Wednesday, January 24, 2018


"Look youngen...... here's a simple circuit on the board.  A source, a fuse, a switch, a load, and a ground, in that order.  If that load... say.... a light bulb.... is lit, we agree its a working circuit with continuity?  Yes?  Good.

Now... saying that's a twelve volt source, can we also agree that an available voltage test would read 12 volts anyplace in that circuit right up to the load... yes?  But... not AFTER the load.... Right?




Oh.... because that side of the circuit is the ground side.  We don't read 12 volts there because the voltage was used by the load.  After the load, it's all ground.... right?

So what does it mean if the switch is closed, the load does NOT operate, and we can read 12 volts after the load on the ground side?

What does it mean?

What does it mean?


No... it does not mean the fuse is blown, (you fucktwat).  NO... it does not mean the bulb is blown Fer GOdsbesaken fracken blacken sacken kibblesnort!!    Look... we can read 12 volts after the load.... that means we MUST have continuity from the source, thru the fuse, the switch, AND the load to get it to the ground side where  we are reading it.... RIGHT?

Okay.... so, before when we tested, we had 12 volts to the load but not after, because after the load the circuit WAS GROUNDED.  Now, we are reading 12 volts AFTER the load because the circuit is NOT........ NOT........ WHAT is it NOT anymore?!?

The battery?  You are telling me we are reading 12 volts after the load because the source is dead?"

There is not enough bourbon for this day.....


Bryn, Isle of Anglesey said...

Hello, just checking in to ensure I've not had a major brain-cell implosion after reading this several times now. I've assumed that you're describing a simple chassis ground circuit where the -ve wire returns to the battery from the load component, with no trickery like diodes or earth ground getting in the way.
Thus : +ve terminal, wire to switch (closed), wire to load (functioning), wire to -ve terminal. All components functional.

If the load suddenly does not operate, and you have eliminated component failure, then the fault must be in the circuit wire from the load ground connection to the battery negative. If your tester is testing voltage only (from load component ground connection to the battery negative), but is unable to carry sufficient current to energise the load during the test, this would give the results you describe. Confirm by disconnecting the -ve battery terminal, test continuity and then current carrying ability from the load component ground to the end of the wire. If bad, replace the wire, ("bad earth", chassis ground) wire & re-energise the circuit. Everything should now work, "bad earth" problem solved.

Sorry if I sound pedantic with this, I'm trying to avoid confusion between US vs. UK terminology, plus my experience is limited to years of working on pre-computerised negative earth bikes & cars.
I don't dare touch my own car these days, when even replacing a battery can leave you inoperative until someone comes along with the right software to reset the engine management computer.... And yes, I check/replace heavy duty chassis/body/-ve earth bonding cables regularly.....

Carteach said...

Yes Brian, perfectly correct (setting aside language/term differences. In that situation, seeing source voltage after the load would show a loss of ground (earth), or path back to Batt Neg.

Getting a student to understand and admit that..... ugh.

Bryn, Isle of Anglesey said...

Thank you; I really did think I was missing something exotic, while my hindbrain was screaming & gibbering "bad earth, Bad Earth, JUST A BAD EARTH....."