Idiot at work

[sv70]

Thanks ThomasG but that is as clear as a Ted Rodgers 321 dusty bin clue,is your method just for a previously electrical job done without disconnecting the negative on battery or every time before you start work on the electrics please.

[Phooey]

Originally Posted by JoeNinety

I don't like the look of item 4 in ThomasG post

Touching terminals should be done with both disconnected - surely ? or perhpas it doesn't matter....

Touching the cables together is just completing a circuit without a battery in place. Touching the +ve and-ve cables will not do anything but ground any residual charge in the circuit. Try it with a small 1.5v battery and a bulb. Nothing happens! Only when you touch the terminals of the battery together will you know about it!

[JoeNinety]

My concern really is the risk of touching the earth to earth somewhere. If you disconnect the + ve terminal as well the battery is out of the loop completely.

Touching terminals with battery disconnected I well understand, it zero's out any residual voltage / storage in ECU's - thus clearing any stored settings in the eeprom or the like.

The battery disconnection procedure is very similar to that required for L322 Range Rovers, apart from the "important" key at Pos II. What is that for ? Again, why would you want to fire up all the systems when you are fiddling (and occasionally missing) to get the - ve terminal back on ?

[ThomasG]

Originally Posted by sv70

Thanks ThomasG but that is as clear as a Ted Rodgers 321 dusty bin clue,is your method just for a previously electrical job done without disconnecting the negative on battery or every time before you start work on the electrics please.

For safety- I do it every time I work on electrics, unless its something silly, like changing a bulb, wiper motor etc.

By rule of thumb, every time you fiddle with:

Steering wheel
Audio unit
Climate unit
Cem(apart from replacing fuses)
Rem(apart from replacing fuses)
Uem definitely
Spokoluzik says that DIM can becremoved and refitted without unplugging battery, but I still do it...
Few like that.

[ThomasG]

Originally Posted by JoeNinety

My concern really is the risk of touching the earth to earth somewhere. If you disconnect the + ve terminal as well the battery is out of the loop completely.

Touching terminals with battery disconnected I well understand, it zero's out any residual voltage / storage in ECU's - thus clearing any stored settings in the eeprom or the like.

The battery disconnection procedure is very similar to that required for L322 Range Rovers, apart from the "important" key at Pos II. What is that for ? Again, why would you want to fire up all the systems when you are fiddling (and occasionally missing) to get the - ve terminal back on ?

Because every time you fiddle with battery terminal, there will be some sparking.
Even if so small that invisible to the eye.
Even if car is fully OFF(by key) there are things drawing power, alarm being a good example, but not only.

These sparks cause voltage spikes throughout the car wiring, and, in extreme cases, can even fry some of electronics.

By turning the key into position II, you are adding quite a few resistive loads connected and switches ON at the time you are reconnecting battery. Bulbs in head and tail lights, for example. Even if only parking lights.

These loads will "dampen" these voltage spikes, this way protecting the electronics.

[JoeNinety]

OK, response accepted, but not convinced, only found this option on a Volvo forum, and the L322 is MUCH MORE volatile in relation to electrics

[ThomasG]

For completely unrelated reasons..

Reasons being here:
https://www.volvoforums.org.uk/showthread.php?t=297162

I started Vida today.

Just by the way I looked up disconnecting /reconnecting battery.

Even Vida says key "must be in position II".
Doesn't say why, though.

[JoeNinety]

Guess it also depends if the car has anything else connected directly to the +ve terminal (either at the battery or under the bonnet), that also trails back to earth on the other side...

[paddy74]

Originally Posted by ThomasG

[...]
Just by the way I looked up disconnecting /reconnecting battery.

Even Vida says key "must be in position II".
Doesn't say why, though.

As you said in your post before, it's to avoid high peaks of electricity and load in the control modules. That is exactly the reason.

[Christerart]

A week or so ago a friend offered to help replace the front off side headlight bulb on my 2005 XC70.

While I was in the house getting something he decided the way to do it was to remove the "brains" of the car - both of them.

But - he did not disconnect the battery (and by the way - you really need a special tool to get those two "brains" back in place).

Anyway, once they were back in place the car wouldn't start. Disconnected the battery and let it sit for 10 minutes, connected it and the car started - but the radio didn't work - every now and then you had a little sound but mostly static and interference.

Looking at replacing the radio I realized I was looking at £200 - 300 for a decent set up and procrastinated for a week and then I came across this thread and paid close attention to what ThomasG wrote.

I figured - maybe if I wipe the memory and follow ThomasG's instruction?

This morning I did just that - and you know what?

The radio is back and sounding and working perfect!!!!

A huge thanks to ThomasG!