Older Bryant 394DAC Limit Switch Issue?

[motorcitydiyer]

I have an older Bryant Furnace 394DAC and the burner will not stay lit. The burner lights and after about 40 sec. it shuts off and the cycle starts over again. The fan works when set to the ON position on the thermosat. If the fan is running when set to the ON position the burner also goes out.

From reading other posts I think I may have a limit control switch issue. I tried to test the limit control switch by putting a jumper across the two wires going to the gas valve but this didn't work. I posted a pictures of the furnance and the red circles show which two wires I put the jumper across. Is this the right way to test the limit control switch?

I'd appreciate anyone's help on how to diagnose the problem.

[HayZee518]

what's the limit switch controlling? what's it for?

[hvactech126]

I just worked on one of these with the same problem, it is the pilot switch, that furnace does not use a redundant gas valve. the pilot switch is what is bad. The furnace should be replaced rather then trying to repair it. It would need a new standing pilot gas valve and pilot assembly to bring it up to today's safety codes.... as it is set up right now, if the pilot get blown out, the main burners wont try to light, but the pilot will continually dump gas (fire hazard)