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Isolator Diode


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Hi all! Newbie here who just bought a 1984 sunrader on the Nissan 720 chassis. I drove it from Salt Lake City Utah to Gainesville Georgia without a hiccup until I got to Georgia. Now I’ve replaced the alternator and battery and alt is not charging the system. I have stalled multiple times in the last week. I just took the alternator out and had it tested to make sure It’s working properly. It passed the system check. 
 

I went into the Nissan 720 forums to ask about the alternator/battery light not coming on when car key is in start position and was told that it needs to be on in order for alternator to charge battery. 
 

my question now... could it be a bad isolator diode?  As you can see mine is definitely looking a little cracked and worn. 
 

would a bad diode make it so that the alternator light wouldn’t come on in the dash? 

5A827717-8A81-4833-85D1-BF5DC843F49E.jpeg

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GET A VOLT METER, learn how to use it!!!  Harbor Freight has them for $7

The isolator has 3 wires. The center wire should come from the alternator. One of the outer wires should go to the truck battery and the other to the house battery. Figure out which is which.

THEN bolt the truck battery wire to the center post with the alternator wire, this is an emergency thing that will let the alternator charge the truck battery even if the isolator is bad.

Use your volt meter to check the voltage at the truck battery with the engine running, should be 14v +. On an old RV a voltmeter is the most important tool you can have. There will be weird wiring from so many previously owners

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Probably not. You do however have a mess there with all of the circuit breakers (little square boxes) so I can't begin to tell you where they are supposed to go. The light should come on with the key. I'm not real familiar with the Nissan alt but they all work pretty much the same. When the key is on engine not running the light looks at the alt as a ground. When the voltage rises the voltage become positive so the ground is not longer there and the light goes out it's part of the equation so it has to light. What you can try is a jumper from the truck battery + to the alt + (large terminal) to bypass the circuit breakers and see what happens. Usually the center post on the isolator is alt + one goes to the camper battery the other to the truck battery. They are like a one valve they only let current flow in one direction. Wish I could help you more but I can't see it from my house.

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19 minutes ago, WME said:

GET A VOLT METER, learn how to use it!!!  Harbor Freight has them for $7

The isolator has 3 wires. The center wire should come from the alternator. One of the outer wires should go to the truck battery and the other to the house battery. Figure out which is which.

THEN bolt the truck battery wire to the center post with the alternator wire, this is an emergency thing that will let the alternator charge the truck battery even if the isolator is bad.

Use your volt meter to check the voltage at the truck battery with the engine running, should be 14v +. On an old RV a voltmeter is the most important tool you can have. There will be weird wiring from so many previously owners

I have a volt meter but need help with how to use it. I will try your setup. I know the wire on the left goes to the truck batter and the one in the middle is directly connected to the alternator and the wire on the right is the one going to the motorhome batteries (yes, there are two of them. One in then engine bay on drivers side and one under the seat in motorhome). 

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8 minutes ago, Maineah said:

Probably not. You do however have a mess there with all of the circuit breakers (little square boxes) so I can't begin to tell you where they are supposed to go. The light should come on with the key. I'm not real familiar with the Nissan alt but they all work pretty much the same. When the key is on engine not running the light looks at the alt as a ground. When the voltage rises the voltage become positive so the ground is not longer there and the light goes out it's part of the equation so it has to light. What you can try is a jumper from the truck battery + to the alt + (large terminal) to bypass the circuit breakers and see what happens. Usually the center post on the isolator is alt + one goes to the camper battery the other to the truck battery. They are like a one valve they only let current flow in one direction. Wish I could help you more but I can't see it from my house.

So those boxes that say 12 volt on them with the red wires attached are circuit breakers? Ugh. I want to make this electrical system simple and go back to what the system originally was. 

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18 minutes ago, Maineah said:

Probably not. You do however have a mess there with all of the circuit breakers (little square boxes) so I can't begin to tell you where they are supposed to go. The light should come on with the key. I'm not real familiar with the Nissan alt but they all work pretty much the same. When the key is on engine not running the light looks at the alt as a ground. When the voltage rises the voltage become positive so the ground is not longer there and the light goes out it's part of the equation so it has to light. What you can try is a jumper from the truck battery + to the alt + (large terminal) to bypass the circuit breakers and see what happens. Usually the center post on the isolator is alt + one goes to the camper battery the other to the truck battery. They are like a one valve they only let current flow in one direction. Wish I could help you more but I can't see it from my house.

I just did what you recommended and voila. Turn the ignition to start and the battery/alt light came on. So... process of elimination would lead me to believe that the diode is no longer directing electricity to the truck battery? Am I correct? 

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Post a pic of your voltmeter.

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The diode is a one way valve it only allows current to flow one direction so the power flows from the alt to the battery provided all is well but not the other way around. Here is where a test light comes in, a voltmeter has almost zero resistance so it can give you a false reading showing no load voltage. If there is current flow the test light will show it because it presents a load that a meter won't. What you have done shows the alt does indeed work now you have to find why it doesn't get back to the battery my thinking it's in that mess of circuit breakers there seem to be some dangling wires to unknown places.

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19 minutes ago, WME said:

Very basic wiring diagram for a diode isolator...https://www.etrailer.com/question-259273.html

Awesome! This makes total sense. It is pretty much what my system looks like. The left post is connected to two batteries that go to the house. So if I was to get something like this —-> https://www.amazon.com/dp/B000OTIPDQ/ref=cm_sw_r_sms_api_fabt1_PvTUFbDZRC6DX?_encoding=UTF8&psc=1&pldnSite=1

 

would I be making things more tidy and less complex? 

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You have a AC/DC voltmeter, an OHM meter (measure resistance) an a milliamp meter all in one box.😁

To use the voltmeter around the RV , turn the dial to 50 DC volts (straight line over 3 dots). RED lead to red+ right lower. Black lead to  - lower left. You will read the voltage on the black scale that goes from 0 to 50. 14v is 4 slashes past 10. Analog meters require a skeptical eye

To check your alternator and everything else, start the engine and measure from the center post of the isolator to ground. Use the engine as ground.

Engine running you should have 14v+ there. Then from each outer post to ground. They should have .7v less than the center post. I.E. center post 14.4v, then 13.7 at outer posts. Then measure at the + post of each battery, you should have less than a .2v drop. Your DC fuse panel should have house battery voltage..

Measure across each of the circuit breakers, post to post should be 0v, post to ground should be isolator outer post v.

V loss can be due to dirty connections, corroded connector crimps, broken wires, inside the insulation. Battery charge voltage is very important.

Use OHM function (humped worm symbol). Use the X1K range, short the leads together and use the thumb dial on the side to adjust the needle to 0 on the OHM scale (green scale)  Then check from the engine block to the - on the battery. Should be 0

To measure shore power, you use the 250v AC range (AC=wiggly line)

 

Just things, weird house/truck light problems are very often bad ground connections, follow the wires and see where they connect to the truck chassis for ground.

Have fun

 

Edited by WME
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Stick with the diode one if you decide to replace it they are much simpler. Their down fall is a slight voltage drop across them but it's no big deal. Dead stationary you should have battery voltage on pins 1 and 2. Now I hate to throw this into the mix but I believe your 84 has a external regulator including a wire for the charge light in this case it's pretty important it is what wakes up the alternator. https://www.google.com/search?q=84+toyota+pickup+alternator+wiring&rlz=1C1SQJL_enUS891US891&tbm=isch&source=iu&ictx=1&fir=_cSYe2n8QedMpM%2CQO9c5xYUcmh5dM%2C_&vet=1&usg=AI4_-kSpVqY4mhe07nE3eQBLDow15Peibw&sa=X&ved=2ahUKEwjjrJCd7ZjtAhWTsDEKHUZYBI8Q9QF6BAgEEDQ&biw=1280&bih=578#imgrc=0bpOuqdb6XfQLM

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Nissan 720, not Toyota PU

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Up's well that opens a different attack plan. Two possibilities fuse #4 and fuse #5 is a good start because there is no indicator light. Fuse 5 is not applicable if it's not fuel injected. The alt has a fusible wire link going directly to the battery from the alt B+ it should be close to the battery very soft isolation they often had a flag on them with an amp rating. Fuse 4 is the exciter feed to the alt. it is what kick starts the alt. when the key is on the warning light sees the alt as a ground when the alt starts working the voltage climbs to match battery voltage and the light goes out because it no longer is a ground so it's a key player in getting the thing charging. The fuse link is always hot and feed B+ terminal on the alt.

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