HSI version of Lennox G24M board. Blower motor won't start. Suspect board?


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Old 04-14-12, 03:21 PM
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HSI version of Lennox G24M board. Blower motor won't start. Suspect board?

I believe I have the weird/rare HSI version of the RAM Electronics Corporation control board (RAM-HIMC4 / 9642-08 H1MC4-01F (C) 1994) as the controller board in my Lennox natural gas furnace. It has three dip-switches on SW1 instead of the 2 that I could find in the G24M "SERVICE INFO" PDF.

Today I replaced the motor, which had been making wonky sounds for about six months, because I assumed that the motor had burned out, when the furnace blower would no longer start at all.

My basic problem is that the furnace blower doesn't turn on, if the black blower motor wire is attached where it belongs on ACB HEAT terminal.

To be sure I installed the new motor properly I attached the black wire to an accessory terminal that was not in use (ACC E4). It works.

So... can I leave the blower black wire on one of the ACC (accessory?) output terminals? Hopefully they are similar 24VDC relay contacts to the regular ACB COOL/ACB HEAT terminal outputs.

The blower motor works when I feed 120 VAC (by attaching the black wire on the motor, to the Accessory terminal on the control board), but it never turns on when left to its own devices, when it's attached to the proper ACB HEAT output terminal, but the relay itself (K2) attached to that output terminal works fine when I feed it 24VDC.

Ignition control works just fine, and the combustion air blower is working fine. The Igniter works fine too. So the furnace spools up, the burner runs, and yet, the blower never turns on.

My first question is if there could be anything wrong about the wiring of the thermostat controls. The thermostat control wiring attaches to a terminal strip with terminals labelled C,B,Y,W,G,R. There is currently nothing attached to the B terminal. The service manual I have explains the purpose of the "early model G24M" board, and shows there is no B terminal on their terminal boards. Mystery #1: What does terminal B do and can it help me with my problem?

My second question is about the dip-switches. Googling around shows some links to a Lennox-tech PDF but sadly the link doesn't work any more. What do the three dip switches in the SW1 dipswitch bank do? (Probably Fan-off timing, like the other RAM board.)

I entertained thoughts of replacing the relay K2 which feeds the ACC HEAT terminal, but it's not the relay that's blown... Some googling shows some people saw other components on their boards that blew up but I can't see any overheated or otherwise damaged looking components....

Manually putting 13.8 VDC into the K4 relay's input coil triggers the relay just fine. So either the 24 VDC signal that triggers K4 isn't getting there or the supply 120 VAC isn't getting to my output relay. My soldering skills are non-existent, but I know my way around a multimeter. Anybody who's see this and knows what to check? There are no obvious signs of trauma on the circuit board except that the light LED1 (which still works) looks like the conformal-coating on the bottom area got a little blackened, perhaps because this board runs the LED a bit hot?

Any help or suggestions or further info on this board and how to make it turn on my blower motor, would be appreciated!

Warren

picture of my board:
 
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Old 04-14-12, 07:48 PM
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Jumper R-G - the blower should come on.

With R-G-Y jumpered, the cooling speed terminal should be energized.

Black is high speed - normally not used for heating but might be required in your application.

It sounds like the board is bad; though the ignitor and inducer work, there could be other problems with it (which you're not aware of) that could render the furnace unsafe.
 
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Old 04-14-12, 08:07 PM
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I jumpered R-G and the blower relay clicked on, but no voltage was present on any of the output terminal spades on the board.

I found another identical board on ebay and bought it, I hope that changing out the board will fix it.

Warren
 
 

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