Checking for Shorts

[Midlife]

Here are my general instructions for troubleshooting electrical shorts.  The instructions were written for installation of underdash wiring harnesses, but it also applies to any other time you change wiring, or have problems.  I highly recommend that you follow this procedure every time before connecting the battery, to ensure you don't let the Smoke Jeannie out.


First, always install  or troubleshoot electrical wiring with the battery disconnected.

Physically install the underdash harness without fuses. 

When ready to check out the harness, close all doors, turn off all lights, accessories, etc.  Then use a digital volt meter to measure the current draw across the battery.  Make sure both battery connectors are disconnected.  Connect the positive battery cable to the starter solenoid.  Leave the negative terminal disconnected from the battery.  Place one probe of the DVM on the negative battery cable terminal, and the other probe on the negative battery post.  Be sure that the DVM is set to amps, DC.  If there are no shorts in the wiring connections, the reading should be on the order of 30 milliamps or less.  Anything above 1 amp indicates some accessory is on; readings above 10 amps (may blow the fuse on the DVM) indicates a dead short.

If you have a short, disconnect all other wiring from the starter solenoid post where the battery connects and re-measure the current.  If current drops, the source is the underdash wiring harness or the harness from the starter solenoid to the firewall.  Reconnect the wiring to the starter post and then disconnect the underhood harness from the firewall and test again to isolate the source of the short. 

If the short is not in the underhood or underdash harness, the problem lies in the alternator or voltage regulator.  Disconnect each of these in turn to isolate the source.

If no short is evident and you see low current, keep the battery negative side disconnected and install one fuse at a time.  Measure the current as above.  If you see high current, there is either a short or an item on that particular fuse circuit is on.  Turn off that item and continue checking.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected.  At that time, and only at that time, is it safe to connect the negative battery cable to the battery.

Good luck!

[Fordication]

I am wearing a pair of blue jean ones right now. :)