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Old 04-17-2016, 11:05 AM   #21
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I can't remember what thread I was getting my info from but I had read that you don't want a spdt switch. The reason was that your low speed fan wire is always connected to the terminal. If you wanted permanent high speed you could also attach it to the terminal. Or in my case put a switch that adds the high speed voltage when I wanted it
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Old 04-17-2016, 11:30 AM   #22
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Originally Posted by jdrain1 View Post
I can't remember what thread I was getting my info from but I had read that you don't want a spdt switch. The reason was that your low speed fan wire is always connected to the terminal. If you wanted permanent high speed you could also attach it to the terminal. Or in my case put a switch that adds the high speed voltage when I wanted it
There is a flaw in that theory. You can not have power to both windings of the motor at the same time. Remember that the subase will do the switching. If your low speed wire is hooked to the G terminal that will be energized through your subase when you call for cooling your subase not you it will supply power to both the y for compressor and green for fan. If you add another wire to G you will try and bring on both speeds at the same time with a SPST switch, You have nothing to drop out the low speed. Either your going to short out the subase or burn the motor up. hope that helps... with a stat that has high and low fan speeds the subase will switch to one or the other settings but not to both at the same time....
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Old 04-17-2016, 06:12 PM   #23
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Originally Posted by 5picker View Post
Makes sense.
Still needs to be SPDT and not the SPST the OP's using.
Yes, you're correct, still need a SPDT. Here's one. Pick your color:

JR PRODUCTS 12625 - Jr Products 12 Volt Black On/On 12625 - RV Plus

Here's another source:

JR Products On/On Switch, Black SPDT 12625

The black faceplate is #12855
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Old 04-18-2016, 10:09 AM   #24
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Quote:
Originally Posted by jdrain1 View Post
I can't remember what thread I was getting my info from but I had read that you don't want a spdt switch. The reason was that your low speed fan wire is always connected to the terminal. If you wanted permanent high speed you could also attach it to the terminal. Or in my case put a switch that adds the high speed voltage when I wanted it
This is correct. You don't want a SPDT, you want a SPST.

The fan leads aren't supplying power to the fan motor directly, it's supplying ground to a set of relays and they're wired so that when only the low speed is connected one relay is energized and that sends power to the low speed windings. Then when you connect the high speed wire it energizes the second relay which breaks power to the low speed and connects power to the high speed windings. The relays are wired in a pseudo-series so that in order to get high speed both low speed and high speed wires have to have to be grounded.

Here's the test procedure from the service manual. (Added bold text for emphasis.)

Quote:
9.3 Analog Tips
Analog control test for Compressor, Fan speeds and Furnace.
All analog controls have 6 or 7 wires from control
board to T-Stat. This test will by-pass the t-stat and harness
in the wall. Disconnect all wires leaning from power
module to thermostat. Check incoming AC and DC to control
board for proper voltage. Follow the tips listed below.
When the ground wire from the control system is jumped
to the listed wire, it should close the relay and activate
that function. The thermostat provides a ground to close
a relay.
Using the wires from the control to test the following

Green plus Tan = Low Fan
Green plus Tan & Blue = High Fan
Green plus White = Furnace
Green plus Yellow = Compressor

Green (ground) to the proper wire should activate the
function from the control system.
If the item ( fan, compressor, furnace, heat strip) comes on
the control module is OK the problem lies in the wires or tstat.
If there is still a problem go to control board testing.
Reference Dometic bulletin A27-8C
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Old 04-18-2016, 11:00 AM   #25
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Originally Posted by Bama Rambler View Post
This is correct. You don't want a SPDT, you want a SPST.

The fan leads aren't supplying power to the fan motor directly, it's supplying ground to a set of relays and they're wired so that when only the low speed is connected one relay is energized and that sends power to the low speed windings. Then when you connect the high speed wire it energizes the second relay which breaks power to the low speed and connects power to the high speed windings. The relays are wired in a pseudo-series so that in order to get high speed both low speed and high speed wires have to have to be grounded.

Here's the test procedure from the service manual. (Added bold text for emphasis.)
I have to disagree because you are wrong. You need a SPDT. They come in two styles: one where there are two positions (ON/ON) and one with a center OFF position. (You don't want center off.) See schematic pic here:

https://www.google.com/search?q=sing...A92o_lViCJM%3A

Then see mine in the attached PDF. It is INSTALLED and WORKING.

This is similar to the one I used:

https://www.radioshack.com/products/...ant=5717515077

Here it is installed:
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File Type: pdf Thermostsat wiring.pdf (121.6 KB, 13 views)
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Old 04-18-2016, 02:14 PM   #26
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But, you have a Coleman and the OP has a Dometic. Different animals!
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Old 04-18-2016, 06:50 PM   #27
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Originally Posted by Bama Rambler View Post
But, you have a Coleman and the OP has a Dometic. Different animals!
The subase that the OP showed doesn't have a ground terminal? Either your relay is N/O or N/C they make no other relay that I know of. Only thing that will melt batteries are a short. Please explain to me what is a "Pseudo-Series" relay is. Yes dometic is different. But by your post and looking at his subase there is no common or ground terminal. make sure your not looking at the HP connections alot of HP's require a ground and supply a terminal for that. Where are you saying this ground wire is supposed to connect to on the subase if I understand you correctly, just so I can rap my mind around this, not saying your wrong, but please define...How would you wire the honeywell stat?...
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Old 04-18-2016, 07:25 PM   #28
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Originally Posted by Bama Rambler View Post
But, you have a Coleman and the OP has a Dometic. Different animals!
Maybe I'm wrong, but looking at his original wiring, there's a high speed fan wire and a low speed fan wire. You have to supply power to one of them at a time. That requires the SPDT. Unless the fan requires ONE wire to be supplied with power for one speed and BOTH wires to be supplied with power for the other speed (doubtful - I've never heard of it), I believe I'm correct.

UPDATE: I may stand corrected. Found this thread that does show switching in BOTH fan leads with a SPST switch for a Dometic. So it would appear it DOES require both leads powered for one of the speeds. Weird.

http://www.modmyrv.com/2008/07/14/rv-digital-thermostat
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Old 04-19-2016, 10:53 AM   #29
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Originally Posted by gljurczyk View Post
The subase that the OP showed doesn't have a ground terminal? Either your relay is N/O or N/C they make no other relay that I know of. Only thing that will melt batteries are a short. Please explain to me what is a "Pseudo-Series" relay is. Yes dometic is different. But by your post and looking at his subase there is no common or ground terminal. make sure your not looking at the HP connections alot of HP's require a ground and supply a terminal for that. Where are you saying this ground wire is supposed to connect to on the subase if I understand you correctly, just so I can rap my mind around this, not saying your wrong, but please define...How would you wire the honeywell stat?...
Lets see if I can explain this. The t-stats in question are basically a switch (actually several switches). it doesn't care whether it's switching a hot or a negative. It's just making contact between two wires. The engineers at Dometic in their infinite wisdom decided to switch the negative (ground) wire to control the functions, so even though there is no ground terminal per-se, you don't need one. The ground just takes the place of the hot in normal installations.

Switching the negative is pretty common in the automotive industry, and that may be the background of the engineer(s) that designed this system for Dometic.

At one point I had a schematic of the relays in the control box. and it was wired so that in order to get the fan to run at high speed you had to have both low and high speed wires connected to ground. The 'pseudo-series' term I made up because the way they're connected you have to have the low speed relay energized in order to get power to the high speed relay. Which then breaks the low speed power to the fan motor and supplies power to high speed windings.

I hope that's clear as mud.
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Old 04-19-2016, 11:36 AM   #30
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Originally Posted by Bama Rambler View Post
Lets see if I can explain this. The t-stats in question are basically a switch (actually several switches). it doesn't care whether it's switching a hot or a negative. It's just making contact between two wires. The engineers at Dometic in their infinite wisdom decided to switch the negative (ground) wire to control the functions, so even though there is no ground terminal per-se, you don't need one. The ground just takes the place of the hot in normal installations.

Switching the negative is pretty common in the automotive industry, and that may be the background of the engineer(s) that designed this system for Dometic.

At one point I had a schematic of the relays in the control box. and it was wired so that in order to get the fan to run at high speed you had to have both low and high speed wires connected to ground. The 'pseudo-series' term I made up because the way they're connected you have to have the low speed relay energized in order to get power to the high speed relay. Which then breaks the low speed power to the fan motor and supplies power to high speed windings.

I hope that's clear as mud.
Don't know if it helps gljurczyk, but I get it now. I know 70-80's Chevy's used to switch their grounds for some things (windshield wipers, for example). Always seems like a dumb way to do things to me, though. As a former aircraft electrician, you don't switch grounds, you use the air frame as the ground wire. Of course, Chevy's only recently started worrying about weight!
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