Suburban SF-30 furnace

[MGM53]

I have a 1999 Ford Gulf Stream Class C. I am having problems getting the furnace to come on. When the thermostat calls for heat, the fan doesn't come on. I tested the fan by hooking up a external 12 vdc source. The motor spins fine. I replace the time delay relay. I get 12 vdc to the relay, but not sure if the 12 vdc goes thru it. Is it possible that the new time delay relay is bad? I'm pretty sure I have the wires connected right according the words on the bottom of the relay and the wiring schematic on the motor. Here is a picture of it. BTW, I had changed out the Coleman Mach thermostat with a Honeywell digital thermostat.

[Old-Biscuit]

12VDC + from a fused source goes to TDR (Red)
*Also ties into Wire that goes to Thermostat (Blue)


Thermostat closes and sends the 12VDC+ back to TDR (Blue)
*also ties into wire that sends 12VDC+ to Sail Switch/Limit Switch (White)
Opposite terminal goes to Ground (green)

TDR closes sending 12VDC+ to FAN Motor (Red)

[MGM53]

So the blue/white wire and the ground wire is the coil part of the TDR? Once the coil heats up, it allows 12 VDC+ to the motor? Since I had switched out the Coleman mach thermostat to a digital one. I noticed all the blue wires are wire nutted together. The Coleman mach thermostat did have a blue wire connected to it. The digital thermostat doesn't have a blue wire connected to it.

[Old-Biscuit]

Blue/White
Blue is DC coming back from Thermostat when it closes calling for heat
White is DC to sail switch
Green ground

DC heats the internal coil allowing DC on Red to go to Fan Motor
Fan comes up to speed closing sail switch...which then sends DC to circuit board for ignition

[Persistent]

Some furnaces check for safety switch positions before starting the fan. Maybe the sail switch or over temperature switch is stuck in the "on" or "off" position respectively.

[Old-Biscuit]

Originally Posted by Persistent

Some furnaces check for safety switch positions before starting the fan. Maybe the sail switch or over temperature switch is stuck in the "on" or "off" position respectively.

Furnaces that have circuit boards with integrated time delay can check (not all do)
Furnaces with stand alone TDR do not

[Old-Biscuit]

Originally Posted by Old-Biscuit

Blue/White
Blue is DC coming back from Thermostat when it closes calling for heat
White is DC to sail switch
Green ground

DC heats the internal coil allowing DC on Red to go to Fan Motor
Fan comes up to speed closing sail switch...which then sends DC to circuit board for ignition

DISREGARD THE Picture/Diagram....I POSTED them in error
DC Relay/AC Circuits for 120VAC Furnace
NOT TDR for 12VDC Furnaces



The posted info is Correct.

[MGM53]

Does the two blue wires at the thermostat have to be connected to the thermostat? Can they just be connected via wire nut?

[Old-Biscuit]

Originally Posted by MGM53

Does the two blue wires at the thermostat have to be connected to the thermostat? Can they just be connected via wire nut?

For a TEST .......YES

DC from furnace goes to thermostat via one of the blue wires
When thermostat closes it sends that DC back to furnace

Connecting the 2 blue wires together will complete the DC circuit bypassing the thermostat which is just a switch



What brand/model of Digital T-stat did you install to replace the Coleman t-stat?????

[MGM53]

I got a Honeywell RTHL3550D thermostat. I understand the theory of the two blue wires. I'm just a little confused on the part of the blue wires connecting to the thermostat. I am a retired service tech. I worked on HVAC system for over 30 years. Seems like it is easier to work on a residential furnace than a RV furnace.

[Old-Biscuit]

How many wires connected to the Coleman Mach T-stat?
What colors were they?

[MGM53]

I had the basic colors. Red(12vdc), green for fan(high speed), grey for fan(low speed), yellow(AC), white(heat), and blue. I connected all wires to the digital thermostat except blue and grey, I just taped them off.

[Old-Biscuit]

Originally Posted by MGM53

I had the basic colors. Red(12vdc), green for fan(high speed), grey for fan(low speed), yellow(AC), white(heat), and blue. I connected all wires to the digital thermostat except blue and grey, I just taped them off.

Blue ----12VDC Neg

Wiring AT t-stat rarely matches up with wiring AT Furnace

Pg 21 has wiring for SF using Time Delay Relay
http://techsupport.pdxrvwholesale.co...ICE-MANUAL.pdf

Check in A/C return for control box and DC wiring to furnace

[MGM53]

I looked in the control box at the A/C. Everything looks normal there.Since I have a older model of furnace. I'm thinking I need to find out where the blue wire would go on the digital thermostat.