Modine Hot Dawg issue

[Matti]

I guess I haven't logged in for 7 years! I've got a 45HD that has been working well for 7 or so years. I turned it on a few weeks back because its getting cold here unfortunately. I go into the garage and I find the exhaust fan running but no heat. I get 3 flashes indicating a limit open. Testing doesn't show any issues although my testing might be flawed. I haven't tested the flame roll out switch yet. If I turn the power off for a minute and back on the unit will often fire up and run fine but next time there is a call for heat it won't run and the exhaust fun stays running. I have cleaned the flame sensor and check the pressure switch tube and exhaust ducting. Any ideas on what I can check next?

 

[The Cobbler]

read this on the web for 3 flashes
Limit switch is open
Limit Operation -
The limit switch is ignored unless a call for heat is pres
ent (W energized). If the limit switch is open and a call
for heat is present, the control de-energizes the gas valve
and runs the indoor blower motor on heat speed, and runs the
induced draft motor. The control will flash “3” on the LED until t
he limit switch closes. When the switch re-closes or the call
for
heat is lost, the control runs the induced draft motor through
post-purge and runs the indoor blower through the selected fan o
ff
delay. The control will return to normal operation after the blower off delay is completed.

 

[D45]

https://www.garagejournal.com/forum/showthread.php?t=374715

This might be your problem .....and solution

 

[The Cobbler]

https://www.garagejournal.com/forum/showthread.php?t=374715

This might be your problem .....and solution

wow, great . I'm going to print that PDF out & put it with my Modine heater for future reference (if needed)

 

[Matti]

I think that article is a solution to a different problem as the unit isn't starting and then shutting down early. I did do some more testing today. The unit wouldn't start and I have 3 flashing lights indicating open limits. There is 24 volts at the roll out switch and limit switches so the error doesn't make sense. Also the unit does run sometimes. I would like to test that there is actually 24 volts to the board but accessing the Molex plug with a multimeter is difficult. Assuming there is 24 volts then either the Molex plug is not making proper contact or the board is defective. The troubleshooting manual also says to check that the limit switches are not touching metal and shorting. Perhaps I could jumper the limit circuit to test this? I'm more of a mechanical guy than electrical.

 

[eddieK]

I think that article is a solution to a different problem as the unit isn't starting and then shutting down early. I did do some more testing today. The unit wouldn't start and I have 3 flashing lights indicating open limits. There is 24 volts at the roll out switch and limit switches so the error doesn't make sense. Also the unit does run sometimes. I would like to test that there is actually 24 volts to the board but accessing the Molex plug with a multimeter is difficult. Assuming there is 24 volts then either the Molex plug is not making proper contact or the board is defective. The troubleshooting manual also says to check that the limit switches are not touching metal and shorting. Perhaps I could jumper the limit circuit to test this? I'm more of a mechanical guy than electrical.

Check that the vent is clear, cap is intact etc

 

[bobbyjean]

Check that the vent is clear, cap is intact etc

yep....a little birdie told me that...bad joke i know....but those thing's will nest in cap or pipe if they can...check it out...actually take the pipe apart back to inducer fan if possible

 

[Matti]

Yes I did check that first. This morning I checked the continuity in the limit circuit. Unplugged the Molex connector and checked it. Everything is fine - no open circuit (6 ohms). I have power through the pressure switch. This should mean the venting is ok. I'm out of ideas.

 

[The Cobbler]

6 ohms? shouldn't it be zero?

 

[Matti]

6 ohms? shouldn't it be zero?

its a closed circuit unless there is a fault. I think there should be some resistance in the switches?

 

[The Cobbler]

its a closed circuit unless there is a fault. I think there should be some resistance in the switches?

I'm not sure, but it seems to me that 6 ohms is a fair bit of resistance.

 

[Matti]

I ordered a new integrated circuit board. Apparently they sell quite a few.

 

[Matti]

Heater now running!

 

[The Cobbler]

great, assume it was the board?

 

[Matti]

great, assume it was the board?

yep bad board