[bikestuff]

Gurus,

I think I am the first to have an basement AC SCS control board burn up with the external relay fix already implemented.

Right now, I am in the early stages of investigation but here are the highlights so far.

I have been away from the coach for a couple of weeks.   I found that the front AC was not running.   Checking the breaker showed that the #1 circuit of the front AC was tripped.   I foolishly simply tried to reset the breaker and it acted weirdly.  After a couple of tries it reset and stayed that way.  (I will talk about this more down below).

I went to the AC unit and pulled off the metal cover where the control board is.

Here is what I found:
   

Note the burned spot in the upper left corner where the AC for the #1 circuit enters the board.

I turned off both breakers to the front AC unit and removed the board:
           

It’s clear that there was a melt down on the AC connector.  Also, the board seems to have already been repaired since one of the traces has been beefed up.

Here is where it gets weird….

The bottom white connector where the #2 circuit enters the board was also loose!!!!  (Like barely hanging on) but there was no visible scoring from arcing. 

Newell has a company that is manufacturing replacement boards for the old SCS AC units.   They are pretty pricy at around $700…. But they are much more robust than the original SCS boards.

   

So…what caused the board to melt?  In my mind there are two possible scenarios.
1) The white connectors had bad solder joints and just gave up.  This caused heat to build up and eventually catch fire.   
2) Either of the fan motors are pulling more amps than they should and the excess current burned the board.

Remember that I already have the external relays so any excess draw from the compressors will be protected by the breaker in the breaker panel.

Since the current for the fan motors still goes through the board, I attempted to troubleshoot them.  I measured the resistance of the wires that go to the evaporator blower (inside) and condenser fan (outside).   Here are the results:

EVAP
C - L (red to white) 6.0 Ohms
H - L (Red to black) 2.6 Ohms
H - C (Black to white) 3.4 Ohms

Condenser
C - L (red to white) 8.0 Ohms
H - L (Red to black) 4.1 Ohms
H - C (Black to white) 4.1 Ohms

All wires showed open between the wire and the chassis.  

My amateur view of these readings say that the motors look good.   No shorts, and no opens.  It looks to me that the SCS board just decided that life was too hard and self destructed.

My plan is to install the Newell version and then measure the current draw on the fans motors as well as the compressors.   I will keep ya’ll posted on the results.

Cheers,
Bill

Ps.  About the breaker.  I should not have tried to reset the breaker until I at least looked in the wiring box of the AC unit.  I know that breakers fail….but if I had it to do over, I would not have been so quick to reset.   It only took a couple of minutes to get into the wiring box and find the burned wire.

[bikestuff]

Gurus,

I said that in the earlier post that the bottom 120V input to the board (the one that did not burn up) was loose.   Here is an image of one of the solder joints on that connector.  

   

Do you see any issue?  (Just kidding). It is obvious that this was a cold solder joint.

My question is…. WHY?

This looks like a mechanical break to me.   Like something was flexing the joint until it broke.  The wires where neatly arranged in the junction box and I cannot imagine that road vibration did this.   I am stumped.

Cheers,
Bill

[Richard]

Hmmm.

Here is some data I collected from the two fan motor spares I carry. I don’t know which is evap or condenser.

MTR 1

W - R. Lo. 8.1
W - B. Hi. 4.1
R - B. 4.1


MTR 2 (this is a three speed MTR)

W - R 7.5
W - B 3.3
W- Blu 6.1

At the next opportunity, I would test all the start caps for the fan motors. The breaker trip, and the heat that caused the burn look like a direct short to me. And I am going to pull the other board for a look see.

I would also test all the hot legs to ground to check for a short somewhere else. Check the leads from the board to the external relays

As far as the bad joint. It could be vibration. It could be the torque applied to tighten the screws, after all the solder joint bears the force of tightening the screws. It could damage from before the relays were applied. The joint you pictured looks more like melted solder than a solder fracture to my eye. I base that on the rounded edges, but that is speculation.

I saw pictures of the board that Newell first had made, and those pictures are different than the board you pictured. Not saying that is bad, just that is not the 1st board that Newell had made.

[bikestuff]

Here is a picture of the back of the new board.  The traces seem to be pretty thick.  The label on the board says it is Rev 1.5

   

Richard,  I think you are onto something.  If someone torqued down on the screws holding the input 120V, they could have cracked the two tiny leads that hold those huge white connectors to the board.

Word to the wise:  If you find yourself looking at the AC board….(with power off)…wiggle the white connectors.  If they move, you might have a loose connection.

[PMCBR]

SCS air conditioner control board professionally reconditioned. From 1997 Newell basement a/c. $500.00

[Latitude 28]

Bill,
A couple of things related to your board issue.  In the past I had my board(s) in and out several times to change capacitors on both units.  I am pretty sure I had tightened my main connections, evidenced by the unit working (did not measure amp draw). The driver side unit quit working after a couple of trips.  Opened the panel up and found the neutral wire had worked loose.  Tightened it up and all was good to go. 
The next thing, I just completed my relay mod this season.  Found the main white nylon lug on both boards were loose and wiggled a bit.  Checked the solder joints and tightened the small nylon bolt inside the middle of the nylon main terminal which made everything solid again.
The Newell board looks real solid would like to hear how it works out.
Steve

[bikestuff]

Steve,

I am 90% sure that none of my terminals had any support other than the solder joints themselves. It is interesting that you had nylon bolts on yours. The new Newell board has 4 solder joints on each plastic terminal.

Thanks,
Bill

[Latitude 28]

Bill,

Here is a picture I took when doing the relay mod sorry I don't have one a little closer but it will give you an idea of my situation.

   

[bikestuff]

Steve,

I think you solved part of the mystery.  I just looked at the burned board and there is no nylon screw on either of the 120V terminals.  Who knows where / when they came out but the connection is definitely not robust without some sort of physical assist to the solder joints.

Gurus….IF YOU HAVE SCS basement AC units, check that you have the nylon screw holding the 120V power terminals to the board.  Missing them could cause you a fire. 

Thanks Steve!
Bill

[Richard]

Just to add a little armchair, hindsight, now I see, engineering to this issue. The problem is exacerbated by the fact that the wiring connected to the terminals is either 12 or 14g solid wire, meaning it is stiff. Any vibration in the wiring is going to transmit to the terminal and onto the solder joint.

A solution could be to use what is commonly referred to as hook up wire, very flexible multi strand wire of the same gauge. You could either just splice it, or install an electrical terminal board with the solid into the terminal strip and the hook up wire between the strip and the board.

I like the euro style with protected screws. https://www.amazon.com/dp/B085W3RGPR/ref...asin_1_img

Or the standby. https://www.amazon.com/ALUM-COPPR-3PORT1...281&sr=1-3