bryant plus 90 won't fire up

[cliff m]

I have an older Bryant plus 90 furnace (Model 398AAW048100)--installed in 1988. Tried starting it up the other day and here's what took place.

The inductor fan immediately came on and the pilot light fired up properly after a few seconds--so far so good.

Normally at this point (within perhaps 20-40 seconds) I hear a repeated clicking sound and the gas valve opens. No sound this time. I wonder if there is some sort of flame sensor--thermocouple??--that might be malfunctioning. Anyone know what voltage signal should be output from the sensor (if there is one) to the gas valve? The gas valve has a number of wires going to it --naturally with no labeling. Any help would be greatly appreciated--Thanks

[HayZee518]

ok, you said the pilot fired up - that's good. in the flame is a sensor, a flame rod or a millivolt generator. they both detect fire in order for the main valve to open. the millivolt generator has a cappilary with a button that goes into the main valve, the flame rod is electronic that uses just one wire AND furnace ground feeding a heat reference back to its controller. try changing the flame rod or just cleaning it up a bit with 320 sand paper.

[hvacwiz]

Your electronic 3 wire pilot assembly needs replaced.