Hi I am stuck on this duralogic Punto 2008 duralogic, It will not crank, and I have one code in the gearbox P0720 -62 vehicle speed. it will not clear and come back on without driving the car, it is a 2 wire sensor. can anyone tell what voltage i should have with the sensor unplugged, I have 1.3 volts on one and 3.2 volts on the other, I was expecting say 5 volt feed and earth?.

I am still not convinced this would cause a no crank condition.


Paul

Re: P0720 no crank

I checked the starter today, that is all working fine, I am not convinceed this code will cause a no crank situation I have found that I have no power to pin 30 on the starter relay, That is powered by relay K 79 ignition circuits relay, I cant find this relay anywhere, I am now told by Autodata it built in to the smart fuse board x28-1, under the bonnet, I have looked at the E learn cd but I cant find anything there, Does anyone have any info on this please.


Paul

Re: P0720 no crank

Hi Paul,

the starter relay is controlled by the TCU (Transmission Control Unit) which controls various parameters before giving its clearance to start the engine. Should one of those parameters not being ok, the TCU wouldn't allow the starter to crank the engine ...

Vehicule speed is one of the paramaters that are checked by the TCU, so if it misses this information, it won't allow the starter relay activation.

BRs, Bernie

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Hi Bernie, Thank you again, you are always there to help and I for certainly appreciate it, But here is that relay?. I pulled the plug on the sensor, I have 1.6 volts on one wire 3.4 volts on the other, That seems strange to me, I would have expected say 5 volts and 0 volts, I would think it is a hall effect sensor with a square wave, I will ohms test the sensor tomorrow. I pulled the plug off
the panda and it still cranked, but it was hard get my hand down the back , the Punto is easy, so the other thing i will do is put multiscan on the panda and find if I have pulled the correct plug off,

Thanks Paul

Hi Paul,

you get the chance to have a non covid19-infected patient to compare with the sick one (sorry it was easy); do you read the same voltage on the cranking-ok Panda ?

Regarding the sensor itself it is probably, as you said, a Hall effect sensor, though they usually have three wires, some manufacturers (Melexis) do have two wires detectors. But I couldn't find any information on what kind of sensor Fiat use for vehocule speed.

BTW, on 1.2 8v, it is located on the differential housing.

BRs, Bernie

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Hi Bernie, Very good, Just a quick one as I have a lot to do today, I just put the scanner on the Pada, and the code came up was P0715 clutch disc RPM, On the punto I think that comes from the crank shaft sensor, Maybe this panda has another sensor at the back of the gearbox as well as the speed sensor. It has a perameter on the diagnostic for a speed sensor.


Paul

Just checked the sensor and another one I have One is 1.6 K ohm and one is 1.1 K ohms.

How can I find a spec on this sensor?.

Paul

Hi I hope everyone is safe and well, Back to this Punto. The History of this car, this was a non runner auction car, It will not crank. I have one fault P0720 Speed sensor fault in the gearbox ecu.

The sensor is good and the wiring is good

The starter is fine if I jump a live from battery positive to terminal 87 on the starter relay it cranks fine. The problem is Relay K79 is not coming in, And I think that is because of the fault P0720

so the wiring if fine and i have tried 2 other gearbox ecu's Now the thing that I am concerned about is pre 2008 the ignition circuits relay is on the main fuse board and this later one K79 I think is in the multifuction control module. so have i got an ecu from an earlier car in it. I know the other 2 ecu's I have tried are from earlier cars, and i get the same fault and no crank Even if the ecu's are the same the software might be different. can anyone out some light on this.

Stay safe every one

Paul

paullap said:

...it is a 2 wire sensor. can anyone tell what voltage i should have with the sensor unplugged, I have 1.3 volts on one and 3.2 volts on the other, I was expecting say 5 volt feed and earth?

Click to expand...

Sensors:
Two wire sensor = 99% it's an induction (reluctance) sensor. Coil on magnet. You can measure resistance (about 1k Ohm seems reasonable, but 1,1-1,6k range might be too much, one of them is off).
Three wire, can also be classic inductive, third wire it's just an earth (screen, shield).
Three wire inductive with electronics inside (amplifier, pulse conditioning circuit). Not easy (or impossible) to just measure resistance.
And three wire hall effect (typical or special with internal magnet - when there's no rotating magnet on the object, like gear etc.).

Classic magnet+coil sensors are "floating" most of the time. That means voltages are "weird" like that (it's not supply and zero), it simplifies a circuit, such signal (initial voltage bias + sensor sine wave) can be fed directly (or almost) to the digital part (frequency counter). It can be different values or the same on both wires.
Unfortunately I don't know what's the correct value for that particular car.

Search for similar threads ("automatic" tranny + no cranking)...https://www.fiatforum.com/grande-punto/479784-grande-punto-will-not-start.html