How is this happening?

thescouselander

Joe_Horner wrote: »

The problem with that is that the sort of resistances that can give problems (0.1 ohm or less) can't be reliably measured with most DMMs. But you get the same effect by measuring voltages with the circuit loaded.

0.1 ohms resistance in a battery earth, for example, will show a voltage drop of a full volt when a 10 amp current (typical high beams) is flowing - you can't measure 0.1 ohm reliably but you can easily measure 1v

Yeah, I get your point but the voltage drop seems to be happening under fairly low loads so I suspect the resistance is large enough to measure. I have seen digital meters give funny voltage readings under some circumstances so nothing is fool proof.

Joe_Horner

Fair comment and we seem to be agreed that, for there to be a (real) voltage between battery and engine there has to be an earth fault - good earths/ connections mean no voltage difference from one side to the other by definition.

If everyone who plays with car electrics could just get that one immutable fact lodged in their mind then they'd find that 90% of fault finding becomes a breeze

thescouselander

Yes, indeed.

marlot

TrickyWicky wrote: »

"No it suggests that as per my post, that was PREVIOUSLY an issue that I resolved by tinkering with a connector...."

You seem more interested in making your point than listening to mine.

You have an earth problem. There should be no potential (=voltage) difference between the engine block and the body.

You're wasting your time swapping ECUs and the like if you haven't fixed the basics. Indeed, as I said, you could easily damage an ECU - they tend not to like huge currents going through them,

I remember in my youth looking at a mini that wouldn't start. When the starter was turning the choke cable smoked. The (visually looking perfect) earth lead was at fault, and the only return for the starter current was through this poor choke lead.