Author Topic: Need help with isolator  (Read 642 times)

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Offline darthekai

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Need help with isolator
« on: December 08, 2018, 05:53:57 PM »
Hey I need a person with electrical background...
I have a ferd with 2 starter batteries and one "house" battery and a big ol alternator.
It has a sure power 2402 battery isolator.
I'm fairly sure it's hooked up wrong but I can't eliminate a "bad isolator" diagnosis.

With nothing hooked up to any of the 3 posts, a diode check shows 0.53 from center to bottom and 0.48 from center to top. It shows OL for the inverse. It shows OL for the case to any of the three terminals.

When stuffs hooked up the way it originally was, it shows a 0.57 result from the middle to the bottom and from the bottom to the middle!
Question 1 = is this maybe because the starter motor (on the circuit on bottom post) is in the circuit and it acts as a rectifier diode kind of?

OK so now I have got a typical manual for all SP Battery isolators and it seems to suggest that the middle post should only have the alt on it.

The way it was wired before was this: an aftermarket light on top post, the house battery AND the alt bat post on the middle and the starter solenoid circuit on the bottom post. After years of trouble free operation during which I never measured any battery voltages, the alt crappedout. Alt guy says the recitifer was blown and said we should check to make sure there's no fusible links in the system that might have gone. There aren't. He fixed it (new alt is like $1600) and I put it back in. The alt cranks out 15 and a bit volts and that's directly applied to the house battery! House battery becomes over charged (13.3v or something) and the starter batteries get 14.3v which is nice. I'm worried that a reason the alt crapped out in the first place that I haven't solved. A large part of me wants to move the house battery off the middle post and up to the top post.

So I do that. Bam no voltage at the alt bat terminal... Or at the middle post on the isolator. I don't understand.
What man put together, man can rend asunder and then also put it together sometimes what's all this left over stuff
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Offline darthekai

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Re: Need help with isolator
« Reply #1 on: January 06, 2019, 12:11:01 PM »
Well, I did some more screwing around.

Still pretty mysterious but ive got at least some repeatable results that make a little more sense.

Inside the ambulance, there is a dash module that controls siren, Thelma brake, lights, anti-theft, etc etc. For SOME uckfayed up reason, if I do not apply battery power to the cable I thought was direct from alt + post, this dash module does not power on. If the dash module isnt powered on, the alt will not output any volts.
Whats messed up is there is a little 2 wire harness coming from the back of the alt. (there is also two large gauge wires, one to frame and one is + output.
One wire is always hot to starter battery
one wire has some resistance to - post.... like 24.5 ohms....
If I had to guess, I would assume that the hot wire is the internal volt regulator reference... who knows what the other wire is?

But you would think, that if the above was true, it wouldnt matter wtf was going on at the isolator.... The alt should always crank up the volts until the starting batteries are up to 14.5V or whatever.

Very strange. I am going to call WORLD WIDE EAST SIDE starters and alternators and see if the dude can provide me a little insight as to how alternators work.
What man put together, man can rend asunder and then also put it together sometimes what's all this left over stuff
Subaru Ford (sold) Toyota

Offline diamondedge

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Re: Need help with isolator
« Reply #2 on: January 06, 2019, 03:51:35 PM »
Alternator would have a smaller low current circuit for energizing / excitation current. The residual magnetism wouldn't be strong enough to induce voltage once the rotor starts spin, once she gets going though the song rotor will build up the magnetic field and it should self sustain


I need to find my textbook

This small current I think comes from the ignition relay/switch or something

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Offline darthekai

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Re: Need help with isolator
« Reply #3 on: January 06, 2019, 07:46:02 PM »
So you think the harness has one wire for acc power for excitation and one wire for batt reference for the regulator?
What man put together, man can rend asunder and then also put it together sometimes what's all this left over stuff
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Offline diamondedge

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Re: Need help with isolator
« Reply #4 on: January 13, 2019, 12:25:46 PM »
So you think the harness has one wire for acc power for excitation and one wire for batt reference for the regulator?

Exactly. Excitation current should be small so that can be tested easily for current from the ignition switch. (Compared to the big juicy running current)

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