Rheem pilot lights but no main burner

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Old 02-29-08, 06:33 AM
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Rheem pilot lights but no main burner

Rheem 80 plus 20 years old this month
Robertshaw 780-715u ignition control

I have been having an intermitant problem that has just become a constant problem. When the t-stat calls for heat, the pilot lights and the ignitor stops clicking but no main burner. The fact that the ignitor stops clicking tells me that the flame sensor circuit is working, right? If I unhook the flame sensor wire while the pilot is lit, the ignitor starts clicking again. Anyway, as soon as the pilot is lit and the ignitor stops, I measure 16 VAC on the main gas valve wire(should be 24 VAC). I measure a good 24 VAC to the pilot gas valve. If I jump a wire from between the pilot gas valve and the main gas valve, the main burner lights and works normal.
I think all of this trouble shooting leads me to replace the ignition control module but wanted another opinion or two before spending the money.
 
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Old 02-29-08, 12:48 PM
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Pls post the full M/N of this furnace, such as: R G _ _ ...
as well as gas valve Mfr & M/N
 
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Old 02-29-08, 04:03 PM
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Similar problem

I think you have a similar problem as mine. Do you have a schematic? I found mine on the back of the furnace cover. Helps a lot.

Wei
 
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Old 02-29-08, 04:50 PM
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Sorry for lack of model info, here it is:
Furnace:
Rhhem RGDC-10ECBJR

Gas Valve:
Robertshaw 7100 DER-S7C
71F-11A-001

Thanks for any help you can give me.
 
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Old 02-29-08, 07:45 PM
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Originally Posted by skarps View Post
When the t-stat calls for heat, the pilot lights and the ignitor stops clicking but no main burner.The fact that the ignitor stops clicking tells me that the flame sensor circuit is working, right?.
Right!

Originally Posted by skarps View Post
I measure 16 VAC on the main gas valve wire(should be 24 VAC). I measure a good 24 VAC to the pilot gas valve. If I jump a wire from between the pilot gas valve and the main gas valve, the main burner lights and works normal.
I think all of this trouble shooting leads me to replace the ignition control module but wanted another opinion or two before spending the money.
Try this before condemning the module:
1. notice that the gas valve has 3 terminals, P, C & M
A Blue wire is connected to the P
A Yellow and a Black are connected to the C
A Red and another Black are connected to the M
2. Disconnect both black wires from C&M at the gas valve,
AND the red wire from terminal M also at the gas valve
3. Call for heat
4. The ignitor will spark, the pilot light-up and then the spark will stop. After the sparking ceases, take a voltage reading, at the module, across terminals MV & MV/PV. If you still read 16V, the module is bad. IF you read 24V, turn the heat OFF and continue to step-5
5. Turn the heat back ON and wait until the pilot is lit and the spark ceases, take a voltage reading as follows: one lead of the meter at terminal C on the gas valve; hook up the other lead to the terminal of the Red wire you disconnected on step-2. If you read 16V, the red wire is frayed...replace. If you read 24V move, turn the heat OFF and on to the next step
6. Reconnect the red wire to terminal M on gas valve. Turn heat ON and wait as before... Take a voltage reading across terminals C and M at gas valve. You should read 24V...if you're reading 16V, the gas valve is defective. Stop call for heat
7. Reconnect the black wires. Call for heat and wait until pilot is lit and sparks cease. Take one more reading between C & M at gas valve. If you read 16V the timer element in the Fan/Limit switch (Honeywell 4064W) is bad, replace the entire fan/limit switch.
 
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Old 02-29-08, 08:33 PM
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Thanks for the reply PFLOR!
I have an upflow furnace so I do not have the black wires connected to the main burner wires at the gas valve. Non the less, I went through the rest of your troubleshooting guide and found the following. With the red wire disconnected from the M terminal on the gas valve it reads 24 VAC. Hook it back up and it drops to ~16 VAC. I would think this means bad coil on the main gas valve that is pulling too much amperage and causing the voltage drop. I still suspect the Ignition control though because I can jump a wire from the P to the M and light the main burner. That is how I have been heating the house for two days. When it gets cold in the house I go down and make sure the pilot lights and then jump to light the main. So this meens I'm actually firing both gas valves with the pilot output from the ICM. If I have a bad gas valve I would think this wouldn't work. Do you agree with my thinking?
Again, thanks for your time and help.
 
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Old 02-29-08, 09:00 PM
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Your suggestion does make sense.

That module should not be too expensive anyway, so it is sound self-advice to go ahead and get a new module (and is less work to replace a module than a gas valve).

And.......if with the new module you continue getting the 16V... ...you know what to do next.

Just playing devils advocate:
The manual for the valve/module combo indicates that the anticipator should be set at 0.7A, out of which 0.5A is reportedly the load (the gas valve) while 0.2A should then be for the module itself (the electronics in it). How many amps is your gas valve pulling?
 
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Old 03-01-08, 11:02 AM
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I checked amperage with the pilot and main jump together and it is @ 1.25 amps.

I also have a 24 vac coil relay that says it takes .1 amps max to energize. I tried energizing that instead of the main gas valve and it would energized maybe 1 out of 5 tries. The voltage dropped to 16 VAC when trying to energize that coil.

I feel pretty confident that it is the controller. I will buy one on Monday and give it a try. I'm going to buy the Honeywell
S8610U3009 which has a 1 amp pilot output and a 2 amp main output. It also has diagnostics on it.

Interesting point. The furnace was installed 2/1988 and had an ICM replaced in 2/1998. Looks like 10 years is about it for this ICM.
 
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Old 03-02-08, 12:40 PM
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fixed!!!

Well, it was the Ignition control that was bad. I took it off and out of curiousity I pulled the cover off to look at the circuit board. When I did that I found a broken soldier joint that connected the output relay to the main valve connection. Fixed the soldier and it is working. I will now order a replacement ICM online and have it on hand for the future.

Thanks PFLOR for your help.
 
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Old 03-02-08, 01:49 PM
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your welcome...glad to read things have worked out for you quite nicely
 
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