Suburban SF-42 furnace problem (spooky)

[rampside61]

Here's an interesting one. The furnace in my Journey is 10 years old. It works perfectly except after doing its job and the stat kills the burner. On cool-down, the fan begins to run erratically. On, off, on, off in a very random and intermittent manor. This will continue until the burner is cool enough to shut the blower off completely. The failsafe keeps this cool-down activity going, so turning it off at the thermostat won't stop it. Observing the stat shows that it blanks out completely when the fan looses power. Since this is going on intermittently every second or two, the display flashes on and off accordingly. Ok, first thought was bad thermostat, so a new one didn't help. So my next plan is to pull the unit, put it on the bench, clean all connections, and maybe replace the thermostat switch in the burner. Has anyone one ever heard of this problem before, and suggestions?

["007"]

Welcome rampside61 to irv2.
While you have furnace open check the fan switch which allows for cooling down the burner.
Enjoy the forums and do post often.

[RV Wizard]

Which thermostat do you have on the wall? I take it, this is what you are seeing going blank. It sounds like you have an issue where the amperage of the motor opens the furnace circuit. This is probably due to a loose connection; possibly a ground on the circuit. If you pull it out and put it on the bench to test you are likely to find it performs as per design. Add a good ground and test in the coach. If this does not correct the problem, add a good 12 vdc + to the furnace. If this does not correct the problem; then it is in the furnace itself, but not likely, due to the T'stat going blank.

[rampside61]

Thanks for the welcome 007. I been reading the forum for a while, but never joined. I thought in this case I would reach out to the group and tap into the experience on this issue though.
The fact that my furnace problem is intermittent doesn't make it any easier to diagnose. One night it’ll come on and have no problem. The next night, I will have this cool-down problem.
Task number one for sure will be to clean all connections first. Looking at previous threads has this and the grounds are the number one issue. Next in line, my unit has two other possible culprits. There is a time delay relay, and a limit switch on the burner.
I plan on pulling the unit out next weekend. I’ll report what the bench and the multi-meter tell us.

[rampside61]

Thanks Mike, I appreciate your thoughts on this.
The new stat is a Coleman-Mach True Air model. It is the correct model and upgrade for my 2 stage basement air. Although it didn’t solve my problem, it does a much better job of controlling the temperature then the old original one, so the replacement wasn’t a total waste.
Right now, my focus is directed toward the limit switch on the burner and a time delay relay (after checking grounds and connections). The main reason for this is that when the furnace kicks on, the fan runs properly, sail switch allows the burner to come on, stat eventually shuts off burner, and then during cool-down, it acts crazy.
It is pretty clear that this furnace has never been pulled before, so I think while the weather is good, now is the time to take care of this. It’s due for a thorough cleaning and check-up anyway. These switches are not that expensive, so I am thinking of just replacing them anyway. Let’s call it a furnace reconditioning. I’ll be going from Michigan to Fla and back in December, and this is the last thing I want to have problems with on that trip.

[rampside61]

I just wanted to post a final update and say that the furnace is fixed. I followed everyone’s suggestions which were all good maintenance things to check anyway. I pulled the furnace which was surprisingly easy (a first on a motorhome), and changed out the thermostatic limit sensor and the time delay relay. Easy parts to find and easy to install. Total out of pocket was around 30 bucks, so pretty happy about that one. I’m not counting the thermostat as it was kind of a different issue. It was an old manual one and unreliable. With the new stat and the furnace working properly, the MH stays toasty warm. I want to thank everyone for their suggestions and know that we ended up with a very positive result.

[Good man]

Quote:

Originally Posted by rampside61 I just wanted to post a final update and say that the furnace is fixed. I followed everyone’s suggestions which were all good maintenance things to check anyway. I pulled the furnace which was surprisingly easy (a first on a motorhome), and changed out the thermostatic limit sensor and the time delay relay. Easy parts to find and easy to install. Total out of pocket was around 30 bucks, so pretty happy about that one. I’m not counting the thermostat as it was kind of a different issue. It was an old manual one and unreliable. With the new stat and the furnace working properly, the MH stays toasty warm. I want to thank everyone for their suggestions and know that we ended up with a very positive result.

Great!, Rampside61,
Welcome to this wonderful community! ...glad you were able to get some good information and solve your issue!
...and THANK YOU for being courteous enough to come back on and "close the loop" of information.
Many of us follow these threads to learn, and we get "invested" in the details. It is so very helpful when we can learn what did and did not work.
It's all good,
Jim