Carrier RTU not proving pilot

zepfan

We have a Carrier RTU model# 48DJD028 from 1995 in which every part in the heating section has recently been replaced. Heat exchanger, inducer motor, burner, ignition module, flame sensor, cables, collector box, pilot runner and gas valve. The unit had been online with the proper manifold pressure for two weeks with no issue. Today there was a no heat call on the unit. The unit would come on and establish pilot flame but never generated the proper MV signal to switch over to the main valve. The ignition module is a United technologies model# 1003-600a, Carrier part number LH33c7010. The instruction manual says that it needs a min. of 0.3 MV to switch over. I normally see with this module a little over 1.0 With everything I tried the reading was 0. I cleaned the brand new sensor, ohmed the cable out, try a completely different flame sensor, and still nothing. I tried running a common ground from the pilot burner, then to the module, and then to the main unit's ground. Still a reading of 0. I tried reversing the placement of the spark ignitor, and the sensor (one is on one end of the burners, and the other is at the other end of the burner) still nothing. You can actually see the pilot flame enveloping the flame sensor, and the sensor turning red, but still no MV reading. It's strange that the unit started up fine when all these parts were replaced, and lasted two weeks and now it will not switch over to the main valve at all. I also tried adjusting the pilot regulator in the gas valve, with no change. The sensor sits about 1/8" over the pilot runner, and is not touching any ground. Any help or suggestions would be much appreciated. Thanks to all

EBEBRATT-Ed

i think i would try a new module, Make sure all connections are clean and tight.

mattmia2

is the reading of what I assume is a thermocouple with it disconnected? Whatever reads the voltage in the module could be shorted. Could try an ohmmeter on the input or a milliammeter between the thrmocouple and module to figure out if it is a bad module or bad batch of thrmocouples.

Tim McElwain

This system is not a thermocouple operated system. Also flame sensors do not produce MV (Millivolts) they produce microamps. Why were all those parts replaced? What was the original problem? Some pictures of the controls would help.

mattmia2

I'm confused by the original post. Looking at the control it uses a single wire flame rectification sensor to prove flame which is labeled "sense". The "MV" terminals on the control are for the main valve. The flame sensor would have to be tested with a milliameter in series, not a voltmeter in parallel.

Tim McElwain

You need a meter which will measure microamps (millioniths of an amp).

mattmia2

Is that why it sometimes doesn't work if it isn't bonded to the electric service and bonding it sometimes helps, it isn't that the circuit the control is measuring with the flame sensor goes outside of the cabinet of the appliance but it is such a low level signal that noise from other sources can be stronger than the current it is trying to sense and the grounding can reduce the noise induced in the wiring and cabinet?

zepfan

Thanks to everyone that responded. Yes I made a mistake in my original post and wrote MV instead of UA. I changed the module today to a Honeywell S86 that can be used for a single or dual rod application (the original UT module had to use a separate ignitor, and sensor) , and the unit is heating now. There must have been something wrong with the flame sense circuit in the original module