Mystery electrical problem

Here one to fuddle your minds - don't even know where to start with this one.

Spent the morning adding a few lights and 12v sockets to my 110 via an auxiliary fuse box and an isolator. All works lovely, so far so good. Bit later I notice my volt gauge showing just over 8 volts and sure enough nothing is working. Turn the key and the glow plug light illuminates at the accessory position and the water temp gauge shoots right around the dial. Pretty confused at this stage.

So I test the battery and here it starts getting really strange - the battery shows 12.8 volts. So next I test the earth and sure enough no earth, but when I disconnect the negative battery lead I can get good continuity from that to the chassis.

So to summarise I get 12 volts at the battery, 8 volts everywhere else and can only get an earth when I disconnect the neg battery lead. Whaaaaaaaaat????

Still sounds like a short or an earthing issue. All you can do is logically trace the problem. You can't be getting 8v everywhere else, not if the battery is 12v.

Start with the + off the battery and follow it, test it at the other end, then move on from there until you find the issue.

4 volts is a big drop, you have a direct short somewhere Isolate the fuses in the aux fusebox to see if the voltage returns back to normal, if it does reinstate one fuse at a time checking the voltage inbetween, how are u testing voltage multimeter or dash voltmeter?
You have obliviously done something wrong...
Bottom line is when it's sorted out get an auto electrician to check over your installation, you adon't won't the vehicle to catch fire.


Sent from my iPad Air on a train

Thanks I'm busy with that now. I also just discovered that with the battery fully connected I get 3 volts between the neg terminal and the chassis. Sounds like a short yes?

discool said:

4 volts is a big drop, you have a direct short somewhere Isolate the fuses in the aux fusebox to see if the voltage returns back to normal, if it does reinstate one fuse at a time checking the voltage inbetween, how are u testing voltage multimeter or dash voltmeter?
You have obliviously done something wrong...
Bottom line is when it's sorted out get an auto electrician to check over your installation, you adon't won't the vehicle to catch fire.


Sent from my iPad Air on a train

Click to expand...

Testing with proper voltmeter. Problem persists with aux fuse box disconnected - still hunting!

You should not have any voltage difference between the battery neg terminal and the chassis - you have a bad earth connection somewhere.

Thanks for the input. Not much further and I have found earth continuity through the switched live that powers the radio and also 12v through the wire from the temp sensor to the gauge as soon as the key is switched on. That's all wrong!