Bad HSI or board?

[Brad.B.]

Hi,
Was on a no heat call, Luxaire LP9C060B12MP12CB furnace.  Inducer was turning but igniter not lighting.  Unplugged igniter, 45 ohms so sounds good? 

With it still unplugged, on the connector from the board I measured 120VAC and this appeared immediately after pushing in the door switch, but this voltage drops to 0 when I plug in the HSI again.  Board issue?

[walker]

you wouldn't expect 120v on the HSI right after you press the door switch.  You shouldn't see any voltage there until all the safeties are proven.

[Admin]

That sounds like a bad board.

[Brad.B.]

Thanks, I thought so.

[Brad.B.]

One more question - Do some boards "look" for the HSI (like maybe a small supervisory currect to detect the 40-80 ohms of a normal HSI)? 

[Sergroum]

I've had similar problems before. A small fault in the HSI prevented proper voltage work in the board. For some reason the board would prove HSI integrity before going through the heating sequence. It's more common on Lennox's el296/slp98 models in my experience. They definitely pulse voltage to the HSI before even sending voltage to the inducer fan.

The resistance on the HSI showed it intact, but changing it regardless resolved the problem still.

[Brad.B.]

Thanks, I've forgotten before to plug it in (the HSI) and the furnace didn't go through anything else.  Someone told me it would do the sequence but just not sense the flame, but I swear on this furnace it was sensing it before doing any other part of the startup seq.

[harshal]

I would check the line voltage in the hsi harness with plugged in.As someone mentioned that the some control board knows if the HSI unplugged.It is usually common in the Trane variable speed furnace.It would give you the fault code of defective HSI.

[Sergroum]

i keep saying voltage instead of current.

[mustang8]

Thanks, I've forgotten before to plug it in (the HSI) and the furnace didn't go through anything else.  Someone told me it would do the sequence but just not sense the flame, but I swear on this furnace it was sensing it before doing any other part of the startup seq.

Hi I dont think its a bad ECB, as i dont see enough evidence of it,
do you hear gas valve clicking after 30 secs of pressure switch closed? if gas valve clicks then its a bad HSI,
But if you don't hear click sound then you ll have to jump pressure switch so you can be sure nothing fussy going at initial start up... and notice if you hear click at Gas valve... If your gas valve is not opening then i would suspect something wrong with ECB... i would not suspect gas valve as they are more reliable then ECB...

[Porcupinepuffer]

i keep saying voltage instead of current.

Then stop doing that and say what you really want to say