Temperature Switch (overheat) - Interesting Triva

[Wotname]

Well maybe interesting to some (like me ).

I removed a overheat temperature switch for checking the other day - it was around 35 years old. It took some effort (breaker bar etc) to get it out of the head water jacket.

When it was first heated to the rated switch temperature (205F), the contacts inside the housing measure about 100 ohms - I figured this was way too high but who am I to know for sure. Cold was open circuit of course.

So I repeated the heating cycle, now it read ~80 ohms - interesting!

Cycled again and now ~40 ohms - heading in the right direction.

Cycled it about a dozen times and the resistance steadied out to be 2 or 3 ohms - much better.

I figure there was some oxide on the contacts and exercising the switch contacts cleaned them up.

Probably should replace the switch.....

[Spot]

We take for granted those sensors and gauges are working, good on you for testing it.
At work we have a person who goes around and checks to make sure gauges and other instruments are in calibration.

[tkeithlu]

"Probably should replace the switch....."

Yeah. That little switch has done its life's work.

[Wotname]

Originally Posted by tkeithlu

"Probably should replace the switch....."

Yeah. That little switch has done its life's work.

Yeah but...that little switch has never done a day's work ever until I tested it

[sailorboy1]

That is interesting. Now i will have to look at the construction of these switches to see why any corrosion would cause the problem. I know mine worked 2 year ago because my belt broke and it alarmed. I also know 10 ago i changed my switch because it failed and always read low.

[Scubaseas]

Originally Posted by Wotname

Well maybe interesting to some (like me ).

I removed a overheat temperature switch for checking the other day - it was around 35 years old. It took some effort (breaker bar etc) to get it out of the head water jacket.

...

Probably should replace the switch.....

Removing the switch may have affected the performance. The guts probably have not moved in ages if ever. Not that uncommon to have a switch like this have to be cycled to get down to spec. They work like thermostats and radiator fan switches. The mechanicals can get "stuck" sometimes (maybe 1 in 50 new ones from my experience) and you have to go over temp once to get them to work on the set temp. Check it tomorrow and if it's good I'd put it back in. Or if it makes you feel better replace it but the new one may overshoot the mark the first time.

[motretu]

you've addressed a problem with mechanical oheat detectors, which is that they rarely get used or tested. not uncommon for mechanical switch contacts to develop a high resistance when not exercised for a long time (years). each time the contacts are brought together, they rub a bit and remove the thin layer of high resistance material that develops when not used. current will do it as well, but the current from a DVM is usually not high enough.

[Wotname]

Originally Posted by sailorboy1

...... I also know 10 ago i changed my switch because it failed and always read low.

Was that low temperature i.e. operated below target temperature or always read low resistance even when cold?

I guess both are failsafe modes which is good thing but I wonder if this a common failure mode.

Always remaining open circuit when above target temperature is what worries me but I dunno how common this is.

[Wotname]

Originally Posted by motretu

..... current will do it as well, but the current from a DVM is usually not high enough.

Yes, I did consider the current flow through a traditional incandescent warning light (idiot light) would be beneficial but maybe a modern LED not so much.

[Wotname]

Originally Posted by Scubaseas

....... Check it tomorrow and if it's good I'd put it back in. Or if it makes you feel better replace it but the new one may overshoot the mark the first time.

I took your advice and tried again today - not good, not bad.
~40 ohms first cycle decreasing to 5 ohms after a few more cycles.

Yes, it will go back in until I hunt down a replacement. It is from Ford Lehman and I haven't much experience in chasing Ford Lehman parts but it can't be too hard .

Designator on the switch body is AIRPAX 8419 & AIRPAX 8406 & CLR205F

Thread OD is 5/8" with a slight taper and a pitch of ~ 1/6" perhaps closer 61 thou.

[Compass790]

My question is, does the alarm work at 40 ohms or 100 for that matter? This may be a dumb question but as you know I'm no sparky. Is it a light or a sound alarm?
Just trying to learn something here.

I imagine you can find a suitable non Ford Lehman replacement if part is hard to come by.

[Spot]

Originally Posted by Wotname

I took your advice and tried again today - not good, not bad.
~40 ohms first cycle decreasing to 5 ohms after a few more cycles.

Yes, it will go back in until I hunt down a replacement. It is from Ford Lehman and I haven't much experience in chasing Ford Lehman parts but it can't be too hard .

Designator on the switch body is AIRPAX 8419 & AIRPAX 8406 & CLR205F

Thread OD is 5/8" with a slight taper and a pitch of ~ 1/6" perhaps closer 61 thou.

It might be 3/8" x 18 TPI NPT.

[Wotname]

Originally Posted by Spot

It might be 3/8" x 18 TPI NPT.

Yes, it is 3/8" NPT.

When I looked at my original measurements I realised they didn't correspond to any known thread - ergo I must be wrong. Another look with wide awake eyes quickly confirmed the thread to be 3/8" NPT.

[Scubaseas]

Originally Posted by Wotname

Yes, it is 3/8" NPT.

When I looked at my original measurements I realised they didn't correspond to any known thread - ergo I must be wrong. Another look with wide awake eyes quickly confirmed the thread to be 3/8" NPT.

What is the warning system? If it's a light a 3/8" NPT grounding sender is easy to come by https://www.summitracing.com/parts/qcr-61-748 as an example. Which is a 235*F grounding switch. You can get them from 165*F to 235*F.

[Wotname]

Originally Posted by Compass790

My question is, does the alarm work at 40 ohms or 100 for that matter? This may be a dumb question but as you know I'm no sparky. Is it a light or a sound alarm?
Just trying to learn something here.

......

The short (and most accurate) answer is - I dunno because I haven't bothered to try it on the bench. I not intending to run the engine up to overtemp conditions to try it on the boat .

In theory, a 2W 12V incandescent warning light has a operating resistance of ~70 ohms so having an additional series resistance of 100 ohms is only going to allow about 5 V at the lamp so it would be very hard to see unless it was inside a cow.

But there is more to the theory, any current flowing through the switch contacts will 'wet' the contacts and reduce the resistance i.e. the 100 ohms will become less once some decent amount of current flows through it. Plus the initial resistance of an incandescent filament is about 10 times less that the steady state operational resistance. So this means the initial current through the switch contacts will be 10 times greater thus 'wetting' the contacts even more.

The end result could well be the switch resistance will decrease enough in the first few milliseconds so that the warning light can be seen outside of the cow.

Best way is to try it on the workbench and maybe one day I will but don't hold your breath .