2 hours ago, martinf64 said:

CANBUS is a differential signal bus system and as such doesn't need to be referenced to the vehicle ground

The supplies do have to be referenced, sending a signal with no common reference would not be likely to work.

59 minutes ago, unofix said:

The supplies do have to be referenced, sending a signal with no common reference would not be likely to work.

No they don't as long as the common mode voltage between the two grounds are within limits. CANBUS uses the voltage difference between the CANL and CANH to determine the logic state, logic 0 or logic 1, that's why it's called a differential system, it doesn't care what the voltage with respect to ground is as long as its within certain limits. It will be very unlikely that the voltage between the two system grounds will be outside this limit especially if one system is low voltage battery self-powered. The only advantage linking the two system grounds gives is improved noise immunity but a differential signalling system gives excellent noise immunity anyway...that is why it is used in both vehicle and industrial applications.

"Do I need an alarm?"

OK, when was the last time you heard a car alarm blaring, and your reaction was:

a) Oh, I must inform the Police, someone is tampering with that person's car...

OR, was it...

b) FFS, where the hell is that owner so they can turn that bl**dy noise off...

7 hours ago, JImpster said:

which robbing swine quoted 300 for a ball joint ???????

 

Local garage we use to check car over on Saturdays check it was safe to drive 

im going to ask my usual guy to look at it when car back here for Christmas 

4 hours ago, StephenFord said:

"Do I need an alarm?"

OK, when was the last time you heard a car alarm blaring, and your reaction was:

a) Oh, I must inform the Police, someone is tampering with that person's car...

OR, was it...

b) FFS, where the hell is that owner so they can turn that bl**dy noise off...

I know ..I had those thoughts myself ...

 

I just want to feel my car is secure As I can make it . ..I was so lucky it was recovered... I won’t be that lucky again 

7 hours ago, Jonro2009 said:

Most likely a Fraud Stealer, I mean Ford Dealer. 

 

@Jo100001my apologies, it's a bad habit to just assume gender. 

No apologies needed ...Most of these car are owned by guys ,.easy assumption to make .... 😀😀😀😀

16 hours ago, unofix said:

Just been thinking this OBDII port movement idea over, and conclude there maybe a better option.

Pins 4 & 5 are the vehicle ground (negative) connection, and pin 16 is the constant 12V positive.

If the negative and positive wires going to the OBDII port were interrupted at some convenient point (out of easy sight) and a double pole switch fitted, and say positioned inside the glovebox as an example, then the port would be effectively dead.

Any device that plugged in to the OBDII port would be unable to draw power from the vehicle if the switch was in the off position. Even if the potential thief's device had its own power source it would still be unable to connect because the Canbus system of the vehicle would not be sharing the same power source.

 

Are you saying that pin 16 is always on even when the car is locked @unofix  and does this apply to the Mk4 ? 

 

3 hours ago, Jo100001 said:

I just want to feel my car is secure As I can make it . .

It's been mentioned before on this thread, but a good old fashioned DiskLok will help you not only keep your car, but also give you piece of mind...(You'll need a size, 'small')

9 hours ago, StephenFord said:

b) FFS, where the hell is that owner so they can turn that bl**dy noise off...

One of my neighbours seemingly cba to fix their remote fob...the alarm goes off every time they unlock the car. 😒

Hello @Wino yes Pin 16 is a constant 12V on all OBDII ports, all models, all manufactures. To all intensive purposes pin 16 is a direct connection to the 12V positive battery terminal.

There are quite a number of car battery chargers that are available that plug directly in to the OBDII port and recharge the car battery using Pin 16 positive and pins 4&5 negative.

Any device that is pluged in to the OBDII port can draw (or supply) power via those pins. Since it is reasonable to expect any device used by a potential car thief to require power in order to operate then my thoughts would be to disconnect pins 16+ and 4&5 - by using a hidden switch.

There is a difference of opinion as to if the thief's device had its own power supply (which is not very likely)  whether it would be able to communicate successfully with the cars systems. My personal stand on that is it would not able to without the use of a common ground. It is something that can be tested and when the summer comes around I may well investigate just for my own curiosity.

Thanks for the reply @unofix . That's more or less what I'd read regarding pin 16. 

I found that during the days when I was able to charge the car via the solar panel plugged into the OBD port that it seemed to only charge if the car was unlocked which is fine during work breaks but not when I go back into work so it left me doubting as to why it wouldnt charge if the panel was left plugged in all day. 

Before going off and disconnecting grounds on the OBM port a few words of warning...if you disconnect the ground and the common mode voltage between the vehicle and external device is above the rated CM voltage for the bus driver/receiver transceiver there is a chance the transceivers are damaged either on the vehicle or within the external device or both. Bearing in mind the vehicle CANBUS connects to various modules that could be an awful lot of expensive damage. I don't believe there will be an issue, particularly between battery based systems, but its something to bear in mind. As previously stated a differential bus, whether CAN, DeviceNET, ProfiBUS, etc, should communicate correctly if the CM voltage is within the range of the transceiver.

I've utilised CAN within various industrial designs, with and without grounds connected, and have never seen a problem. However saying that I would always advise and recommend the signal grounds are connected for additional noise immunity, particularly over long CAN bus lengths, and to help reduce any CM voltage issues 😎

Hello Martin, thank you for the information on the Canbus.

I take your point about potential damage to modules if the CM voltage is exceeded. Anyone trying to steal the vehicle would have a device that either expected to draw power from the OBDII port, or if self powered would be at a 12V level (possibly 9V if using a PP3 battery).

At any time, with or without the grounds commoned together, connecting excessive voltage (>20V) in to the port may cause damage to devices on the network.

 

2 hours ago, unofix said:

Hello Martin, thank you for the information on the Canbus.

I take your point about potential damage to modules if the CM voltage is exceeded. Anyone trying to steal the vehicle would have a device that either expected to draw power from the OBDII port, or if self powered would be at a 12V level (possibly 9V if using a PP3 battery).

At any time, with or without the grounds commoned together, connecting excessive voltage (>20V) in to the port may cause damage to devices on the network.

 

Depends upon the transceiver chip utilised on the printed circuit boards. In the early days of CANBUS the transceivers were only rated for 12V systems so connecting >20V would indeed damage the chip however these days, and bearing in mind we now have 24V & 48V vehicle supplies, the transceivers are much higher rated transceivers to protect against a 24V/48V short to the CANBUS. In fact Texas Instruments produce transceivers rated for a bus fault up to +/-70V powered or unpowered.  The transceivers must also not affect bus communication if the module it is located in is in an unpowered condition...an important requirement for a multiple device bus system.

Now the issue you have is you don't know what transceiver the manufacturer has fitted to the board....?