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Finding bad grounds
Start with the main battery cables. If you have a voltmeter you can measure the voltage drop from one end of a cable to it’s other end with everything turned on (first with engine off then running). There should be none (or less than 0.1 volt). If you suspect one of the cables you can jumper across it with another known good one (like a battery jumper cable) to see if problem goes away (or voltage drop decreases). Then remove the cables, clean both ends and places they attach to with sandpaper. Bend them along every inch to see if they feel cracked or old. If so replace them. Remember, even a 0.1 ohm resistance from a hairline crack, corrosion, or missing strands in a cable that's running 30 amps will drop 3 volts which will produce problems. Battery cable internal cracks, corrosion, and poor connections are the #1 cause of electrical problems. Put them back on securely tightened. Smear Vaseline or grease over terminals (especially the positive) to prevent corrosion.
If satisfied with battery cables, remove alternator main cable and do the same. Be careful not to touch any metal as this one is hot. Disconnect battery before attempting this. Also make sure you have zero resistance from the alt. casing to battery negative, engine block, and body.
If that is ok then start looking for bad harness/device grounds using pictures from the manual to locate them if necessary (under Component Locations – Grounding). If you have any voltage on any harness/device grounds (to battery negative) with things turned on, you have a problem there (poor connection, broken wire - remember wires can be broken inside their insulation but look normal from the outside). The location of an internal break in a wire can be found by measuring continuity from one end to farther and farther places in the wire using a safety pin (attached to other meter lead) stuck thru the wire's insulation. Wiggle harnesses/wires/connections to see if problem changes and measure voltage at the ground points or resistance thru suspected wires. If a wire is shorted you will have to find it - installing another wire across it will not help. I have an article in Tech Guide electrical on finding shorts.
If you have very weird symptoms (eg. headlights never go off, or turning on/off one device causes another device to turn on/off), check for open grounds on the devices affected (also any isolation diodes that are an open or short circuit in both directions). Without the device ground the current will find another way to ground, often through another unrelated device which can cause strange happenings! Also check all fuses as sometimes a device may be getting power through a different fuse when its one is blown. If an aftermarket radio or alarm power was tapped into a not true power source, strange things will happen. A not true power source is one where you measure +12V but it’s because you are on the load side of something that’s turned off (ie. open circuit voltage). As soon as the device is turned on there won’t be +12V there anymore since it causes a voltage drop. Often using a light bulb in addition to a voltmeter is a good way to find true power sources if you don’t have wiring diagrams (light will be dim on not true sources).
Last edited by luv2rallye : 06-24-2008 at 04:58 AM.
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