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what happens if ECU is not earthed? 

 

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 Post subject: what happens if ECU is not earthed?
Posted: Thu Mar 11, 2010 11:29 pm 
Tyre Shredder
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hey all, was just laying in bed thinking about issues with my car, and realised i havnt earthed the ecu since my engine swap, just been sitting out of its mount, just wondering if that effects the ecu at all, such as would it lose memory due to not being earthed etc??
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Thu Mar 11, 2010 11:42 pm 
Getting Side Ways
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i dont think the ECU case actually has a current running through it mate.
i think it should have one wire that needs to be earthed but not the case

 

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 Post subject: Re: what happens if ECU is not earthed?
Posted: Thu Mar 11, 2010 11:43 pm 
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sorry, thats what i meant, i havnt hooked the earth wire back up since i swapped engines and put the J3 chip in.
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Thu Mar 11, 2010 11:49 pm 
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that will not be helping you. it needs to be earthed

 

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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 12:09 am 
Getting Side Ways
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lol yeah connect it up
what was the car doing without it?
like how did it run?

 

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in the shed - .595" lift cam, GT4202, forged AU engine, ems 4424 ecu, glide & 5500 stall - next-bellhousing

XH ute daily - DOHC BA XR6 engine, T5, AU3 ECU, J3 chip, wideband & logger, ported BBM, 3" intake

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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 11:16 am 
Parts Gopher
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While it will probably still run OK without it, it won't be running the best.
There are lots of signal grounds the ECU can use to sink current back to earth but there will be issues with voltage drops and stray induced currents in signal paths.
The injectors are high current, and negatively switched and will probably cause the most issues (injectors are inductors and have back-EMF). EF/AU EDIS systems will be worse, I'd be suprised if they idled properly.

The ECU will be confused by the pulses and variations in ground potential to some degree.
It will learn around some of it and compensate but the major effect will be increased fuel consuption as it loses accurate insight into some of its inputs.
Factory earthing is borderline anyways and the underbonnet wire gauge +12v to injectors/TPS/coil is poor.
It's hard to measure the gains but I try to uprate the +12v and earths anyways esp running decent earths direct to the dash (the green screw under the top dash trim) and ECU.

Also in theory the radiator is worth earthing with a small strap - it floats off-earth otherwise and is probably subjected to electrochemical potentials, increasing corrosion.

Anyhow, chuck an earth on that ECU :)
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 1:07 pm 
Tyre Shredder
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main problems im having atm are idling issues, hunting, stalling when put into gear etc. ill disconnect the battery for an hour tonight to reset the ecu and earth it all back up properly. Ill let you know how it all goes. thanks for the info guys!
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 1:21 pm 
Getting Side Ways
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check for vacuum leaks first mate

 

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in the shed - .595" lift cam, GT4202, forged AU engine, ems 4424 ecu, glide & 5500 stall - next-bellhousing

XH ute daily - DOHC BA XR6 engine, T5, AU3 ECU, J3 chip, wideband & logger, ported BBM, 3" intake

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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 1:24 pm 
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the ecu must be earthing throught the case or something else, because if it was not earthed at all it wont run...
but the ecu earthing through the case is not ideal... earth the wire asap...
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 3:32 pm 
Parts Gopher
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lowie57 wrote:
main problems im having atm are idling issues, hunting, stalling when put into gear etc. ill disconnect the battery for an hour tonight to reset the ecu and earth it all back up properly. Ill let you know how it all goes. thanks for the info guys!


NP, wouldn't suprise me if it's behaving oddly - while it will still run without the case ground connected, the "ground" it's seeing will be all over the shop due to the injector pulses, ISC, etc all affecting the negative reference.
An ECU reset is a great idea as it's probably learned wrong info (HEGO, TPS etc).
This will clear the KAM and you'll be able to get a clean set of diags off it.
As mentioned, vacuum is also one to check - inlet manifold, fuel pressure reg hose etc.
Spray some (SOME) butane (upside down camping gas can) thru a squirt straw around the inlet manifold gasket at idle, if the revs flare a bit you've got a leak near where you sprayed last.
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 3:49 pm 
Tyre Shredder
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the case has been sitting on the carpet on the floor of the car.. so not sure how it would be gettings its earth :?
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 5:08 pm 
Parts Gopher
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Haha yeah that won't be so good, you won't even get case ground back through the bracket!

It'll be using signal grounds for return. There's a few of them, IIRC the DOL, the knock sensor, and probably a signal earth reference back through near the ignition and/or BEM.
Due to the currents flowing the ECU will probably be seeing a very erratic earth potential, probably pulsing at a volt or two above ground :(

Interesting to see how it goes for ya..
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 6:05 pm 
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The case earth is a "safety earth" to make sure there is no potential difference across the actual case that could short out something or leak voltage caused by induced currents generated by the ECU itself or otherwise.

 

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 Post subject: Re: what happens if ECU is not earthed?
Posted: Fri Mar 12, 2010 11:57 pm 
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so i reset the ecu and wired it all back up, started the car and it seemed to idle OK, but as soon as i put my foot on the pedal it stalled with a splutter... then wouldnt start again, and if it did it would put put put and stall, also water and black smoke were coming out the exhaust.. guess its done a head gasket.... ehhhh so over all the head aches with this car!! lol
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 Post subject: Re: what happens if ECU is not earthed?
Posted: Sat Mar 13, 2010 4:41 am 
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lowie57 wrote:
so i reset the ecu and wired it all back up, started the car and it seemed to idle OK, but as soon as i put my foot on the pedal it stalled with a splutter... then wouldnt start again, and if it did it would put put put and stall, also water and black smoke were coming out the exhaust.. guess its done a head gasket.... ehhhh so over all the head aches with this car!! lol



Did the replacement EL xr6 engine that you fitted come with a warranty or a cyclinder compression test. Looks like you have swapped over a s**t engine with another s**t engine?

 

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