Garland runaway

[ectofix]

I can’t offer much diagnosis of such an unusual problem over the internet.  ESPECIALLY when it’s an intermittent problem (THOSE are so aggravating).

Also, I’m not certain about what temp control you have.  HOWEVER, you did mention a potentiometer, so my guess is it’s a solid-state control board instead of a digital or computer panel.

 

Anyway, I found a MCO-G parts manual to peruse to get some idea of what you got: 

I see that it uses an RTD probe. 

– There are many types of RTDs, but one commonly used type would have 100Ω at 32°F. around 110Ω at room temperature…and some relatively much higher resistance at normal oven temps. 

Obviously the one I just described has a positive temp co-efficient (PTC) – so when temp goes UP, the resistance goes up. 

Otherwise, a probe that has a negative temp co-efficient (NTC) – temp goes UP, resistance goes DOWN.

– I cannot tell you which type Garland uses…since I don’t know.  But if happens to employ a PTC one, then degradation of its leads passing through the oven’s wall might have allowed them to short together (or to ground?), then cause an extremely low resistance signal from the probe and therefore making the temp control call for heat when it shouldn’t.

 

That’s just one idea. 

Otherwise I would indeed suggest there might be a problem with either the potentiometer – which can quite effectively be tested using an analog ohmmeter, or there’s a wiring issue in your temp control/heat circuit. 

Don’t EVER discount the possibility of a wiring issue.  Go through your wiring METICULOUSLY before deciding it’s not the problem.  I say that because folks tend to target problems with components, then they go nuts spending a bunch of money on replacing parts…only to discover that the problem still exists.

I wouldn’t consider the temp control board as a possible culprit until you’ve eliminated the possibilities I’ve suggested and you can “make” it act up to catch it in the act.  SO…aggressively OPEN and SLAM the door during your testing to see if you can replicate the circumstances.

[rationaltechnician]

Hey Sundog

Its been a while since I worked on those Garland convection ovens but I remember a little of the 300 controller (IM300).  It is a solid state controller.  As ectofix says, intermittent problems are the most challenging.

 

I’m guessing an SSR on the board is sticking.  If I recall, Garland convection ovens use a door switch which stops  heating demand and convection motor (stops fan when door is opened so you don’t burn your face).  So the heating request SSR is activating and deactivating as the door is opened and closed because of heat loss but then may not be de-energizing.

 

If there is a sticking relay on the board, it could be the cause of your problem.  The SSR might be sticking closed.

 

150F is a difficult temperature for an oven to hold but with the digital controller and RTD probe, this oven does a pretty good job.  An open or shorted RTD would give you a probe error LED on the panel.  I’m ruling out the RTD in my guess.  Just make sure it is still mounted correctly as ectofix recommended.

 

The gas system is a redundant safety.  It should not be energized if the board and ignition module did not intend it to.  The ignition modules have an internal safety that looks for a condition that should not be there before ignition trial.  If it detects a flame when it should not be there, it does not energize the redundant gas safety valve.  Because of this, I think the board would not send a heating request to ignition module and ignition module would not start an ignition trial (excuse my choice of words – I have a habit of simplifying).

That being said, a sticking SSR on the board would not be affected by this and eventually a high limit should trip.

 

Lastly, if you find the 300 controller is bad, you might need to convert it to a 200 controller (300 NLA).

 

Hope this helps and good luck.  Let us know what you find in the end.  It will put closure to this thread and we can all learn from it.@

[sundog]

Thanks for the replies,

 

Most have echoed my thoughts already, but did give me some direction as well.

 

I went to see the unit today, checked all the wiring didn’t see anything obvious no nics or cuts, worn spots etc. Metered the probe it seems to be within tolerances at appropriate heat levels.  The POTS seemed good, but for just a moment it seemed to go a bit nuts, but it only happened once and I had to reposition my leads shortly thereafter so it might have been a fluke and my leads may have not been just on the POTS.  

 

I found out also that it is NOT only when they open and close the doors, thats how it started but it has happened since in the middle of the night when no one was around.  The oven was set to 150F to dry out pork skins, when he came in later the next morning the oven was still at 150F but the pork was black, leading him to believe that at some point during the night the oven shot up to 400F at some point.  Cant say that I can disagree with what hes saying.

 

As was suggested parts are no longer available for this unit, it needs to be converted to 200 series.  I spoke with garland and they suggested what I was already going to do, replace the POTS, as its relatively cheap, and if they still have issues, decide if its worth the cost of the upgraded board.

 

I’ll check in and let you know what happens after I replace the POTS.

[olivero]

Sounds like a thermostat issue to me, Those RTD’s like what Ectofix mentioned, they can be super flaky, sometimes they work, sometimes they don’t, makes it a nightmare to figure out.

 

If it’s got to do with temperature fluctuations or irregularities, the first place I look is the thermostat, sometimes it stands out but most times, you gotta look and think it all through in your head.