How to test for bad earth?

[Jonathan Kay]

The 3,000 rpm criterion isn't crucial, it just needs to be well above idle... open the throttle by hand while you measure the voltage across the battery terminals. Or treat yourself to those croc clip leads for the multimeter.

Jonathan

[revilla]

If your battery voltage is very low it's possible that the ECU operation is compromised. I haven't had much experience with the Emerald but with the standard MEMS you get some odd effects with low battery voltage. The failure of the fuel pump could be nothing to do with the fuel pump circuit itself. It could just be that the ECU isn't operating normally and isn't commanding the pump to prime by turning on the relay. If I were you I would fix the things that are known to be wrong first and then see where I stood.

[anthonym]

7.8 volts was on the wrong side of the Grand St Bernard (now some years ago) after the alternator failed outside the (closed) Italian factory that made it. Most frustrating as I ordered a new one from England. End of that trip :-) She fought bravely. I use Brise now. 

[anthonym]

@Andrew far as I know as of now it all works and is ok. However, I have said that sequentially four times in a row and been found wrong. So exploring what and how to monitor. I concur that there's nothing wrong with the pump, having tested it off the car last week. Every time it has been the pump 12v supply side that failed, all points (connections) between the battery and the pump. My "time-to-fix" each problem has become exponentially shorter with each event, largely due to you and Ian's original instant detailed emergency responses/training last week. There haven't been any voltage drops other than to zero, which is relatively easy to find.

Nothing like necessity to encourage learning.

Anthony

[anthonym]

@jk isn't that going to give the alternator charge?

[Jonathan Kay]

That's the point.

• The voltage at rest tells (a bit) about the battery.
• The minimum voltage during cranking tells a lot about the battery AND an estimate of the risk of some electronic component not working properly, see #26 above. 
• The voltage at 3,000 rpm (or thereabouts) tells you about the charging system.

What are the values?

This collection of tests isn't only diagnostic for primary faults. If you're relying on having a good battery in any other tests and it isn't then they may be misleading.

Jonathan

 

[anthonym]

well, when I looked at rest it was 12.8 volts after being charged, alternator was 13.7 and cranking don't know yet. These numbers from my Stack display. When driving I'm sure I've seen 14.1 to 14.2 

edit: after my above mentioned 7.8 volts experience I tend to keep an eye on this.

[revilla]

Those numbers sound a lot healthier.