American Standard Gas Pack Blower Problems

New to the forum, though I've used it as a source of information and inspiration several times over the last few years.

I have a problem with my natural gas furnace. It's an American Standard gas pack, model: YCC036A1L0AA, circa 1990. (I expect to have to buy a new unit in the not-to-distant future, but I'd like to keep this on limping along for another season or two, if possible.)

Recently, it's been presenting a problem, which I thought I had figured out, until a new problem presented.

What was happening for a little while until a few days ago:

Upon calling for heat from the thermostat (fan set to auto), the inducer motor would start up immediately. Then either:

a) the ignition control would begin clicking, the burners would light, the blower would come on, the thermostat would eventually be satisfied, the gas would drop out, and the blower would stop after 30-45 seconds [GREAT!]; or

b) the ignition control would not click, the blower would come on, the thermostat would never be satisfied, and the system would continue to run until the thermostat was turned off [BAD!].

Sometimes it would fire, other times it wouldn't. If I tried a couple of times in a row, it would finally work. I assumed it was failing because of a bad high limit switch (disc switch L200-60). I confirmed this, I think, when I caught it in error once and measured 25.3V across the switch. The system fired up a few minutes later, and I measured 24.2V across it, as if it had become unstuck and allowed the system to process.

Just before I bought a replacement switch (which is very hard to find), this began to happen:

Upon calling for heat from the thermostat (fan set to auto), the inducer motor will start up immediately, the ignition control will not click, and the blower will come on. Then, eventually, the ignition control will click, the burners will light, the thermostat will eventually be satisfied, the gas will drop out, and the blower will stop after 30-45 seconds.

So, it seems now, like it may be a bad fan limit switch (a separate disc), in addition to, or instead of, the high limit switch.

I'm not sure how to check the fan limit switch (in relation to checking whether the high limit switch is stuck closed or not). The high limit is a three-wire switch (black, red, and brown). Brown goes directly to the fan limit, a second brown wire then connects the fan limit switch to the (?fan relay?).

What else should I check? What other info might help trace this problem out?

The system is currently working predictably (as just described), though it sometimes takes a few minutes for the burners to light and warm air to enter the house. So, I want to know exactly what I should check the next time I open it up, for fear that I will disturb its fragile equilibrium .

Thanks in advance.

 

Sounds like a relay or control board issue. I understand you would like to limp it alone but given that it is 25 years old, I would stop dumping money into it and bite the bullet and replace it. You'll be better off.

 

Thanks, JUrban, for the reply. I'd love to replace the whole unit right now, but don't have the means. What I do have is the wherewithal to to buy and replace a few parts, if it means I'll have heat through the season.

Any advice on tracing out a faulty relay? I'm also open to the idea that this may be an ignition control problem. Any insight into troubleshooting why the system waits so long to start sparking ignition?