electrical theory & practical application question - resistance

[qikcpa]

my brains a little fuzzy on this...

if i disconnect the negative battery cable but leave the positive connected and then run an ohm meter between any red/red&blk wire and a suitable body ground, should i expect an open circuit?

i caused a small electrical fire yesterday and was just wondering if this was a valid way to check for shorts before applying a current...or if the clock or a switch being on will invalidate the reading causing you to never get an open circuit and be sure there are no shorts.

thank you.

[E Sully]

Once the battery ground cable is disconnected there should be no current flow. You will get resistance readings between the power wires and chassis grounds due to the devices that are grounded by the brown wires, but no voltage reading.

[McLaren-TAG]

Easy way is to do just what you did by disconnecting the battery ground, and then simply checking your positive leads with your meter set to continuity. One lead on the hot and one lead on chassis ground. If you read continuity on a positive, then you've got a short.

You can't do the same on ignition wires since some do show ground at rest especially if they're relayed.

[E Sully]

McLaren-TAG, this is not correct. You will still get some reading on the continuity meter since there will still be a circuit through the light bulbs, clock, radio, etc. to the chassis through the common feeds in parallel and common brown wires connected from the devices to the chassis. This is what qukcpa was referring to in his last paragraph.
Finding a dead short is a lot more involved and requires isolating each circuit.

[McLaren-TAG]

I'm answering in the context of the OP which is a way of checking out for shorts. If you have continuity to ground in a 12+ lead, that's a good wire to be suspect about. You can also gauge the level of resistance and make a pretty good guess as to whether it's a circuit or a true ground short.

[masterdave]

+1 for what E Sully has said. You will get a +12V reading from any power wires (wires coming from the battery) and a solid chassis ground. In theory you should not but the chassis is a big enough conductor and is relatively 0 volts compared to the battery that you will get a reading of around 12V. You must also remember that all the chassis ground are connected so to test any circuit Sully is right you must isolate it or you will get some funky grounding loops going on. Which circuit are you testing we may be able to offer more help if we know what the problem is.

Regards
Dave

[dad911]

If you want to check for shorts before applying current, put a test light between the negative terminal and ground. Bright light = short, but it won't draw enough current to do further damage.

Pop fuses until the light goes dim, and you'll know which circuit is shorted.

If you are using an ohmeter, disconnect the battery totally. Too easy to blow it out on a live circuit.

[T77911S]

Originally Posted by dad911

If you want to check for shorts before applying current, put a test light between the negative terminal and ground. Bright light = short, but it won't draw enough current to do further damage.

Pop fuses until the light goes dim, and you'll know which circuit is shorted.

If you are using an ohmeter, disconnect the battery totally. Too easy to blow it out on a live circuit.

finally!

remove/repair any burned wiring first. then do the above.
make sure interior lights, glove box lights, trunk lights are not activated and nothing else is on or this will give you a false reading.

[bfunke]

Dad911 is right. You'll usually fry your OHM meter by hooking it up to current.

[Porboynz]

I agree with dad911, the light bulb method is quick and easy. Using an ohm meter anywhere near 12V will end in tears. You can use an ohm meter to check earths once the battery is disconnected. In theory with the ignition off and the clock disconnected the 12V lead should be isolated and read hi ohms, but if you are fault finding what should be often is not.