How To Wire D+ Signal?
When hooking up our DC-DC isolators and chargers, there are multiple ways to connect the D+ signal wire. The D+ signal wire is used as an on and off switch for these devices, its main purpose is to send a 12V+ signal to the device when the vehicle is running. This is necessary for devices without a voltage-sensitive relay (VSR) so the unit knows to turn on when the alternator is running and off when it is not, so it does not drain the starter battery. There are multiple circuits in a vehicle that are only powered on when the vehicle is running, we recommend splicing into one of these for the D+ signal.
The first step is identifying which circuit is active when the vehicle is running, which can be accomplished by reviewing the vehicle's electrical diagram or testing for DC voltage with a multimeter. Our first recommendation is to check on the alternator itself, most alternators will have three wires connected ( BATT +, BATT -, Ignition circuit), identify the constant 12V+ which will be live all the time. To test for voltage set the multimeter to DC voltage on the dial, once that is set find a common negative on the vehicle either the battery negative post or an area on the chassis and place the negative lead from the meter there, then splice into one wire at a time to expose the bare wire and with the positive lead from the meter test on that area. Once the 12V+ constant is identified take note of that wire, then turn on the ignition and test the other two wires for a 12V+ signal, once that one is identified, that wire will be the hot in run/start ignition circuit. With the ignition circuit identified the D+ signal wire can be spliced into that wire.
The second place we recommend to check is in the engine bay fuse block, by reviewing the fuse layout diagram on the vehicle the accessory/auxiliary fuse should be identifiable. Once that fuse is identified test for a 12V+ signal at the fuse by checking for DC voltage with a multimeter, once that is set find a common negative again and place the negative lead from the multimeter there, then place the positive lead of the multimeter to one of the test points on top of the fuse, when the vehicle is off there should be no voltage present, next turn on the ignition and test for a 12V+ signal in which there should be voltage present. If an accessory/auxiliary fuse is not identifiable the above steps can be followed on any fuse in the block to identify an active circuit when the vehicle is on. Once the active 12V+ signal is identified when the vehicle is running the D+ signal wire can be spliced into that connection.
When the D+ signal is spliced into one of these circuits and the rest of the installation is complete on the device, the unit should turn on and off with the vehicle. This is a brief introduction on how to wire the D+ signal wire to a vehicle and we always recommend this is done by an experienced installer or professional. If you have any other questions or concerns feel free to call or email our customer support team here.