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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I am trying to repair a 75 k furnace MN 2865C766 SN

Customer Question

I am trying to repair a 75 k Coleman furnace MN 2865C766 SN###-##-#### ***** has a Robershaw ignition control MN HS780 and I think the SN for that is 34NL-306A. I get call for heat, the inducer motor starts running and it keeps running but that's about it.
There are a few moments when its running that I hear a slight buzz for a few seconds. during that few second buzz I am reading 18 volts going to gas valve. It only lasts a few seconds and the inducer motor keeps running. One problem with this furnace is that
the front chamber is with the burner and igniter is is sealed so I cant see whats going on.
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


So far it sounds like a bad ignition control module, bad igniter, or gas valve.

If you can attach a well lit, well focused close up photo of the furnace wiring diagram, I can mark it up with trouble shooting instructions.

Use the paper clip icon at the top right of the tool bar for that.

We can go from there.

Customer: replied 1 year ago.
okay I will try to get over there in the next hour or two and take a picture. should I be able to attach thatfrom my smartphone?
Expert:  Phil replied 1 year ago.

Hello again, generally you have to load the picture into your computer and post from there, that is especially the case with Apple products.

You can also post the picture to and post the link to the picture here.

Customer: replied 1 year ago.
Expert:  Phil replied 1 year ago.

Thanks for the great pictures

let me know what you find, we can go from there.

Expert:  Phil replied 1 year ago.

Set the thermostat in heat mode and up to 90F for this test.

Customer: replied 1 year ago.
That was the first thing I did yesterday. And the iducer motor started, and an occasionally 18 volts to gas valve.mostly the inducer motor just keeps running and that's it. The furnace is about 5 miles away from me and access is not easy. There is someone living in the apartment too and she's very cold
Expert:  Phil replied 1 year ago.

It might take a while for you to get parts for the furnace, it would help if you got her a couple of portable electric heaters and plugged them into separate 120 volt circuits for her.

The igniters fail frequently on those, see if you can pull the igniter out and look at it, if it has a thin black line across the face of it, its bad. You can also use an ohm meter to check across its two wire connections when it is unplugged and see if it has an open circuit or not (its bad in that case)

Check for 24volts across the low voltage transformer, if your meter reads 24 volts there, but not across the gas valve, then it is likely that the ignition module is bad.

I am sorry to hear that access is so bad, I spent 30 years of my early career crawling though attics and under houses to get to such poorly chosen equipment locations... One outfit dug a hole under a house to put the equipment in... the hole filled with water every year.

Let me know how all that goes.

Customer: replied 1 year ago.
I tested the igniter. It is not open I got some continuity across it. I was measuring the output of the transformer it's only 17.6 volts. and I also see what appears to be a relay at the top that occasionally bosses for a second I think that's what I was hearing it seems like when it buzzes I get a quick burst of voltage across the gas valve. could it be as simple as a bad transformer? When I get home I am going to send you another picture
Expert:  Phil replied 1 year ago.

Hello again, it could be a bad transformer but it is rare they fail that way, usually they put out 24v or nothing, is possible your meter read is off, due to poor contact with the probes or some other reasons, but still... it could be a bad transformer.

Most relays buzz a bit, I wouldn't jump to any conclusions on that basis.

If the gas valve stays open you will have a steady 24v across it...

we can get into the tall weeds speculating. It will be best if we get the igniter working first, then go from there. tell me what voltage you get to the female side of the igniter plug, be sure you have good electrical contacts.

we can go from there

Customer: replied 1 year ago.
I attached photo of the igniter and a picture of the transformer and relay. tomorrow I will test the voltage going to igniter. Im pretty sure meter is accurate, I was working on another furnace today where I was measuring 25.6 volts consistently. My only thought is that if the transformer for some reason is not putting out sufficient voltage then it might not be enough to trip the relay except for an occasional moment.
Expert:  Phil replied 1 year ago.

Thanks for the pictures, the igniter 'looks' like its OK...I will be looking forward to your test.

You can fit a clamp around amp meter on the coil wire of the relay to see if it gets constant coil power, and you can remove the switch leads from it to see if the contacts are closing or not.

I will be here all day tomorrow.

Customer: replied 1 year ago.
ok. I will have to find my clamp around meter. it's somewhere. I'll be in touch with you tomorrow for sure
Expert:  Phil replied 1 year ago.

You will have to wrap the wire you are testing around one side of the clamp in order to increase such a slight amp can splice in an extended wire to do that,...10 loops provided 10x the amp reading.


Customer: replied 1 year ago.
I lucked out. I replaced transformer and it fired right up
Expert:  Phil replied 1 year ago.

Good work!

Thanks for letting me work with you. If you still need help let me know!. Expert is not paid for assisting you until you rate the assistance. Feel free to follow up for30 days at no additional cost.