Bryant Plus90 won't ignite main burners

[richdog33]

I am having a similar problem to cliff m (but for some reason, being a newbie, I can't reply to his post). I have a Bryant Plus90, 1988 - Model #: 398AAZ036080.
I turn on the heat, induction motor kicks in. Ten seconds later, the ignitor sparks and lights the pilot. After about few minutes of the main burners not kicking on, the pilot goes out, followed by the induction motor.
I've probed around to see what has power where, and came up with the info in the attachments:

http://www.richbloom.com/images/Wiring1.jpg
http://www.richbloom.com/images/Wiring2.jpg
Note: Red star: 24V +
Blue Star: Common (ground)
Yellow Star: 120V

I assume the pressure switch is Ok because the pilot lights. I also assume the flame sensor is good because the ignitor recognizes the flame of the pilot and stops sparking.
I haven't checked gas pressure (yet- I will if necessary), but I have gas to the pilot- which leads me to another assumption that I have adequate gas pressure.(?)
I'm stumped. Does anyone see a ground where there should be power (or vise-versa)? Should the white wire from the pilot have power? Anything else I should check?

Any help would be appreciated!

Thanks,
Rich

[richdog33]

This has to be everyone's hopeful success story... I didn't spend a dime!
After mulling over the drawings, and deciding what I needed where (voltage-wise), I concluded that I should have power on the white wire at the gas valve (from the pilot assembly). So I removed the white wire from the gas valve and fired the system up. After the pilot lit, and I showed power on the brown wire (pin 5 at the gas valve), I jumpered 24VAC power to pin 1 (the suspected white wire's connection) on the gas valve and POOF! the main gas to the burners came on. Now I knew it had to be the pilot assembly (there are diodes on ILO and HFR, so it can't come from there! (refer to Wiring2.jpg)). I removed the assembly, ready to get a new one tomorrow. After looking it over, I removed the bracket that mounted it to the burners, and noticed some corrosion between the two mating surfaces (the circuit is grounded through the equipment (i.e. the mounting bracket)). I sanded both sides down, reinstalled it, and POOF! again.

Problem solved!

I edited Wiring1.jpg to show what wire should have had power.

[richdog33]

I wonder how long I have had this problem? I have had intermittent furnace problems for the past few years, usually when I tried firing up the system for the first time each season. I also had a problem where the furnace would occasionally not fire up in the dead of winter (the past two years). Over the years, I have probably paid about $600 in service calls and parts, but the problem was never fully fixed. It makes me wonder how much the professionals out there are really educated on the electrical diagnostic side of furnaces (at least local to me) rather than just hanging parts?

Just a thought.

[roddhall]

Thanks! Your posts saved me the cost of a new pilot assembly. Every detail of your problem exactly matched one I was having until yesterday. Proves my first rule of repair ... take it apart and look for something that doesn't seem right.

[richdog33]

I'm glad to see my problem could help even one person. These forums really are a valuable asset to people like us... non-professionals who want to save a few bucks!

Rich