Rheem Classic 90 .. burners for 7 seconds then stops

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.

 

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A good flame sensor reading when the burners are lits is typically 5-6 uamps DC current. Usually the flame sensor circuit starts shutting off the burner about 1 uamp DC or less.

Since the flame sensor current is low and drops to zero, that is likely the issue causing the burner to shut off. Since you have cleaned the flame sensor and checked the wire to the flame sensor to insure that is OK, the problem is likely a circuit board that is defective and needs to be replaced.

What is the make and model of the circuit board or ignition control module on the furnace?


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Ummm. The burners wont turn on unless the pressure switches are closed and have 24 VAC to them. I would suppose you are not measuring that propely. Please describe in detail how you are measuring that voltage.

 

Took a quick look last night for the board number and and there are a ton of number on this board. I'l check that out again tonight.

From this thread (http://www.doityourself.com/forum/ga...-lit-help.html post 10): I put the meter on VAC and measure the two pins on the pressure switches. They read 24 VAC before the cycle starts but as soon as the cycle starts (and completely through the 7 second burner shutoff), the meter reads ZERO VAC.

 

Connext one test leads from the multimeter to the C terminal on the circuit board or a bare piece of furnace sheet metal as ground. Set the multimeter to measure AC voltage greater than 28 Volts --- 50, 100, 200 or whatever.

Use the other test lead to measure the voltage at the pressure switch terminals. You should have 24 VAC or thereabouts when the thermostat is calling for heat on one terminal, and at both terminals when the inducer motor has powered up and the pressure switch is closed.

 

Thanks very much for your assistance.
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

 

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

 

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).