What is the safest way to disconnect the batteries in the vehicle?

For all my gas vehicles, I've learned to disconnect the ground on the battery before servicing electrical components. By doing this, any accidental short between the ground terminal and the frame will not create a current path.

I'm about to replace the glow plug relay on my 7.3 F350, and I'm confused...

The procedures I've looked at call out disconnecting the positive battery terminals!

Shouldn't I disconnect the ground connections first? It seems to me that disconnecting the hot battery terminal has a high risk of creating a short to ground.

But then, there are two batteries, so where is the real ground?

How are the two batteries in a diesel connected? Is ground between the two batteries? Or are they "stacked" above ground? Would really hate to melt one of my wrenches...

 

12 volt negative ground.. negative to disconnect..aka the BODY..

duel batteries.. both ground cables..

 

I always just unplug both connections.

 

None of those procedures came from the factory service manual (or from someone who knew what they were doing).

Yes.

They are in parallel. Disconnect as per Chuck's direction.

Unnecessary. You'd still need to follow the proper sequence should you decide to do the extra unnecessary work.

 

Cool! I didn't know if it was a 24 volt system. Now I know to expect 2 parallel 12V connections. This was definitely a case of me not knowing, and at least knowing I did not know.

I'll disconnect both grounds before replacing my GPR. Of course, I will verify with my meter first, just did not want to melt a wrench or burn my hand.

 

It's also a good idea to ROTATE your batteries, swap them since they don't both see the same loads (especially when starting) so it helps to prevent the pass-side from going bad "faster" than the driver side.

The general rule is replace them in pairs, so it's a PITA to have to replace both because 1 goes bad faster, hence swapping them once/twice a year (I do it with each fuel filter change - 10k miles).

As stated, remove BOTH negative ground cables from the batteries..

 

I was not sure that practice was still going on in the diesel world today...
but was Common years ago.

and its true.. my opinion.

the battery with the shorter cable to starter works HARDER...