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Question DetailsAsked on 11/18/2014

Pilot light won't stay lit after releasing bypass button. Thermocouple produces 20 mV. Valve coil = 121 Ohms..

This is in a used hanging garage (natural gas) furnace; its a Modine model PA75AB 01 11. Upon initial install, the pilot would not stay lit after releasing the bypass valve so I replaced the Thermocouple. I now have 20 mV while the pilot is lit (and I hold the button; again I release the button and the pilot goes out immediately). I did not test the previous thermocouple with a DVOM; in hindsight it was probably fine. I also have about 121 Ohms across the coil of the gas control valve which I think is low, but "normal". Not sure what my problem could be. The pilot flame is nice and blue, about 3/4" tall and heats the thermocouple properly based on its output voltage. Any ideas???

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1 Answer


That is an old reliable unit heater model. I could not call up the troubleshooting guide at Modine to see what the readings are supposed to be, so can only throw out a few possibilities. I presume you are leaving the pilot light button down for a good minute to give the thermocouple ample time to fully heat up - in case 20mV is not its full voltage ?

1) overtemp limit switch has gone bad with age, so not letting gas valve open - check for continuity at room temp with volt-ohm meter

2) thermocouple voltage is wrong for unit - check thermocouple model number compatibility at manufacturer website

3) thermocouple tubing is bent inside the threaded connector, so instead of the tip going up into the hole and contacting the gas control valve electrically, it is going in a bit sideways or not all the way because it is either contacting only at the edge of the tip (tilted a bit), or is hanging up on the threads in the hole and not going all the way in (this is very common). I always slide the threaded connector back on the tubing a few inches, straighten out the tubing, then insert it into the hole, making sure it is making good center contact - then pull the connector up and thread it into the hole. Prevents hangups of the tip going in.

4) ignition detection control (if your model has it - some 75's did, some not) has failed

5) control board/sensors not properly grounded - will cause it to fail to sense the 15 second try-to-ignite circuit (if you have auto ignite, probably do) so it will not ignite at all.

6) holder for thermocouple not tight, so outer casing is not grounded - many makes/models need that to be well grounded so it can sense a differential voltage between that and the center electrode.

7) fan blockage switch is blocked or failed

8) manual reset switch failed

Modine does have parts listed on their website (but not all available) - maybe you can get the troubleshooting guide to comeup - did not work with my browser at least.

Other than those thoughts, I can't help, I am afraid.

Answered 6 years ago by LCD

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