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Old July 18th 19, 07:45 PM posted to,
Arlen G. Holder Arlen G. Holder is offline
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First recorded activity by DIYBanter: Apr 2019
Posts: 134
Default How the heck does a typical home transfer switch work?

On Thu, 18 Jul 2019 08:11:52 -0700, Jeff Liebermann wrote:

Methinks your first problem is finding the box that controls the
automagic transfer switch (which contains the sensing, timing, and
switching logic). Something like this:
"Automatic Transfer Switch Controller Tutorial"
It's probably inside the generator enclosure. Just look for another
rats nest of wires. The thresholds, timing, interlock, etc settings
are usually adjustable.

Hi Jeff,
I agree with you, and I can clearly see that there is a rat's nest inside
the generator, which must be doing the initial sensing of the power in the
first place, as you mentioned.

I think my task is EASIER than looking at that rat's nest inside the
generator though, since all I need are the wires coming OUT of the

I'm not sure yet where to FIND those wires inside the transfer switch, but
I think the first (and only?) place I need to look is at what the pins of
that "plastic solenoid" do...
since that solenoid seems to send the purple and blue wire 12VDC to the
"double-fisted solenoid" to switch power from the mains to the generator.

I have two guesses:

1. The +12VDC that runs the relay is probably missing because the
starter battery in the generator is dead, or the fuse that protects it
is blown. That might explain the missing fuses. It might also be
that State Electric took one look at the mess and ran away. There
might be some lower voltage coming from the charger trying to charge a
dead (shorted) cell.

I understand and appreciate this assessment, where there's LOTS I didn't
mention (which is always the case in such things), mainly the fact being I
"think" it was me who pulled those fuses long ago and forgot to put them
back (I think I was testing them but I don't actually recall).

Also, you're actually correct that the battery in the generator WAS dead,
since I had disconnected it to charge it, and then I had left it
disconnected where the charge eventually bled off. I actually had to
jumpstart the generator when the power last went out, but I have since
charged the battery (I'm gonna put quick connect clamps on the battery at
some point, which will help in the charging process since I have multiple
spare batteries I swap in and out of that generator).

2. Every controller I've seen has a self-test and/or test-run
feature. You should be able to test the transfer switch with the
test-run button instead of reworking the wiring.

This is good to know, for two reasons:
1. This is dangerous stuff so having safe tests is required, and,
2. Most of what I read suggested testing MONTHLY (which is crazy frequent)

Also, I have some suggestions:

1. Don't play with the transfer switch with the utility AC power
applied. The life you save may be your own. The undersized wires
feeding the transfer switch should go to a double breaker on the main
panel. Flip it open, check that there is now no AC on the contacts or
anywhere in the rats nest of wires, and then troubleshoot.

Thanks for that advice, Jeff, as I'm well aware of the power, but I'm not
sure yet how to test a transfer switch. It does seem prudent to test the
transfer switch ISOLATED from BOTH the mains and the generator.

Preventing the generator from turning on should be easy as it has a power
switch and it requires the battery so it's easy to prevent it from turning

I'm not totally sure simply turning OFF the mains will isolate the transfer
switch - but that's simply because I'm currently ignorant of the wiring
diagram (which is one reason you said to do that first).

If the power goes from the power pole to the utility meter to the main 200
Amp breaker switch, and THEN to the transfer switch, then doing all tests
with the main 200Amp circuit breaker off is prudent. (Obviously I'd
doublecheck with the Fluke DMM.)

2. Draw as schematic diagram and label everything. If this were my
headache, that's the first thing I would do.

Yup. I agree. I had wanted from this question on Usenet to first get a
general idea of how these transfer switches work - which - I think I kind
of now have - but the exact wiring of every connection is still needed
before I can effectively troubleshoot.

Generac sent me the owners manual for my 09067-9 generator, which contains
exploded diagrams, for example, here's the exploded diagram of the 09067-9
Generator Control Panel:
And the wiring diagram for the 09067-9 generator itself:

And here's the exploded view for the 79848A transfer switch:
And the wiring diagram for the 79848A transfer switch:

3. If you know a local electrician, who won't turn you in to the
county, have him look at the wiring and make some recommendations.

Once, I called a well pump guy, and told him I had a problem, which he
fixed, but I told him before he came out to charge me based on the fact I
would be standing there right next to him as he debugged, asking him
questions while he did the work in front of me.

It was then that I noticed he simply replaced entire circuit boards, simply
by the process of pulling them out, putting the new one in, and finding
that it worked, so he was about to take the old circuit board "home" with
him, where I said if I'm paying for the new one, I want the old one (I
still have it). Some day I'll figure out specifically what's "wrong" with

A similar thing happened with the heater repairman, who simply replaced the
main circuit board, but he insisted that there was a core charge which _he_
wanted back - and that he's have to charge me for that - so he got the core
charge, not me.

I learned from that that these guys replace the entire board rather than
figure out what's wrong ON the board.

Given that replacing things seems to be what the repair techs do also, in
this case, I think I have three options, two of which are what many people
use, while the third option is the approach I'm currently trying:
1. Replace everything, one by one, until the damn thing works
2. Pay State Electric or Spiess Electric to fix it (in my presence)
3. Debug the damn thing (after first figuring out how it works)

Personally, I like to debug first, where simply UNDERSTANDING how the
circuit works usually causes the offending part to SCREAM OUT that it's

To that end, I'll follow your advice and start marking up the panel with a
label of the purpose of each of the myriad connections.