Weird Electrics

[blind_elk]

I've been having problems recently where the car doesn't want to start cleanly without a jump start. It is symptomatic of a bad earth. So today I disconnected the main earth cable to clean the points of contact with the engine / chassis.

At some stage, I accidentally touched the earth cable to the chassis and got sparks. Checked with the multimeter, and I have 12V differential between the strut nut and battery -ve. There is no voltage between +ve battery and chassis. So somehow the chassis is "live".

Any suggestions as to where to even start looking. Could it be something in the alarm, because I noticed when touching the cable to chassis that the door locks appeared to be switching.

Any help greatly appreciated.

[t_revz]

At some stage, I accidentally touched the earth cable to the chassis and got sparks. Checked with the multimeter, and I have 12V differential between the strut nut and battery -ve. There is no voltage between +ve battery and chassis. So somehow the chassis is "live".

Was this with the main battery negative still disconnected or had you reconnected it?

[blind_elk]

Actually, thinking about it further, what is happening is perfectly logical.

The car has certain items that draw permanent battery current, eg clock, radio battery backup, alarm - they all rely on having permanent power from the battery. They all earth to the body of the car, and under ordinary circumstance would have a completed circuit through the chassis and the earth strap to the battery -ve. But since I've disconnected the earth strap from the chassis, the circuit is incomplete. Touching the strap to the chassis completes the circuit, and the resultant current flow causes a few sparks at the strap / chassis contact point as the initial contact is made.