Troubleshooting Battery Isolator Installation

Question:

The write-up states This battery isolator protects your starting battery power so that no matter how weak your accessory battery becomes, First question is does it work the other way too, does it protect the house battery if the engine battery goes dead like if you leave the headlights on? Second question is if BOTH batteries are dead and you connect charging cables to the engine battery terminals should the house battery get a charge? The basis of these questions is 2001 Roadtrek which is relatively new to me went out this morning and both batteries completely dead after about 10 days not used since last moved. Hooked charging cables to engine battery and the both batteries started getting a charge which surprised me. I look at the isolator and the wiring looks questionable. It appears the alternator and the engine cables are attached to the same post. I would have expected 4 posts so wires to the 4 posts. Does this look wrong to you? I am guessing here but from other work he did I am thinking the isolator may have been acting up so the previous owner jury rigged the wiring, which leads to my last question. Do you have instructions on how to test an isolator that I can try before ordering a new one? Thanks, realize it is a long question set and I should add you will find me in your customer database for multiple previous orders.

asked by: Michael J

Expert Reply:

Thank you for all of the details and photos! It looks like you have the Deka Battery Isolator # DW08771 installed and you should have only 1 wire going to each terminal. I believe you're correct that the previous owner just winged it so we need to get that fixed. If we test the isolator and it's bad then I recommend replacing it with the one mentioned above and wiring everything as it should be.

To test the isolator what you need is a multimeter like part # PT89ZR. Set the multimeter to the "diode" function and then connect the red wire to the terminal that connects to your alternator, then connect the black wire to either of the terminals that goes to a battery bank. If you don't get a reading then that means the isolator is shot and needs to be replaced.

Battery isolators are designed to stop the accessory battery from draining the start battery when the vehicle is off. Typically you will see the weaker of the batteries pulling a charge from the stronger of the batteries. When it comes to charging them, they should charge at the same time - it won't be a trickle effect where one completely charges and then that starts bleeding over to the other. The only way to make sure your start battery is completely charged before the accessory battery is to disconnect the accessory battery from the system so the start battery gets all the juice.

Hope this helps you out!

expert reply by: Jon G