Ruud furnace with Honeywell controls, pilot won't ignite


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Old 11-14-12, 06:35 PM
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Ruud furnace with Honeywell controls, pilot won't ignite

The ignition module is a Honeywell S8600H and the gas control valve is a Honeywell VR8204M. Thermostat is known good. On call for heat, the ID fan starts, the ignitor sparks and there's a VERY brief flame. Listening with a stethoscope, I can tell the PV is closing right after it opened. The ID fan continues to run.

I don't see 24 V across the PV to PV/MV connection, but doesn't it have to be present for the spark to happen and the PV to open?

What's my problem? Could it be bad ground at the sensor? Can someone tell me the diagnostic sequence to follow to isolate this? Thanks?
 
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Old 11-14-12, 10:03 PM
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The spark and pilot gas should turn on together and stay on together until the pilot lights. At that point the spark should shut off and the main burner gas should be turned on.


Measure the voltage from PV to C ---- it should be 24 VAC when the inducer motor comes up to speed and the pressure switch closes.

If the 24 VAC isn't turning on and staying on when the spark is going, replace the ignition module.
 
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Old 11-15-12, 12:52 PM
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So the ID fan pressure switch has to engage BEFORE the 24 VAC is present? I need to check that either way.

But how is the PV actuating, as evidenced by the sound of the solenoid and the short flame WITHOUT having the 24 VAC signal? To put that another way, if those two things are happening, can't we assume current is present even if we didn't have a meter (I do). Just a question to educate myself.

Unit's on the roof, so I may be a while between communications.

Thanks.
 
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Old 11-15-12, 01:14 PM
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The pressure switch is closed any time the ignition module is causing a spark.
 
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Old 11-15-12, 01:33 PM
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I've checked the ignition module again. The only time there's voltage between the PV and PV/MV is when the igniter is sparking. It's not 24 VAC, it maxes my meter on the 250 VAC scale. This doesn't seem right from everything I've read and the discussions I've had so far.

Can an ignition module fail in a way that allows current to bleed to the wrong terminal?

I'm going to start shopping for a replacement module, but want to know if I'm diagnosing this correctly before I pull the trigger. Everything I've done so far keeps coming back to the module.

Thanks.
 
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Old 11-15-12, 03:02 PM
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The high voltage of the spark often causes errors in reading voltages on a multimeter.

Disconnect the voltage supply to the module so that the spark doesn't start. That allows you to measure the voltages up to that point without interference.

And please provide a description of the sequence of operation as described earlier. Random measuring of voltages is not a diagnostic method. Careful observation of the sequence of operation of the furnace is the beginning of an accurate diagnosis of what is happening.
 
 

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