Having trouble with diode battery isolator

wired up my battery isolator (dual diode type) and then found that I could not start my truck.
I wired it as in this diagram


does the wire for the starter go to the alternator then to the battery instead of having a dedicated cable straight to the battery?

 

came up with a simple solution:
remove +wire from alternator and cap it off. this wire splits and also goes to the starter.
run new wire from alternator to battery isolator. run the other wires from isolator to batteries as previously stated.

and do the big 3 while I'm at it. win win win

 

truck starts, but now I'm not getting any voltage from the alternator. With the engine running, I get ~12.7V across the isolator(alternator to primary battery), and 0V between the alternator and ground. Also, my "check battery" light is on on the dashboard. Does this have something to do with the alternator being internally regulated? does anyone know of a work around to this? there are 3 small wires that go to the alternator along with the large battery wire. is one of these used as a signal wire to the alternator?

ANY info or help would be greatly appreciated here.
Does anyone at least have the wiring diagram for the charging circuit on 2005+ 4.0L auto tacomas?

 

The grey wire from the alternator feeds the Body ECU for the charge led.
The white wire goes from the Alternator to the battery.
The Green or Pink comes from the IG1 or IG1 No.2 fuse.
I think there is another wire White w/Blue Strip for the 130A Alt with towing Pkg.

I'm not sure where you are seeing a wire that goes from the battery to the alternator then down to the starter. I don't see that on the circuit schematics at all... ???????????????

You should have one large wire Black w/Red Strip from the positive terminal going directly from the Battery to the Starter Connector. A smaller Yellow w/Green Strip wire from the Start Relay to the starter coil winding.

I would be careful before you burn something up. Just my opinion.

 

Why didn't you just buy an isolator from a parts store there only about $40? When you say doesn't start is it because the battery is not charging? If your alternator wire is between the two diodes it will not sense a battery voltage it need this to charge that is why the store bought isolators have 4 terminals one is an ign feed to the alternator B+ from the key switch. If there is no voltage at the B+ terminal on your alternator with the engine off it won't charge.

 

Thanks both of you!! this is very helpful info. I have 1 grey, 1 green, and 1 white wire at my alternator
I don't have any schematics, I must have traced the wire incorrectly.

I bought an isolator from a parts store. I made the mistake of buying a 3 terminal instead of a 4 terminal. I didnt know the denso alternators required a voltage reading from b+. The terminals on my isolator are labled "alternator", "battery 1", "battery 2". After looking in the isolator manual it has specific instructions for Toyotas that suggest i need an isolator with a 4th terminal labeled "exciter." Is this "exciter" terminal connected directly to the "alternator" terminal? The manual states that I should connect the ignition wire to the "exciter" terminal. If so wouldnt it allow the secondary battery to be charged by the primary battery through the ignition? Here is the diagram from the isolator manual for further clarification:

It didnt start because I accidentally disconnected the starter from the battery thinking it was the alternator.

If I add a jumper from the battery or the ignition to the B+ terminal, it will bypass the diode for the primary battery, allowing the primary battery to charge the secondary battery(which is what i dont want). I want to keep both batteries completely separated from each other. If i were to pull out the alternator could I dismantle it and disconnect the voltage sensor from B+, and add 2 jumpers with diodes from both batteries to the voltage sensor(allowing current to flow from each battery to the voltage sensor but not back to either battery)? In theory it solves the problem

 

Thanks PSU!

 

If you are electrically savvy you can add a diode between the "ign" wire on the alternator and the B+ terminal this will supply 12 volts to the B+ terminal with the key on so that it will start charging that will do the same thing the 4th terminal on an isolator does.

 

In order for all this to work properly your alternator B+ should go directly to the "A" post on the isolator only. The Diodes require a break over voltage there is about .7 volts lost to heat that is why the isolators have fins. So if you directly join the alt. to the truck battery the extra battery will suffer (path of least resistance). The "E" terminal supplies voltage to the alt B+ terminal because it no longer will have standing voltage because the isolator diodes block it. But being a diode they only conduct in one direction so it cannot feed back to the ign system when the key is off. There is very little load on the "E" terminal the only thing it does is present voltage the battery sense terminal in the regulator without it, it will not charge. Frankly the simplest set up is a standard issue continuous duty relay joined between the truck battery and the extra one operated by the key switch that way you do not have to alter your truck wiring. This is standard fair in motor homes or anything the needs aux. batteries some people say the solid state isolator is better some say the relay I am a firm believer in the K.I.S.S. principle and also don’t like the ideal of losing power to heat or messing with perfectly good wiring.

 

I am thinking a heavy amp relay would do the same thing without tapping the IGN wire, just run a second POS wire from the alternator to a relay and then to the second battery. The second battery then powers the AUX block which is completely isolated from the truck's electrical system. Put a second voltmeter on the second battery and a switch to the relay. The truck's ECU wouldn't know it's charging two batteries right?

 

I found a voltage sensitive relay that I think will suit my needs.
http://www.princessauto.com/pal/en/Battery-Accessories/12V-Battery-Isolator/8437394.p
I may use one of the diodes after the relay to prevent the auxiliary battery from charging the starter battery during vehicle operation. Hopefully that will be enough to keep both batteries from affecting each other? I am an electrician by trade but I'll admit my knowledge of batteries is quite limited.
I agree with the KISS mentality. Just wanted to explore every option before I went out and bought more parts.

[edit] I realise that adding a diode to only one battery charge lead will cause in imbalance in the charging circuits but I do not think it will be significant enough to cause the auxiliary battery to not reach a full charge. The measured 14.4V output of my alternator - at most 1.7V drop across the diode is still 12.7. I have yet to measure the fully charged voltage of my auxiliary deep cycle batery. If I can not attain a full charge using a silicon diode then I may also experiment with a schottky diode which has a significantly lower voltage drop across it(0.15-0.45V compared to 0.6-1.7V across a silicon diode).

 

That is often called a combiner Blue Sea and many others make better ones then that, that read the voltage on both batteries before they combine and charge the primary until it reaches its full charge first before combining the batteries these are made primarily for boats something that your life may depend on starting. Why make it complicated? The secondary battery will not “charge” your truck battery yes there may be an exchange of a few tenths of a volt as they equalize but your are only talking about a very small percentage that will not do any harm to either battery. If you are going off road your best bet is to make it rugged and simple. There are thousands of RV out there with a simple relay nothing more that keep the coach battery charged and isolated with zero issues. The alternator will provide you with all the charge you need for both batteries. The relay type also gives you the advantage of being able to give a boost to a weak truck battery from the secondary battery just by turning the key on. It is fool proof key off no connection between the two and very simple non invasive wiring.