Wade Posted June 22, 2016 Share Posted June 22, 2016 Can anyone explain this to me? I took my ohm meter to my isolation relay and, with no power connected to the relay circuit, there was zero ohms resistance. It would appear to me that the batteries are not isolated. However, my starting battery was somewhat discharged and at a lower voltage than the coach battery. If I measured voltage to ground on the starting side it was a lower voltage than on the coach side. Both readings were close to the actual readings of the battery. I then pulled the starting battery and put in on a charger for a bit as I was making a wire to run the relay. While the starting battery was removed, I got no volts on the starting side and the correct voltage on the coach side. Got power to the relay and put the starting battery back in. Makes a significant clunk when power applied. Voltage equals (or at least closer to equal) on each side when engine is running. How is it that I can have zero ohms between the battery terminals on the isolation relay yet have different voltages, even zero volts when one battery is removed? Thanks! Wade Quote Link to comment Share on other sites More sharing options...
Maineah Posted June 22, 2016 Share Posted June 22, 2016 You kind of lost me there. If there is not voltage applied to the relay windings (small terminal) it will have an open circuit. If voltage is applied then the relay closes to join both batteries together. The relay has no effect on the truck battery the only thing the relay does is separate them with the engine off to keep the coach battery load from discharging the truck battery.It is nothing more than a electrical switch just like a light switch. Quote Link to comment Share on other sites More sharing options...
Skydancer2992 Posted June 22, 2016 Share Posted June 22, 2016 If there is a small leakage across the relay, the ohm meter will see that as a closed circuit or zero ohms. The resistance of the relay is still large enough to maintain a difference of potential to ground on either of the relay. Quote Link to comment Share on other sites More sharing options...
Maineah Posted June 22, 2016 Share Posted June 22, 2016 The battery contacts have no connection to ground or the windings. If you want to check the relay it is voltage you want to check. Remove one large battery wire from the relay check the voltage to ground on the connected one then with the key off check the voltage at the empty post it should be zero. Turn the key on check them again this time both posts should read the same voltage.If that is true the relay is fine. Quote Link to comment Share on other sites More sharing options...
Wade Posted June 22, 2016 Author Share Posted June 22, 2016 D'oh... Of course that is the correct procedure. I think what happened in my little test, due to the fact that zero was the recorded voltage on the other side with the truck battery disconnected, tells me that it is working to isolate. Now I just have to pull the other one off, apply power, and confirm that it is locking in. Thanks for stating it so clearly! Wade Quote Link to comment Share on other sites More sharing options...
DanAatTheCape Posted June 23, 2016 Share Posted June 23, 2016 another way is to hook up the "out" side so power is going to the house battery. with engine off , the isolater(solenoid) should be open. Measure voltage at both batteries. then start engine and check that voltage increases at both batteries. Should be about 14 at the start battery, and a little less at house battery. I get about .1 volt drop at every connection. Also verify what power source is activating the solenoid - my 93 winnie warrior gets power from hot side of windshield wiper circuit. Note that my solenoid looks like a Ford Started solenoid. However there is BIG difference - the Ford is set up for high draw short usage. The RV is set up for low draw long usage. They are NOT interchangeable. Your mention of a big clunk alerts me to the possibility that someone may have replaced the original with the wrong part. Also note that when the solenoid is activated, voltage should flow both ways - essentially the 2 batteries are hooked up parallel. The good news is that the solenoid is a low cost easy to replace part. I had to replace mine and I also replaced the CB's too. The 2nd pic shows the original parts. I got the CB's at NAPA - they were only a few dollars. Note that the solenoid grounds to the chassis. I made a grounding wire and double grounded mine - my new ground wire ran from one of the mounting bolts to another ground on the fender well that looked good to me. Quote Link to comment Share on other sites More sharing options...
DanAatTheCape Posted June 23, 2016 Share Posted June 23, 2016 (edited) http://www.tekonsha.com/products/brake-controllers/breakaway-kits/battery-switch/TzaIxkVx1cJts1kUSaaQjKeizfQnI3kfSdgi49ek4l0= https://www.amazon.com/Tekonsha-7000-S-Terminal-12-Volt-Continuous/dp/B003VAV50U http://www.napaonline.com/napa/en/search/?text=7823006 looks like the CB's cost more now - from this source. A quick google search - looks like they are quite available.... Edited June 23, 2016 by DanAatTheCape Quote Link to comment Share on other sites More sharing options...
Wade Posted June 23, 2016 Author Share Posted June 23, 2016 I am pretty sure it is the right relay. Looks just like the one I had on my old Dodge that I used to have to replace every few years;-) The voltages id do as you said IIRC. Will test again when I clean up the terminals. Thanks for the info and links! Don't think I have a CB installed so that might be a good idea. Wade Quote Link to comment Share on other sites More sharing options...
Maineah Posted June 23, 2016 Share Posted June 23, 2016 It should have them on both sides. The ones pictured are manual reset yours are most likely auto reset If you think it's some thing you need to deal with first go to the auto parts store and buy two new ones you will break the old one trying to get the terminals off you might buy replacement terminals too you may never get the old ones off. They are common parts you'll need a 35 amp one. If the relay still works it's the right one if you use a starter relay it may last a day before it cooks. When you remove the terminals you will find they are clean underneath it's just the posts that get rusty. Once you have replaced them paint them in what ever color you like! Quote Link to comment Share on other sites More sharing options...
DanAatTheCape Posted June 24, 2016 Share Posted June 24, 2016 I bought mine at a rv shop - needed it quick. if you go to an auto parts store make sure you don't get the one for a starter set up (high draw. short duration). You need a constant duty low draw unit. They look very very similar. Quote Link to comment Share on other sites More sharing options...
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