the transmission have its own ground? Is it not physically connected with conductive metal to the engine?) can cause a fire. Is there any merit to this claim?

This is the video: https://youtu.be/bbgY7hygM0c?t=31s

Answer 1

I don't know if the correct way to word this would be to say that there are "separate circuits" but essentially it helps keep the power clean, and reduce noise. Given the amount of computers and delicate sensors, you want as clean as possible. By allowing for multiple grounds closest to each crucial "noise maker" if you will, you're able to ensure cleaner power.

Electricity want's to take the shortest possible path. While everything is essentially in the same circuit, having grounds in different locations mean nothing is "fighting for a clean ground".

The other aspect of this is that it's easier to run a wire individually than to have to run a wire to each grounding location. It would be less neat, cost more in wire, and have an aspect of completely disrupting service as everything would lose ground at the same time.

Answer 2

Just to expand on the accepted answer a bit: imagine that you have a sensor, say a temperature sensor, and something power-hungry like the headlamps, both sharing the same ground.

All ground connections have a resistance, whether large or small. If our ground point has a resistance of 0.1 ohms, which is not impossible given a corroded terminal bolted into a corroded engine block, and the headlights are drawing 10 amps, then that will make our ground connection a full VOLT above "true ground". Assuming our temperature sensor measures linearly over, say, -50 to 150c (real temperature sensors aren't linear, but it makes the example easier to understand) then that 1 volt drop could cause our temperature reading to be incorrect by (200/5=)40c, which is a huge amount!

It's also worth saying that even small fluctuations in the ground can induce noise in sensor readings.

Generally, car manufacturers are most concerned about keeping the engine management grounds separate from all the other ones, since engine management components are the most susceptible to this sort of interference. If you look on your cylinder head, you will most likely find a dedicated ground for the ECU and fuel injection components, giving the sensors their own ground path directly back to the ECU.

Answer 3

Follow the money.

My background is working at an original equipment manufacturer (OEM) in Detroit as an automotive design engineer and program manager. Electrical wiring is expensive. While there are some technical requirements (avoid ground loops, accessibility, performance and radiated energy) often the driving question is:

Will the manufacturer save more money (wire length) with multiple grounds or a reduced number of grounds?

Hint: Generally less # of grounds = longer wiring runs —> more expense.

Don’t mean to discount the other answers, but ask yourself, which is more likely a reason for an answer to the question:

Why’d they do that?

Answer 4

As others said, having multiple smaller grounds provides redundancy, likely costs less than a single large one, and tends to lower overall electrical noise.

However, another consideration I haven't seen mentioned is that there's a point of diminishing returns when increasing the size of engine to chassis grounds. No matter how thick the wire is, current flow will still be limited by the very small area of thin sheet metal its ring terminal is bolted to. So by using multiple grounds to spread the load over more of the relatively non conductive steel chassis instead of concentrating the load on one small area, the expensive wire is better utilized.

As for whether forgetting to connect a ground could be a fire hazard, it certainly could under the right conditions. If there isn't a good electrical connection between the engine and the chassis, it's likely that current (which could easily be several hundred amps) will take unintended paths such as through other significantly undersized wiring or even something like a throttle cable. If a large current gets drawn through such a high resistance path, that unintended conductor is going to get extremely hot and could melt/ignite itself or surrounding combustibles.

I've personally seen a car with a broken engine to chassis ground strap that wouldn't run unless my scanner was plugged in and it somehow drew so much current through its OBD port and my scanner that it melted the OBD port and its wiring, so dangerous things can definitely happen if the engine to chassis grounds are compromised.