Honeywell S8600M - Pilot burner continuously cycles, no main burner ignition

BrandonK

Hey all.

I'm at a real loss here. I have an older (1990) Tempstar furnace in my home (one of two furnaces). Two days ago it stopped operating. The plan is to replace this with a modern 96
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The ignitor sparks, pilot immediately lights, but a third of a second later the pilot goes out. A few seconds later it starts the cycle again. It will do this forever.

If I apply 24v directly to PV on the valve (while the S8600M is cycling) the pilot stays ignited, the spark stops and the main burner immediately lights. After some time the fan and limit switch kicks the blower on as expected. Inducer blower and vacuum switch are operating as expected as they directly control power to the S8600M (when call for heat, the inducer turns on, pulls the switch which then sends power to the Honeywell controller).

Video: https://i.imgur.com/OdB1ceN.mp4 - This just shows that manually powering PV lights the pilot, keeps it lit as well as the short-cycling of the pilot without it jumpered.

I pulled the pilot assembly out and scotch-brited the flame sensor. It was dirty, but not what I would think as "not working" dirty. No change, exact same operation.

If, once the main burner is lit, I remove the spark wire (which also functions as the flame sense wire) from the S8600M, the main valve immediately closes as it should. Reconnecting the spark wire (with the pilot valve still jumpered) the pilot immediately lights, followed very quickly by the main burner.

If the flame sensor was faulty, the S8600M would never send power to the main valve, which it is doing, correct?

The only thing I can come up with is that the S8600M itself is faulty. I've been told "those are the most reliable furnace controllers ever made", so I'm surprised, but on the other hand this is also 32 years old.

Does anyone have any suggestions on what else to look at or test? Any other thoughts, or are you in agreement that the controller is the only culprit? On the plus note, a replacement S8610U is actually less expensive than the flame sensor / ignitor module and Amazon has it in stock!

Teemok

If the ground loop wire/path from the pilot back to the ignition module (burner ground) is good. It looks like the module. But you won't know for sure till you can swap it out. The old one served its time.

Teemok

Can you keep the pilot lit with direct 24v at the right moment? If you can't it, it could be a bad gas valve coil. I haven't seen one fail like that if it is the gas valve.

BrandonK

Teemok said:

If the ground loop wire/path from the pilot back to the ignition module (burner ground) is good. It looks like the module. But you won't know for sure till you can swap it out. The old one served its time.

Ground wire between the module and the sensor/ignitor appears to be good. Low resistance according to my meter. 

Beyond that, I'm assuming it MUST be good as disconnecting the spark wire (which serves as the return for the flame sensor) immediately shuts down the main valve. Since the module is the one opening the main valve once the pilot lights, it must be detecting the flame, which says both the spark wire and ground are good. 

BrandonK

Teemok said:

Can you keep the pilot lit with direct 24v at the right moment? If you can't it, it could be a bad gas valve coil. I haven't seen one fail like that if it is the gas valve.

Yup. I have the pilot valve being fed by the inducer blower switch. Once the inducer blower spins up, the switch closes and sends power to both the controller and the valve. Pilot lights immediately and then the main valve opens (being controlled by ght S8600M) and the main burner lights. The S8600M must know that it's lit as it stops sending spark once the main valve opens. 

With it jumpered this way the furnace works as intended, perfectly fine. Obviously if the pilot never lit that would be an issue as it would be sending gas out of the pilot non stop. I'm just testing theories tonight before I go throwing money away. 

Teemok

That sounds like the relay for the pilot is bad. Because when you do it's job the module then proves the pilot and then opens the main. If it was a flame proving problem, when you hold the pv open you would get a standing pilot flame and the main would never open. Easy fix. Make sure you check for cracks in the HX walls where the flame contacts and have a carbon monoxide detector in house.

BrandonK

Teemok said:

That sounds like the relay for the pilot is bad. Because when you do it's job the module then proves the pilot and then opens the main. If it was a flame proving problem, when you hold the pv open you would get a standing pilot flame and the main would never open. Easy fix. Make sure you check for cracks in the HX walls where the flame contacts and have a carbon monoxide detector in house.

Thanks. I appreciate the confirmation to make sure I'm not going mad.

I ordered a S8610U from Amazon, it will be here Wednesday, we'll see how it goes.

mattmia2

Does it have an inducer or damper it needs to prove?

Check the voltage at the input to the control when the valves open/try to open.

BrandonK

mattmia2 said:

Does it have an inducer or damper it needs to prove?

Check the voltage at the input to the control when the valves open/try to open.

No damper.

The inducer switch is actually what provides power to the S8600M controller. Without the inducer running the controller can never start.

mattmia2

Did you put a meter on the control, does it still have power when the pilot drops out?

My first question about voltage was maybe a contact or connection is bad and there is enough drop in the safety chain that it can pull in the pilot valve but not the main valve.

BrandonK

mattmia2 said:

Did you put a meter on the control, does it still have power when the pilot drops out?

Controller has constant 24v (as long as the inducer is running), yes. I even took 24v straight from the transformer and put it on the controller (bypassing the inducer switch) and it still operates exactly the same.

The PV relay puts out the appropriate 24v, but only for ~300ms, then it drops out for a few seconds and starts again. It's not random, which makes me think it's not a failing relay or dirty contacts, it's very cyclic. But at this point I have no idea what else it could be. Inducer proving is already done. Flame sensor appears to work as it opens MV when manually jumping PV. Removing the spark wire immediately shuts off MV. None of it makes sense other than a failed controller.

Teemok

Maybe the PV relay inside the control module closes with monetary force and it energizes the PV coil for a brief moment but the resistance of the bad contacts doesn't maintain the current needed to hold the PV valve open. The proving circuit sees the flame fail and it recycles. Could be a bad solder joint I guess. I don't pull them apart to find out, it's just not worth it. 33 year old Tempstar? Celebrate it's long service and swap it out as soon as you can. You don't want to push it to HX failure. The parts aren't all that expensive and many can still be had but it's not good investment.

mattmia2

Does it have any bklinky lights or is this before they realized people in the field need to know what was going on?

Teemok

Tip: make sure vent damper plug is in place and send 24v not to the 24v terminal but to TH-W terminal.

BrandonK

Teemok said:

Tip: make sure vent damper plug is in place and send 24v not to the 24v terminal but to TH-W terminal.

There is no TH-W terminal, nor is there a connector for vent damper on my S8600M. They are unpopulated on the PCB.

BrandonK

mattmia2 said:

Does it have any bklinky lights or is this before they realized people in the field need to know what was going on?

Unfortunately no blinken lighten.

EBEBRATT-Ed

You might check the S8600M to see what the minimum flame signal current should be. Ican't remember but it's something like 1.5 microamps

Teemok

There will be, on the new one.

BrandonK

Quick update if anyone searches and comes across a similar issue.

A new S8610U solved the issue. Direct replacement, no changes to the DIP switches. Since I don't have a damper as Teemok mentioned the 24v coming off of the inducer switch went to TH-W instead of 24v. Fired right up.

Of course, it's been 70 degrees for the last two days lol