Rheem Classic 90 .. burners for 7 seconds then stops

[CheapScotsman]

Rheem Classic 90 Furnace
Model RGRA-04EMAEB

This unit is 12.5 years old with zero problems till a day or two ago. Now we are getting no heat

 

Here is the current sequence

1. The thermostat is calling for heat
2. Draft blower starts
3. It appears to have a sparking igniter and that goes off
4. the main burners fire
5. About 6 seconds later, the main burners shut down
6. Waits about 30 seconds
7. Sparking igniter goes off
8. the main burners fire
9. About 6 seconds later, the main burners shut down
10. Blower motor starts and runs for 3 minutes then the above cycle repeats then it enters a 1hr lockout.

Troubleshooting so far ...

The two Green LEDs are solid ON
The Yellow LED is OFF

The flame sensor has been cleaned and there is ZERO build up on it.
Continuity of the flame sensor wire has been tested and confirmed
Current generated by the flame sensor indicates less than 1.5 uAmps on initial burner fireup but within about 2 seconds it is back to zero.

I've checked both pressure switches (one for the drain and one of the vent) and they both read ZERO volts for the entire cycle.
 

 

Any suggestions would be greatly appreciated. Thanks very much.

 

[Daveb1972]

Haw was the flame sensor cleaned? You need to use an abrasive to do it right. Does the flame cut out instantly or fade (like fuel pressure dropping)? What is the light code explanation (on the furnace)? 1,5uA is generally the minimum to retain flame, so check for burners being clean (they are the other conductor) and that the flame is covering the sensor. If these don't improve it you are likely looking for a new board. The circuit that provides for flame sensing can fail leaving this issue as well. I've had the same problem before that resulted in a flame sensor cleaning even after I was told it was cleaned, you MUST use an abrasive to clean them, plumbers cloth works well, but and sandpaper should do the job.

Don't be afraid to post a video of the burner starting up, can sometimes be VERY helpful.

[CheapScotsman]

Thanks for your reply

 

The flame sensor was cleaned with steel wool and is very clean

The flame cuts out pretty instantly about 5-7 seconds after starting

There is no light "code". There are 3 LEDs on the control board. The two green ones are always ON while the yellow is always OFF.

During the 7 second main burners on, the full apperas to be full on and the flame sensor is embedded in the flame.

 

I am using a digital multimeter inline to measure the flame sensor current. In the cycle above

On the first fireup, it reads ZERO uAMPS MAX til flameout in 7 seconds

On the second fireup, the current reading jumps all around but didn't get above 1.5 uAMPS. Within two seconds the multi-meter is showing ZERO uAMPs for the the rest of 7 second burn till the burner cut off

 

the cycle is very consistant in its repetitiveness, however, the current draw is not. I run another cycle and get MAX 2 uAMPS for the first two seconds or flat ZERO for the first two seconds or maybe even 4uAMPS for the first two seconds ... Note that these are max. During those first two seconds the meter might show .2 then 1.5 then .8 then .4 then 1.2 ... but at the end of about two seconds, it is reading ZERO consistantly. The flame runs for another 5 then cuts out.

 

I'll take and post a video tonight.

 

Thanks for your help.

[Daveb1972]

Replace the board. The flame sense light on that one is the yellow led. The board generates a small a/c signal on that line then watches for dc micro amps if the board isn't generating a stable ac signal, then dc micro amps will be all over the place dropping to zero without cause. I presume the burners are clean, and the board c connection has a good connection to the burner. I'd run a jumper from c on the terminal strip to the burner, if the uA isn't improved by that; replace the board. ( one last thought is the hot ac line for the furnace power actually the hot line? Polarity can affect this, and if any house electrical work was done recently it could've been swapped hot/neutral. Check for adequit ground too)

[CheapScotsman]

Buners are clean (at least that is how it looks to me). Nothing has changed on either the furnace nor house wiring. I'll jumper the cable as you suggested and try and put a video together tonight.

 

Thanks for you help.

[CheapScotsman]

I jumpered the C Terminal to the burners and it made no difference.

 

While I try and get a video that works ... some additional troubleshooting ...

 

With the meter set at VAC and one lead on the C terminal (ground) on the circuit board.
 
When calling for heat (before inducer motor starts),

• The vent pressure switch reads 24 VAC on one terminal and 0 VAC on the other
• The drain pressure switch reads 14 VAC on one terminal and 0 VAC on the other

When the inducer motor starts,

• The vent pressure switch reads 24 VAC on both terminals
• The drain pressure switch reads 0 VAC on both terminals

When the igniter starts (and through the entire 7 second burn cycle)

• The vent pressure switch reads 24 VAC on both terminals
• The drain pressure switch reads 24 VAC on both terminals

Edited February 20, 2013 by CheapScotsman

[CheapScotsman]

The control board has bunch of numbers but the most relevant seem to be 1028-927, 1028-83-9271A and 62-24140-01

Edited February 20, 2013 by CheapScotsman

[Daveb1972]

Just verify L1 to Gnd is 120vAC, and L2 to Gnd is near zero. Then replace the board.

[jb8103]

Just verify L1 to Gnd is 120vAC, and L2 to Gnd is near zero. Then replace the board.

 

Dave, I got respect for your diagnostics, and 99% agree with you here, it's almost certainly the board - but I'd try swapping out the flame sensor first. I don't trust the damn things and that goes double for cleaning 'em. What's to lose, eight bucks?

[Daveb1972]

Only replaced a handful of them in my days. But I see the point.

[CheapScotsman]

Guys ... thanks for all your help

 

Detailed examination of the board indicates scorch marks under a couple of the resistors so it definitely appears that something went on
the board

 

I've replaced the main control board and the furnace is now working perfectly (again).

Edited February 22, 2013 by CheapScotsman