Inside / Outside Temp: How does it work?

[Zeppelinium]

Steve,

That 18K thing looks like a disc ceramic capacitor to me, but there are thermistor packages that look like that. Most of the thermistor packages look like a glass resistor or a small tube/probe with two wires coming out radially.

I don't think these units are using thermistors. Most everyone is reporting 500 degrees, which would mean low resistance and possibly a short circuit. I think it is more likely that the external sensor has a broken wire, which is an open circuit.

All that aside, I agree with fltlevel510 that the best bet is to replace the unit with a currently available substitute. You can get a very accurate dual-temp device a Harbor Freight for about $12. The external sensor is on a lead that's about 6-8' long, plenty long enough to thread through the firewall to an outside location. The only problem is making a presentable bezel/faceplate and installing it in a way that you can get to it to replace the battery every year or so.

Zep

Originally Posted by Keyair

So, I figure that it has 2 Thermisters in play... one outside somewhere and one inside.
...
I turned my attention to the unit and disassembled it looking for the inside thermister...



It seems that Thermisters come in many shapes and sizes...
Electronics are not my thing!
Could it be the small brown round unit with the black tipmarked L8k or 18K on the top left side of the above pic?

[pwhittle]

Keyair,

You should be able to locate the internal thermistor by tracing the other side of the switch from the wires to the external thermister. It may be another connector to get it away from any heat generated by the display, although that type of display is not particularly hot.

Tracing is a little tough to do from the pictures you already loaded, but should be fairly easy to do in person.

I suspect that the variable pot's are to adjust the temperature readings.

If any segment of the display is dim, try to leave it on as they get brighter with use and dimmer with lack of use.

Let me know if you are ever in the Atlanta area and we can take a look together. These types of circuits are from an era when I worked in hardware/software integration.


Paul

Originally Posted by Keyair

Pulled mine out of the dash to see if I can get it working...
Here is what it looks like..

So, the only thing wrong with it(I think) is the external "Thermister" has failed.
As I said in a previous post, mine reads +500deg when powered up.
I shorted the sensor (grey) wires together, and it reads -40c or -28f when I do that.
I know that a "Thermister" is short tech-speak for a "thermal-resister".
If I can find the correct unit, I can fix it, but I am not a electronic person, so maybe someone who undersatnds this stuff can explain what I need?

[Keyair]

Thanks Paul.
I never thought about the fact that it might have had a external sensor for the inside temp...
When i next pull the dog house off I am gonna chase the external temp thermister.
More soon...

[Keyair]

Well, what a surprise....
In a previous post I had traced the wires to the external thermister from the dash, thru the firwall and into the fwd part of the engine compartment. It then was bundled with the main harness and went rearward on the drivers side bundle...
So, yesterday, I pulled off the dog house, found the wire and followed it down the harness, around the rear of the engine, and down under the passenger side floor....
I got under the M/H to trace it further... and there it was.........
The wire terminated just rearward of the engine, next to the bellhousing.....
SNIPPED OFF!
No Thermister.

So back to guessing...
But it does explain why both sensors read the same!

Zep and Paul...
You guys know more than me about this stuff...
I spent some time looking at the unit thru a magnifying glass, looking for part numbers, and looked them up on the internet... no joy yet!
I agree with you Paul, the pots on the board i also assumed were there to tweek the readings for the external thermisters, so lets me pull it out again see if I can trace the onboard thermister.
Also, I will see what voltage the unit is outputting to the sensor wire, and what the readings are when I put them together.

I know I should give up, and get something that will replace it... but I am a crusty old gog on a bone!

[Zeppelinium]

Originally Posted by Keyair

...So, yesterday, I pulled off the dog house, found the wire and followed it down the harness, around the rear of the engine, and down under the passenger side floor....
I got under the M/H to trace it further... and there it was.........
The wire terminated just rearward of the engine, next to the bellhousing.....
SNIPPED OFF!
No Thermister. ...

Surely (Shirley?) with your "map" someone else could find the thermistor and report any markings...

[Keyair]

So, I am awake last night... can't sleep, so I decide to sift thru my photo's, and try to track this dang Interior Thermister...
My Logic was that if I could find the one on the board, I could match it and use the same for the exterior Thermister...
I carefully tracked the trace from the switch to the soldered connection for the Outside Sensor wire...

Paul actually NAILED IT!
I followed his lead... and looked at where the 2 Grey Exterior wires were...
Using the zoom feature on my Photo program....
And there it is... Obvious when you look!

Here is the back of the board...



Zoomed in a little... The wires for the exterior sensor are on the right...
but what is that thing soldered to the board in the black circle?
A twisted wire... soldered to the board... Odd...


Zoomed in more, and it hit me!
There is the broken off wire for the interior sensor... You can see the sub of wire, and grey insulation on the lower edge!



Clearly the Interior sensor is external, and had a pair of wires!
Holy Smoke!

Now I just need to understand what value Thermister to buy!

[DaveFL]

"Could it be the small brown round unit with the black tipmarked L8k or 18K on the top left side of the above pic? "
------------------------------
From here it looks like a capacitor.

RES 18K OHM 10W 5% WW LUG - 1630184-3

the one outside will probably be the wire looking like it was twisted together and maybe a sleeve of shrink wrap or a small 1/4 button like it was dipped in epoxy.

the inside one should be away from heat generators on the circuit board.

Good suggestion was find cheap HF has them at about $8 but they require a AA battery, could be velcro mounted in place of original, as puriest old one repaired would be better.

[Keyair]

Dave,
Maybe I need to recap and clarify..
The Unit works, but gives like 500 deg reading... shorting the 2 grey wires together gives a different read, but I dont remember what..
I traced the 2 grey wires that I assume were for the "External Temperature Sensor/Thermistor" through under the dash, thru the firewall, and into the engine bay, where they were snipped off! I assume they went rearward and the sensor was mounted somewhere away from engine temperatures.
Next, I puzzled over the "Internal Temperature Sensor/Thermistor".
I spent a lot of time looking at every componant actually on the Circuit Board, and could not ID any as it. It did seem odd that it would have been mounted on the Circuit Board inside the unit housing inside the dash where it would get a false reading!

My study of, and tracing the circuits on the back at peoples suggestion led to spotting the place where the wires were!

Now, I am a dumbo with electronics, so I am reading up, but it seems I need an NTC Thermistor, but I need to know what Voltage and Amperage the Circuit Board is outputting before I select the correct Thermistor.

It would be easier to replace the whole dash unit, but I just would preferto fix it if I can!

[DaveFL]

Your circuit board has a couple of adj. pots to the right of the black switch, probably to set temperature reading. If you find the specs, you might be able to use this one for a cheap fix.

K Thermocouple Temperature K Sensor 1M | eBay

[crazeevw]

Steve,
Where is this located on your moho? One the dash? If you get back to your Airstream, love to see a photo, as mine has a hole in between the upper cabinets @ front seat area. Beginning to think maybe that's (temp gauge) whats supposed to be there.....
Thanks, Derek