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Intermittent pilot control module(mk)

Mike Kraft_2
Mike Kraft_2 Member Posts: 398
Have some gremlins I'm trying to find in a new install.The ignition procedure is clear although some of the workings are a bit abstract.(for me.....thats easy :) )Basiclly the fan proves,ignitor sparks,gas valve allows pilot flow and the pilot lights.Next the flame sensing rod confirms pilot and the burner lights.

My understanding of the ignitor, is it is the ignitor and sensing rod in one.When igniton is happening an AC current is sent, after ignition the AC current converts to microvolts DC current.

My query is how does the sensing rod receive or know the flame exists.And when it knows what does it do?Is it opening the gas valve as a thermocouple would?It was explained to me that the current somehow makes its way to the drafthood(The flame completes continuity?) and continuity is transfered through the appliance chassis.Am I close?

Help me please..........cheese


  • Bikerdadh
    Bikerdadh Member Posts: 10

  • Bikerdadh
    Bikerdadh Member Posts: 10

    Cheese, your close. On an intermitent spark system, the pilot lights and at that point a signal is sent through the flame to the flame rod, flame sensor(whatever you want to call it). The flame is actualy a conductor of electricity, it is also a rectifer. It converts the AC curent to DC. ( Had it explianed many years ago as FM. " ------- Magic", I'll let you fill in the blank) When the ignition module receives the DC Microamp (not Microvolt) signal back, it simply energises a relay inside which sends your 24volts to your gas valve main coil. All this happens in about 7 seconds.
    You mentioned some gremlins on a new install. I don't know quite what they are but many of this type of system are polarity sensitive.( make sure the 120 hot and the ground are correct) Also a good ground on the 24 volt side is a must.
    Good Luck , Bikerdadh
  • Excellent description.

    The wire usually fails in the high voltage spark mode. The ground path through the flame to the pilot, back through the pipes, to the ground on the control, usually shows up weak on the sense mode.

    A good test is to grab the pilot itself with a jumper wire alligator clip, and ground it through the jumper to the ignition control ground screw, itself, providing a "zero ohm" path for the signal. If this test drives off the gremlins, the ground path needs to be cleaned and tightened up.

    This happens a lot. It's a pretty common call, at work.

  • Mike Kraft_2
    Mike Kraft_2 Member Posts: 398

    Thanks bikerdadh!The bulb is getting a bit brighter.It actually makes more sense now that I read the words.I must grin(G) FM was also the term used to describe it to me.I'm a bit more familiar with RTFM!

    So how does the DC microAMP current find its way back to the module?

  • Bikerdadh
    Bikerdadh Member Posts: 10

    I noticed you used a S8600 number, I believe that is a honeywell number... Come to think it doesn't realy mater Honeywell whiterogers, robertshaw etc.. The Module uses one of two ways to sense signal.
    They either use a remote sensor, on the module you will see a terminal that says sens, sensor, fs (a couple of labels used) Some have a wire that comes from that terminal to the sensor. The AC signal is sent out of the terminal through the flame rectifies to DC and back to the terminal usualy caled "burner ground"
    The second way and most comon on Boilers is the use a "local sensor". It works the same way but the AC signal is now sent through the hi volt spark cable, rectifies through the flame to the pilot assembly and back to grount through the pilot tubing, burner etc... .
    A rule of thumb is 1 microamp is needed for the module to sense flame.
    Believe it or not the most common problem on older jobs is a dirty pilot. Next I would say is the wire from the module that comes from the terminal "Burner Ground" Make sure where it connects to the chassi, it is clean and makes goog contact. I have in many cases taken that wire and attached it directly to the burner airshutter screw on the burner with the pilot assy. Also a good ground is to the gas valve, if it is a honeywell they give two terminals on the valve body for ground.
    All your doing is making a loop leaving the module through the flame and back to the module. and if you think about how little the amount of current, it gives you an idea of how important all your connections and a good sharp flame on your pilot are.

    Sorry so lengthy, I hope it helps. I would look more at obvious on a new install like polarity though.
  • Mike Kraft_2
    Mike Kraft_2 Member Posts: 398

    I had a reading of 3 microamps eliminating the flamerod sensor.I am speaking with the manufacturers tech service.The issue or should I say the gremlin is short cycling.

    Manometer is 13.9" on inlet and burner is 8.5".A little high on inlet but acceptable.Description:fan proves, ignitor sparks, pilot lights and main burner fires.(this is a sealed combustion LP)On a cold start the flame drops off and ignition repeats ....cycling quickly.When I allow it to repeat its cycle ,the burner length of burn time increases ever so slightly.After 2-3 minutes (tops)the burner fires fine beautifully.As the call for heat continues (high limit and diff. temp reached)and the burner fires it occasionally cycles one time.

    Pressure switch,flame roll out,transformer and L7148F control all seem to check ok.They are sending me a new control but I am skeptical but will try.

    The part of this that was messing with my mind was the rectification of the flame.I have a much better understanding of the FM side of it:)Thanks bikerdad!

  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    Noel & Biker...

    were exactly right, so I don't need to repeat what they said, but I will emphasize how important a good, solid earth ground is for those (and other) spark modules. If the pilot lights, and the unit "clicks" constantly, it's a dead give away of a bad ground.
  • Bikerdadh
    Bikerdadh Member Posts: 10
    Try This

    Cheese, the gremlin sounds more like a venting problem than a rectification problem. I have had a similar situation and it was dificult to catch. The unit would fire and run for maybee 40 to 60 seconds, then dropout and cycle again. Some times it would run for the night, and sometimes not. A simple check I used was fire the unit as soon as I got spark I knew the presure switch made, I then used a jumper and jumped the normaly open contacts on the presure switch, watched the unit. It did not short cycle. Turns out the switch was just on the edge and would break for just a split second and cause a short cycle. If you have an incline manometer or a magnahelic guage you coud verify this but the jumper will at least get you to the right area.
    A Little heads up you want to wait until you know the pressure switch makes before jumping it out, some units look for a normaly open switch before letting the unit begin its cycle.
  • Cheese I am concerned about

    inlet pressure of 13.9" W. C. and then 8.5" W. C. at the outlet. Inlet pressure should be 10 to 11" W. C. That 13.9" means that the pilot has that much pressure on it as the pilot is not regulated. This could be causing the pilot to pull away from the sensing rod causing your problem. I would check with the LP provider and have them reduce the second stage regulator to 10" W. C.

    I am going to send you some literature on Flame Rectification and also procedures for checking intermittent pilot systems. Everyone has given you some good answers so getting it all in a written form you will have it for future reference. The literature is free no obligation, glad to help out. E-mail me your address, I think I have it but want to make sure.
  • J.C.A._3
    J.C.A._3 Member Posts: 2,981
    Timmie, you're a gentlelman among scholars!

  • lchmb
    lchmb Member Posts: 2,997
    just a thought Tim

    I would have the gas company check lock up pressure while they are doing this. Another thought is to have the gas company set the regulator to 11" water column not 10. Most US unit's are designed to run at 11" with a 1" drop across the gas valve..
  • Mike Kraft_2
    Mike Kraft_2 Member Posts: 398
    Re:Inlet pressure

    The inlet pressure tolerance is 11"-13".The burner side is 8.3".Lp co. is leaving regulator pressure as is until the other appliances are set.Timmie your help and literature is much appreciated.Please enclose a bill:)The manufacturer was fine with pressures.I am not questioning your concerns or advice ..............just giving you the facts mam :)

    Thanks......Joe Friday

    PS I am getting a reading of 3 microamps(DC) on the sensing rod 1-2 is what they are looking for.
  • Eric_10
    Eric_10 Member Posts: 17
    flame rectifacation

    This is the "magic"
    The control module sends an a/c current thru the flame rod or igniter depending on set up.When air fuel are mixed within burners ionization takes place. ionization is negetivly charged. when a/c {alternating current] moves thru the flame finding its way to burner ground it alternates from positive to negitive[alternating current].because of ionization in flame[negativly charged] the positive side of the a/c current is removed or magnitized to the ions. whats left is anegatively charged current [d/c] which finds its way to burner ground. with your meter on microamp setting in series with flme rod you will read current. if the ingniter is also the flameprover put your meter in series with burner ground. good luck.
  • Everyone forgot to mention

    that what makes this all work is that the ground area ratio to rod area has to be 4 to 1 or greater. If the gound area is not at least 4:1 it will not rectify the AC signal.

    The other thing to be aware of is that the AC signal is generated from within the electronic control.

    Normal microamps should be 2 to 10. If it is less than 2 then clean the sensing rod, clean pilot, make sure you have a good ground.

    Now here is one for you, WHAT IF MICROAMPS IS GREATER THAN 10? What is that an indication of and what would you do?
  • Eric_10
    Eric_10 Member Posts: 17
    flame rod

    Would it be possible the rod is too close to burner ground?
  • Mike Kraft_2
    Mike Kraft_2 Member Posts: 398
    The ugly maybe...............

    with the thermistor or sensor on the L7148F control.Timmie went through his mental archives and pulled a recent problem with the sensor communicating with the resistor.Seems resistance would not allow the system to run.But would cycle due to heat demand.The more it cycles the warmer the diode becomes and cylces become longer until cycling disappears.Now that the system is up and running I cannot make it do it again.Honeywell apparently addressed the issue approximately 1 year ago.I'm thinking I may have a leftover that slipped through:(.

  • Glen
    Glen Member Posts: 855

    Ma should be a function of grounding area ratio - and if that's the case then the pilot flame is too big. On industrial burners we can control flame signal by "clipping" FR length to bring signal within specs.
  • rich pickering
    rich pickering Member Posts: 277

    Is there any way of testing the strength of the spark other than how far it will jump a gap? I had a pizza oven that would not light, but the spark looked good. Changed the control and it lit perfectly with no other changes made.Does anybody know if Allsons transformer tester would work?
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