Tempstar furnace clicking, igniting and then shutting off

[hunnypuppy]

This problem started today, the motor comes on, about 20 seconds later I can hear a clicking and the sparks rattling away, it seems to ignite a pilot flame which burns for a 2 seconds and then it shuts off. And then the cycle repeats itself.

It's a Tempstar model NUGE075BG01. Attaching a link to the video showing exact issue. It's just stuck in this loop for hours before I realized it.

What's the likely issue here? (I'm very handy but have limited experience with furnaces). I also can't seem to figure out if this model has a flame sensor or a thermocouple.

 

[pjones]

This problem started today, the motor comes on, about 20 seconds later I can hear a clicking and the sparks rattling away, it seems to ignite a pilot flame which burns for a 2 seconds and then it shuts off. And then the cycle repeats itself.

It's a Tempstar model NUGE075BG01. Attaching a link to the video showing exact issue. It's just stuck in this loop for hours before I realized it.

What's the likely issue here? (I'm very handy but have limited experience with furnaces). I also can't seem to figure out if this model has a flame sensor or a thermocouple.

Thermocouple is used with standing flame applications. You have spark ignition, not standing pilot.

It looks like you are failing to prove your pilot flame. Travel down that road and you should find your issue.

 

[user 4883]

clean or replace the flame sensor. Might need to replace the sensor and ignitor as a single component.

 

[hunnypuppy]

Can you make out where the flame sensor is on this unit/model?

 

[hunnypuppy]

Thermocouple is used with standing flame applications. You have spark ignition, not standing pilot.

It looks like you are failing to prove your pilot flame. Travel down that road and you should find your issue.

How does the model know that the pilot flame is lit? I can't find any flame sensor here. Attaching a picture, it's got the Spark Wire (electric as you said), the GND wire and the Gas pilot tube. No flame sensor, what am I missing?

 

[hunnypuppy]

Okay here are the results I've found so far:
When the thermostat sends a heat signal to the furnace

S8600M -> 24V terminal -> yes 29v

The inducer starts up, and the igniter starts clicking:

With Gas turned off -> S8600M -> PV Terminal -> 29V intermittent
The clicking continues for a long time since the gas has turned off, so it looks like it's detecting the the pilot flame didn't turn on and keeps sparking the ignition

With Gas turned on -> S8600M -> PV + MV + MV/PV Terminal -> 29V intermittent
After the spark, the pilot lights up (like in the video above), as soon as the pilot lights up a second later the sparking stops (like in the video above). So it's detecting that the pilot was lit because it turns off the spark (in the previous case when the gas was turned off, it kept sparking so it seems to know if the pilot is lit or not).
So there's voltage being sent to the PV, MV and PV/MV terminals. Is it normal to fluctuate between 0 and 29v rapidly (the Multimeter is set to AC 200V) rather than a constant 29V?

I didn't open the spark ignition unit since it appears that the S6800M can detect if the pilot is lit or not.

I also checked the pressure valve voltage (it's a 2 wire relay system), it seems to switch between 0v and 21V. When it tries to spark ignite it 21V, when the pilot ignites it goes to 0V (relay closed if I'm understanding this correctly).

The Gas Valve is VR820M 1075 model.

Any thoughts on where the issue may lie? Thanks

 

[hunnypuppy]

One more development, I had an identical unit so I tested the voltages on it, it's giving me a constant 28V on PV from the time the power is applied by the thermostat, up until the furnace lights up and remains constant at 28V (+- 0.5v)

Then I checked the voltage on this problem furnace, the voltage starts out at 29V when thermostat calls for heat. As soon as the sparking sequence starts it fluctuates like hell (i.e. goes out of range of the Multimeter, is just fluctuates so randomly it causes the multimeter to hang up). Then I checked the wires, so ensure it's a solid connection. bypassed the pressure switch just incase it was creating the issue and the multimeter would go crazy as soon the sparking started. Then it would go to 0V on PV until the next cycle. Then I disconnected the spark ignition wire from the S8600M (now without the actual spark there should be no current draw and hence no fluctuation IMHO), first of all I was surprised to hear the high speed clicking noise even though the spark wire is disconnected (is that normal??) and the fluctuations didn't stop.

This is making me think the S8600M is defective. Any thoughts?
If I need to replace the S8600M, I can't find the model anymore. Does anyone know the exact replacement for it (wire to wire) for this setup?

 

[hunnypuppy]

I would like to thank everyone who helped me out. It was a successful diagnosis and replacement. To highlight a few things along the way

1. Start with testing the 24V and 24V (GND) voltages. The key here was that while it started out with 28V, as soon as the spark ignition kicked in the voltage started fluctuation wildly so that indicated a module failure (on the control unit the voltage was steady/constant)
2. The Honeywell S8610U is the correct replacement module for the S8600M. I did check the manual as well to ensure that the settings are correct. The key things to look at are Pages 2 which talks about the DIP switch settings SW1 and SW2 when replacing the S8600M (different variant models as Yuri pointed out) with the S8610U3900. The default factory settings for the Switches are a perfect match for the S8600M so don't change them. The second thing was look at page 8 which talks about the wiring when you move from the S8600M to the S8610U. It's a perfect 1 to 1 match with the exception of the 24V connection from the S8600M which needs to be connected to the TH-W input instead of the 24V in the S8610U if you don't have a Vent Damper installed. The detailed wiring diagram can be found on Page 13 as well. If you have a Vent Damper installed, see the manual, again pages 8 and 12 (google what a Vent Damper is if you don't know, it's basically a unit connect to your Exhaust Vent and most systems no longer use Vent Damper as they are expensive and ineffective). Here is the link to the manual: https://www.supplyhouse.com/Honeywell-S8610U3009-Intermittent-Pilot-Control-4584000-p
3. After installing do a heat exchanger inspection, it look good to me with no deformaties, I was able too look at it through the burners section using a high powered torch. You can also do the burner test I shared above.

Just a quick point to note, when installing the S8610U, it's best to install it flat on it's base, there's one side which should never be covered (it's written on it and also mentioned on the manual). Also if you have a separate flame sensor/ignition then you need to cut off the factory installed Wire Jumper to the sensor connector and instead connect your separate flame sensor to it. In my case it was combo ignitor and flame sensor so everything was factory default for me.

The new module has 2 LED lights on it for flame and operation which are very helpful to diagnose issues.

I hope this helps other folks. You can buy the S8610U from Amazon for about $95 https://www.amazon.com/gp/product/B00ECV3H3K/?tag=skimlinks_replacement-20