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Discussion Starter #1
Hello All,

I've joined up to see if you can help me with my dads corsa. its a corsa C (i think) the reg is Y-reg its a 1.2 basic model

i've tried a search for a similar fault but i cant seem to find one

The problem is basically it wont start from cold - turns over fine but just wont fire up - if you bump start it it'll fire up almost strait away then once its warm even after 20 secs from cold it'll start from the key

ive tried a new battery - still no good :(

and also tried a good earth to the ecu and it makes no difference:(

the ecu light somethimes comes on and ive read the code and it comes up crank sensor but it's not long since had one (for a different fault) - code was cleared after that!
but could this be the crank sensor???? i thought if it was the crank sensor it'd either work or not? and what relevance would the engine being cold have??

i've thought about swapping the starter motor but i just cant see it being that (could it) - kinda clutching at straws now tho lmao

any replies or ideas appreciated :):beer:
 

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right, it does seem a weak crank sensor or signal, what you have to remember is a crank sensor is an inductive pick up coil, which means it picks up the influence of a magnetic field being manipulated by the crank toothed wheel and this generates a small ac signal voltage, this voltage is dependend on the speed of the crank toothed wheel and the strength of the magnet within the sensor head, and the distance it is loacted from the toothed wheel cogs, on the other hand magnets can get weak over a period of time when subjected to very hot temeperatures, hence why sensors can becone weak rather than just work one minute and not at all the next minute.

Another way to look at an inductive sensor is that it is like a small electrical generator, the faster the toothed crank wheel spins past it, the more the voltage it will produce, the lower the speed lesser the voltage, on cranking it can produve very small voltage perhaphs just beloe the border line (threshold level for the ECU to recoganise) and so from a few hundred milivolts to as much as 100Volts at higher RPM!

(Obviously don't get confused, the voltage is a small ac signal which is conditioned to extract square wave pulses from it, representing the cogs on the crank wheel which is what the ecu needs and some sensors have built in conditioning amplifiers within the sensor heads , while others use a conditioning amplifier external to the sensor, inside the acrual ECU) if the incoming signal has become very weak then this cannot be conditioned at very low voltages.)


on a cold engine, there are greater frictional losses, as the oil is cold so the turning resitance is greater, than it would be on a hot engine, so when you crank the engine when it is cold the signal frm the crank sensor is just under the threshold for the ECU to see and or condition that signal into usefull pulses. Conversely on a hot engine it cranks just that wee bit faster and produces a slightly stronger signal just enough for the ecu to recoganise and firse the engine away.

Hence why when you push start the car, the engine spins faster than the starter motor would spin it when it is cold, and so it starts, yes I would actually strongly suspect its crank sensor or it may have not been installed fully home or may even be loose and helped itself dismount slightly from its rest position, so either try to take it out, clean it around its seating area, and then push it back all the way and tighten and see if this cures the problem, if failing that your only other choice would be to get a new one.

Oh and welcome to the Mig web
 

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This has been happening to a lot, and many of them is down to the starter motor (due to the cold freezing weather)

Give it a tap with a hammer see if that does the trick mate.
Either that or what is mentioned above.


Regards.
 

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remove the clocks put some foam in to pad out the plug for the instruments so its a better connection and all should be fine common fault on earlyish c's use to do loads when i was a vauxhall good way to test when cranking car push hard on instruments and you will probably find it will start cos you are making a better connection failing that run an earth from ecu on side of engine to battery earth that was another 1 hope this helps.
 

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Discussion Starter #7
thought i'd report back, i know it's been a long time but today i swapped the crank sensor with another corsa and it started fine:cool::cool:

so big thanks MSG:beer::beer:

i've ordered a genuine one :)
 

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You are welcome mate, sure a new sensor should solve your problem and give you trouble free motoring for future.
 

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If any of you are having problems with starting cold I've sorted it.. My brother has a 1.2 Corsa D Limited Edition, never started when cold. Changed camshaft sensor and crankshaft sensor still didnt start. Realised the ECU was saying the car is hot all the time the reason why it did that is because the thermostat temperature sensor was stuck open so telling car it was hot even though it was -1.. changed tt sensor and started straight away.. haven't had a fault with it since..
 
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