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  1. #21
    Join Date
    Mar 2009
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    DFW, Texas
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    71

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    Quote Originally Posted by Bodybagger View Post
    Disconnecting the battery is primarily to prevent accidental starting of a vehicle that's being worked on, with the worst case being started in gear.
    It would also have prevented a recent fatality around here. A mechanic was working on a large truck on a construction site, and was under the vehicle when someone else jumped into the truck, started it, and started to drive off.. But I am sure it can also be blamed on the lack of proper tag out/lock out procedures to me...


    Quote Originally Posted by Bodybagger View Post
    As far as the computer is concerned, they go bad on their own. If one goes bad after you've welded on the car, you will be blamed. It makes perfect sense, even if it had absolutely nothing to do with it. And ESPECIALLY if you disconnect the battery... after all, that amounts to "tinkering with the electrical system."
    Also, if some components are kind of marginal, but the ECM has learned to deal with it adequately, when the battery is hooked back up, and the ECM has to learn the system from scratch, some previously not apparent near failures may now show up.

  2. #22
    Join Date
    Apr 2009
    Location
    Oswego IL
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    664

    Default Damaged computers....

    I have seen 2 computers with the CAN function on the computer which have failed due to welding on machine. Again for your protection I always disconnect the computer when welding on anything with a computer. All what needs to be done is disconnection of the battery.
    Kevin
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  3. #23
    Join Date
    Sep 2009
    Posts
    19

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    I will interject here and explain what happened when I was doing some tig welding on a high end chopper.

    Not too long ago I had a customer come out and he wanted me to make a part on site with bike and all. For bracketry, I fit up everything on the bike and bolted it down and then went to weld the part to my brackets that were bolted to the chassis. All went well and I took the part off after welding and he put it in a bag and got ready to start his bike......Nothing happened.

    Long story short, the high frequency affected a small on board computer that had been installed to kill his bike from too many DWI's!! The HF scrambled the computer. He did some things to the unit as it was a timed unit that would send a signal to his PO officer. He was able to reset it and blow in the breathalizer and all was good.

    Moral of the story, High Frequency can and will affect very sensitive computers. As for grounding work or a stray arc frying it, not likely unless you have a bad ground.

    Grant

  4. #24
    Join Date
    Feb 2008
    Posts
    893

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    HF will get into anything and everything. I would not recommend using HF around any RF or computing device.

    But let me ask you this... the power leads into a PCM are only 2 of a hundred or so. So this begs the question... why wouldn't you disconnect the other wires? They are there acting like a hundred antennas applying high RF voltages to the circuitry due to the RFI from the use of HF.

    The 12 volt input into a PCM is the LEAST susceptible place for voltage transients. The power block consists of a voltage regulation and filtering circuit including capacitance to smooth out transient voltages and short AC noise, such as RFI.

    And batteries have a HUGE capacitance that shorts RFI. If anything, that battery is absorbing voltage transients and disconnecting it defeats this inherent protection.

    If you are worried about HF getting into the computer, disconnect it from the wiring harness and you will have also disconnected it from the battery.

    If a storm is coming and you are afraid lightning will damage your TV set, it would be downright silly to unplug it from the wall but leave it connected to a big outdoor antenna mounted on the highest part of the roof, wouldn't it? The power company has lightning arrestors on its lines (analogous to the battery and filtering circuits in the PCM), but the lead-in wire from the antenna is unprotected (analogous to the many sensor inputs for the PCM).

    If a PCM died after you welded on the car, the fault was caused by voltage transients on the sensor inputs, not the battery cables. I'd swear that on a stack of bibles. The sensor inputs are the weak link, and the fault would have occurred just the same with or without the battery connected. The only thing that would make it more susceptible is if the PCM were actually amplifying the sensor signals while welding... as in welding while the car is running (or at least while the key is on, running or not).

    80% of failures are from 20% of causes
    Never compromise your principles today in the name of furthering them in the future.
    "All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko
    "We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal
    "Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

  5. #25
    Join Date
    Jul 2008
    Location
    Delhi, Ontario:
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    1,971

    Question To Disconnect or Not ??

    Quote Originally Posted by Bodybagger View Post
    HF will get into anything and everything. I would not recommend using HF around any RF or computing device.

    But let me ask you this... the power leads into a PCM are only 2 of a hundred or so. So this begs the question... why wouldn't you disconnect the other wires? They are there acting like a hundred antennas applying high RF voltages to the circuitry due to the RFI from the use of HF.

    The 12 volt input into a PCM is the LEAST susceptible place for voltage transients. The power block consists of a voltage regulation and filtering circuit including capacitance to smooth out transient voltages and short AC noise, such as RFI.

    And batteries have a HUGE capacitance that shorts RFI. If anything, that battery is absorbing voltage transients and disconnecting it defeats this inherent protection.

    If you are worried about HF getting into the computer, disconnect it from the wiring harness and you will have also disconnected it from the battery.

    If a storm is coming and you are afraid lightning will damage your TV set, it would be downright silly to unplug it from the wall but leave it connected to a big outdoor antenna mounted on the highest part of the roof, wouldn't it? The power company has lightning arrestors on its lines (analogous to the battery and filtering circuits in the PCM), but the lead-in wire from the antenna is unprotected (analogous to the many sensor inputs for the PCM).

    If a PCM died after you welded on the car, the fault was caused by voltage transients on the sensor inputs, not the battery cables. I'd swear that on a stack of bibles. The sensor inputs are the weak link, and the fault would have occurred just the same with or without the battery connected. The only thing that would make it more susceptible is if the PCM were actually amplifying the sensor signals while welding... as in welding while the car is running (or at least while the key is on, running or not).
    Bodybagger, Hi; I think I got most of that ?
    Are you saying Not to disconnect the battery?
    But if you disconnect Anything you should Disconnect the Computer Entirely ?

    Confused in Delhi ? ........... Norm

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  6. #26
    Join Date
    Mar 2006
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    Ocean City, Maryland
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    951

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    Where i work we work on and weld on aerial construction equipment, [buckets, digger derricks etc]. There has been alot of debate on if battery/electronics should be disconnected. Caterpillar will tell you to disconnect their ecm before any welding. Our trucks have a considerable amount of electronics, remote start/stop, throttles, emergency power etc. We have ruined one baby board for these functions when some welding was done on the road. Its true, on the Fords it has to relearn the PTO function after disconnecting everything, but you only have to test drive it for it to relearn. We ground close, disconnect batteries, [usually 3] and unplug the ECMs on the CATS. The Fords we dont disconnect the PCM only the batteries
    We've welded on plenty of trucks or equipment , excavators/ trenchers and never seemed to bother anything. I think it doesnt hurt anything to disconnect the battery or use some sort of surge protector. So in my opinion, a good, clean, close ground is most important, if you can disconnect an ecm easy, i'd do it, and it usually only requires one battery cabe end removed. I guess I'd error on the sie of caution versus risking cooking something, probably get away with no damage hundreds of times, but i dont want to replace one $1500 ecm.
    Scott
    HMW [Heavy Metal welding]

  7. #27
    Join Date
    Mar 2009
    Location
    DFW, Texas
    Posts
    71

    Default

    Just for the sake of discussion, if the ECM is disconnected, you now have all the sensors with long antennas attached... So you may be exposing hall effect crank angle sensors to HF energy... And what about completely isolated computers like ABS controllers? Don't some of the GM and Audi cars have as many as 20 separate computers in the mix?

    In general, I agree that disconnecting the battery allows a lot of circuitry to float that would otherwise be constrained to their operating voltages. Disconnecting the ECM from the harness is likely the best way to protect that expensive chunk of hardware...

    Probably the best CYA practice is to follow the manufacturer's instructions, if you think that puts you in a good place to point any issues back at the customer and manufacturer...

  8. #28
    Join Date
    Feb 2008
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    893

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    You raise an excellent point, kabraby. The sound system, climate control modules, ABS, and other systems have their own processor and they are digital, which means they are just as sensitive to things like RFI and ESD. And when's the last time you ever heard someone say they blew the climate control module from welding?

    Why is it always the PCM that people claim to be ruined from welding.

    I remember a conversation I had with my father in law about some of his books. He was a physicist retired from the US Air Force. He had this one book from the 1960's about electromagnetic propagation using Maxwell's equations and I started talking about modern NEC (Numerical Electromagnetics Code, not NFPA 70) for antenna design. He could not tell me any specifics, so he sufficed to say "imagine how many volts you'd see on an antenna when the radio station is 10 megatons."

    Well, I don't know how many volts you'd see but I'd bet it's enough to make a lot of unhappy PCMs.

    80% of failures are from 20% of causes
    Never compromise your principles today in the name of furthering them in the future.
    "All I ever wanted was an honest week's pay for an honest day's work." -Sgt. Bilko
    "We are generally better persuaded by reasons we discover ourselves than by those given to us by others." -Pascal
    "Since we cannot know all that there is to be known about anything, we ought to know a little about everything." -Pascal

  9. #29
    Join Date
    Mar 2006
    Location
    Ocean City, Maryland
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    951

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    Quote Originally Posted by kbraby View Post
    Just for the sake of discussion, if the ECM is disconnected, you now have all the sensors with long antennas attached... So you may be exposing hall effect crank angle sensors to HF energy... And what about completely isolated computers like ABS controllers? Don't some of the GM and Audi cars have as many as 20 separate computers in the mix?

    In general, I agree that disconnecting the battery allows a lot of circuitry to float that would otherwise be constrained to their operating voltages. Disconnecting the ECM from the harness is likely the best way to protect that expensive chunk of hardware...

    Probably the best CYA practice is to follow the manufacturer's instructions, if you think that puts you in a good place to point any issues back at the customer and manufacturer...
    I agree with you, most new vehicles have many circuit boards, controllers or processers that control everything. Certainly not pratical to unhook them all.
    Scott
    HMW [Heavy Metal welding]

  10. #30
    Join Date
    Mar 2006
    Location
    Southwest Wa.
    Posts
    32

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    Quote Originally Posted by Sonora Iron View Post
    I think every up fit manual Iíve ever read says disconnect the batteries before welding. Some even say unplug the computer.
    I have always felt itís good practice to keep the ground close to your work.
    yep. me too

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