conventional thermostat failure modes

[Tmoose]

I've had (wax pellet?) thermostats stick partially open, and one whose metal frame broke, leaving it open.

Is there a "stuck closed" failure mode to worry about, too?

Dan T

 

[IRstuff]

Unlikely, but could happen, I suppose; relay contacts can weld themselves together, but usually due to arcing or overcurrents, though.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[btrueblood]

Tmoose,

The stuck closed failure for the wax pellet actuator would be due to diaphragm rupture and loss of the wax fill. Not a common failure mode, but can happen, most especially if the diaphragm gets overheated (but it would take temperatures over 250 to 300 F for that to happen), or lots of cycles at middling-high temperatures.

 

[evelrod]

Interesting. I've had two failures in something over a half century of driving. Oddly (perhaps?), both were "stuck closed"!

Rod

 

[MikeHalloran]

I've also had two stuck closed.

I've discovered a few stuck open or broken open, mostly during teardowns for other reasons.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Were the ones that "stuck closed" one of the typical modern types?

http://www.hondacivicrepairs.com/views/honda-civic-thermostat.JPG

thanks,

Dan T

 

[btrueblood]

Not seeing much "modern" about that...looks just like all the others I've seen over the years.

Regarding stuck closed failure mode, and diaphragm ruptures...it's a tricky problem. The best seal for hot water is EPDM. But EPDM is attacked by paraffins (e.g. hot molten wax). So the diaphragm is usually a nitrile rubber, sometimes a Viton or similar. It's basically then a race as to whether the diaphragm rots away due to hydrolysis attack from hot water, or gets dissolved/softened by hot molten wax.

There are some more "modern" designs with a viton diaphragm, and a seperate rod seal or wiper of EPDM.

Oh, another failure mode is a stuck-open one, and happens when the piston is allowed to stroke a little too far, and the diaphragm inverts. Reverting the diaphragm is sometimes possible, but takes a lot more force than the return spring usually has in it. This mode is usually an infant mortality failure (one cycle), unless something else breaks or deforms first.

 

[patprimmer]

I suspect when compared to stuck open, stuck closed is much more likely to get the operators attention rather quickly, one way or another.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[murpia]

The only time this ever happened to me, the thermostat stuck closed. A coolant hose blew off on the motorway. The AA guy removed the thermostat, refilled the coolant and off I went minus the thermostat and completed my journey.

Was an 80's car.

Regards, Ian

 

[NickJ67]

I have seen both, but fail open or early open/not fully close are by far the most common.

As Pat remarked, fail closed does tend to get immediate attention!

Regards

Nick

 

[thruthefence]

"stuck closed is much more likely to get the operators attention rather quickly,"

Ya got that right!