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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

  • Having a ground wire run to all the points would cost more in wire than multiple short wires. – NitrusInc Mar 28 '18 at 15:37
  • Redundancy, costs, preventing devices from lifting the ground, easy access ground point for other installations, grounding of chassis parts themself, noise, there's a multitude of reasons for it. It's bad design if devices are dependent on a chassis ground point itself though. Those easily corrode. It should always also contact a (shared)wire running to the negative side of the battery/alternator. – Bart Mar 28 '18 at 20:24
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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.

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    Very good answer. I’d also add that with the advancement of sensors and troubleshooting/diagnostic systems, multiple grounds provide a more robust diagnostic system. It is possible to have a single shared ground, but that would mean (as you noted) single point of failure, and the computer would only be able to supply 1 ground fault code (e.g. “General Ground Fault”), not for any single sub-system... not a very helpful code. – kyle_engineer Mar 28 '18 at 17:33
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    The shared single ground is the metal body / chassis / frame of the car... – Solar Mike Mar 28 '18 at 18:26
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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.

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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?

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