Troubleshooting Heating – Older Sunline Units

Continuing with my series on troubleshooting packaged units prior to the introduction of Simplicity Controls with diagnostic flash codes, the older Sunline units used a Johnson G67, G770, and/or G775 ignition control.  There were no diagnostic flash codes or lights on this control or on the lockout relay board.  One of the biggest ‘mistakes” a tech could make was recycling the power or thermostat before going on the roof and doing a few checks.  If power was recycled or the thermostat was reset, by the time a tech gets to the roof, everything may be working.  Below is a step by step approach to help diagnose “no heat” calls on the Sunline units.  This also works for the new UTEC replacement control for the older Johnson “G” series ignition controls.


  1. When arriving at a NO HEAT CALL, first check the thermostat to assure that it is set for HEAT and is calling for heating.  DO NOT shut off or move the switches on the stat, as this will reset the control in the unit. 
  2. Without interrupting the power to the unit, carefully remove the access panel to the heating section. Check for the following:
    1. indoor blower is operating
    2. combustion blower is running
    3. unit is NOT firing 
  3. If the above conditions have been met, the unit is in a lockout.  Only 4 things can cause a lockout condition:
    1. A rollout has occurred
    2. The unit has proved and lost pilot flame signal.
    3. The centrifugal switch on the end of the inducer has stuck open.
    4. Either the operating limit or auxiliary manual reset limit has opened. (on the older units, these were located behind the horizontal duct cover for the heating section)

 BEFORE SHUTTING THE POWER OFF TO THE UNIT, if the centrifugal switch is stuck open or the limit is open, simply check for 24 volts at terminal THS and ground on the ignition control. If power is NOT present at this point, the centrifugal switch, limit, or auxiliary limit is open. These safeties are wired in series to the ignition control. Check to see which safety is holding the unit out. Keep in mind, the auxiliary limit is a MANUAL RESET LIMIT. This has been in units for the last 10 to 12 years.  Units produced before this time only have the operating auto-reset limit.  If 24 volts IS present, the control is in a lock out due to flame failure or roll out. 

If the unit is off on roll out switch, check for 24 volts between R1 and ground on the ignition control.  If NO voltage is found, the roll out switch is open. Cycle power to the unit; let the roll out close and check to see if a roll out condition exists.  Correct as necessary. 

If 24 volts is found, the problem could still be in the roll out switch.  This is an automatic reset roll out and it may have reset itself before you arrived.  The ROLL OUT WILL put the ignition control into a lock out.  Recycle power to the unit and, again, check for a roll out condition.  Also, disconnect the wires from the roll out and, using an ohmmeter, check across the terminals.  It should read “0” ohms (switch closed).  If you read anything other than “open” or “closed” roll out, the contacts in the switch could be corroded causing a nuisance roll out lock out at the control. (read my post on RTU nuisance lock-outs). Replace the roll out switch.

 If the lock out has occurred due to loss of pilot flame signal, 24 volts will be present at THS and RO but not present at PV or MV.  Several things can cause this problem:

  1.  Check for a loose or corroded connection at the Sense #4 terminal on the control.
  2.   Verify that the flame sensor is no more than 1/8”  away from the pilot runner tube. Make sure the  sensor is not touching the runner tube. Check porcelain insulator for cracks or shorts to ground.
  3. Check the igniter position. Check  the igniter just as you did the sensor above.
  4. Recycle power and listen for the spark sound.  If no spark, check the “nail” connection on the ignition control and the end of the ignition wire to make sure it is making contact with the nail connection on the control. Cut the ignition wire back and make sure you can see ‘wire” and then re-attach to the “nail” connection.
  5. Verify that the burner assembly is properly grounded.  It should have a green wire from the ground strip on the ignition control and the gas valve.
  6. Pull the burner assembly, remove the runner tube, remove the orifice, and make sure it is  clean.  Take a piece of #14 gauge stranded wire  and push it through the runner tube making sure   there are no obstructions inside the tube such as spiders and webs.
  7. Recycle power and see if 24 volts is present at PV at the control with the gas valve wires removed.  If it is not present and power is present at THS and RO, REPLACE THE control. If power is present at PV, put the wires back on  PV & MV and check for 24 volts at the  terminals of the gas valve. 
  8. If power is present at the gas valve terminal, and no gas is going to the runner tube, verify that gas is present at the unit.  If gas is present on the inlet side of the valve, check the pilot pressure. Pilot pressure on the runner tube should be 4 to 5 IWC as it is unregulated.  There is a needle valve for the pilot and turning it clock wise decreases pressure, while turning it counter clock wise increases pressure. The opposite of a pressure regulator.
  9.   If power is present at the valve and we have spark but no pilot gas, replace the gas valve
  10. If we have spark, pilot gas, a pilot flame but no main burner, we have not proved pilot flame.  Nominal micro-amps should be   .5 µa . If the control does not see the signal, it will not send power to MV to open the main valve.  If the proper micro-amps are present and there ins no power to MV, replace the control.  If 24 is present at MV and the gas valve and it does not open, replace the valve.

Verify both inlet and outlet pressure of the gas valve.

  1. On all current units, the correct manifold pressure is 3.5 IWC.  On some older 7 ½ ton through 12 ½ ton units, with two stage valves,  High fire manifold pressure should be set at 4.1 IWC.
  2. Inlet pressure should not exceed 13 IWC.

 If the combustion blower does not come up to speed or is not running, power will not be present at THS on the ignition control. The blower must spin at 2500 RPM before the switch will close.  If the motor is running and the switch does not close, replace the combustion blower motor. 

If the combustion blower is NOT running and the thermostat is calling for heat, verify that the DMC contacts are closed and power is present at the blower.  If voltage is not present, check for 24 volts at terminals R and G on the DMC (on older units, there was a draft motor relay instead of a solid state board. Check for 24 volts at the coil of the draft motor relay.).  If 24 volts is present, replace the DMC or relay.

If the main blower is not running or the belt has broken on the blower, the unit will shut off on the limit or the auxiliary limit. Place the “fan on/auto” switch on the thermostat to the ON position.  If the blower does not run, check to see if 24 volts is present at the blower contactor (either 2M or 3M depending on the unit size). If 24 volts is not present at the coil, replace the lock out relay (thermostat) board.

If 24 volts is present, and power is present at the load side of the contactor, replace the blower motor.

 Place the fan switch back into the “Auto” position and cycle the unit to call for heat.  Within 30 seconds of a call for heat, the blower should start.  On older units, there is a time delay relay in the unit.  Check for 24 volts at the coil of the relay.  If power is present, check to see if the contacts of the relay are closing. If they do not close, replace the time delay relay.  In extremely cold weather, since this was a “warp” switch relay, the contacts may be slow closing or may not close due to the cold ambient temperatures.  These older units can be retrofitted with the newer solid state blower timer used in current production. If the contacts close, check for power at the blower contactor coil. If power is present, replace the contactor.

 On newer units, the blower is controlled by a solid state blower timer (BT) in the unit, At the initial call for heat, 24 volts should be present at terminals #2 and #5.  After 30 seconds, 24 volts should be present at terminal # 1 and ground.  If it is not, replace the BT timer.

Take the time to do them step by step.  Make use of the wiring diagram if necessary. Hopefully, these checks will help you diagnose heating problems on our older units.

About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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16 Responses to Troubleshooting Heating – Older Sunline Units

  1. Tim says:

    I have noticed some grounding issues when switching from Johnson control to ut control have had to add a new ground

    • if you follow the instructions with the UT control, yiou do need to run a ground and that is why the green wire is in the kit. The ground not only acts as the “common” side of the 24 VAC, but it also is critical for flame rectification

    • Tim Conway says:

      Is there an adapter tee available to check pilot pressure and adjust as the unit is sparking.? the pilot line is 7/16-27 thread try and find a fitting to adapt to it.

      • Tim — I made one out of 1/4″ aluminum tubing, a “snap-off” fitting for the gas valve side, and a compression tee. Then, just unscrew the pilot line ouit of the gas valve and attach it to one part of the tee with a piece of rubber hose and hook your manometer up to the branch of the tee that’s left. crew your tee into the valve, hook up you hoses and turn it on.

  2. dadtoaaaaaa says:

    Helped a tech in the field get a unit back online using your article…Thanks for sharing

  3. Dan says:

    I have an intermittent issue with one of these units where the ignition spark stops sparking as the pilot valve is energized. It only sparks for about a second then locks out. I have replaced the ig. Module and checked the pilot orifice. Also cheked connections and grounding. When it does stay sparking it lights pilot and main valve without issue. Have you come across this before?

    • what is your inlet gas pressure to the unit? If pressure is too high, the pilot valve may be trying to open but can’t against the high pressure. The result is high amp draw dropping the voltage which will lock out the control. The other possibility is a bad gas valve with a weak pilot solenoid.

      • Dan Fraser says:

        Replaced the gas valve and it worked for a couple weeks and then locked out again. It’s propane fired inlet is 11″ . Manifold is 9.5″ I’ve ordered a flame sensor and ignitor though it’s lights well when I cycle it on and off.

      • if the unit goes into a “hard lock-out” nd you have to recycle power to get it to start, the problem could be the rollout switch above the b burners. Ohm out the switch — if you show any resistance other than a “dead short”, there is corrosion in the switch dropping voltage to the R1 terminal on the ignition control and it is going into a lock-out. Also — be sure to check for a cracked heat exchanger actually causing a “roll out” condition.

  4. Ron Roberts says:

    I’m having an issue on a York package rtu…I replaced the gas valve, spark rod, and flame sensing rod. I have good has pressure and spark but no flame…3.5 ” wg going thru to the tubes and the spark rod only sits in there one way…it did try to light a couple times and I noticed the flame kinda pulsing and weak. Not strong enough to establish a good flame signal. Any suggestions? Ron

    • Check the pilot pressure — Pilot pressure on the runner tube should be 4 to 5 IWC as it is unregulated. There is a needle valve for the pilot and turning it clockwise decreases pressure, while turning it counter clockwise increases pressure. The opposite of a pressure regulator. also, make sure there is no restriction in the pilot crossover tube restricting the gas in the tube.

  5. jk says:

    i have a york roof top i am trying to trouble shoot, pilot comes on, gas valve sometimes doesn’t come on until it reset the unit 2-3 times

    • What is the inlet gas pressure? Too high and the valve tries to open but pulls high Amps locking out the control. If it is ” high pressure gas” there should be an external regulator. Make sure the Vent is clear and measure the pressure. Should not exceed 11 IWC.

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