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Ambient temp sensor, DIY replacement with generic thermistor possible?

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Freight
Posts: 95
Joined: Dec 26, 2024 3:19 PM
Location: NY

Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Freight »

Looking at prices for the real deal, got me wondering if it can’t just be replaced with a generic temperature probe or substituted with, for example, an e39 temp sensor? Mine is pretty inaccurate so which irks me.

does anyone happens to know the details of these sensors on our e28s? Are they simply 12v NTC thermistors? It would certainly be easy enough to take a dead oem sensor, modify it to take a thermistor, and rewire/seal it up
gadget73
Posts: 1308
Joined: Nov 22, 2017 10:30 PM
Location: New Jersey

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by gadget73 »

I don't suppose the manual has an ohms vs temperature chart?  If it does you can figure out what it is. 

NTC Thermistors - Calculate Beta Values | Ametherm

will figure out the temperature curve, at that point you can pick a replacement based on ohms at whatever the standard temp is (25C I think?) and the beta value. 


If the temperature vs ohms chart looks more like a straight line instead of a curve, it may have a fixed resistor strapped across it.  I found that with one of my cars.  there was a change from early vs late, and the difference is one had a resistor, the other did not. Simply clipping the resistor made it work correctly.  If that fixed resistor has drifted that may be why the display is wrong though. 
Mike W.
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Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Mike W. »

There was a thread long ago on this, someone might even have been making replacements. IIRC it was a thermisistor, but I don't recall the specs. How much have you searched?
Freight
Posts: 95
Joined: Dec 26, 2024 3:19 PM
Location: NY

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Freight »

gadget73 wrote: Feb 25, 2025 9:26 AM I don't suppose the manual has an ohms vs temperature chart?  If it does you can figure out what it is. 

NTC Thermistors - Calculate Beta Values | Ametherm

will figure out the temperature curve, at that point you can pick a replacement based on ohms at whatever the standard temp is (25C I think?) and the beta value. 


If the temperature vs ohms chart looks more like a straight line instead of a curve, it may have a fixed resistor strapped across it.  I found that with one of my cars.  there was a change from early vs late, and the difference is one had a resistor, the other did not. Simply clipping the resistor made it work correctly.  If that fixed resistor has drifted that may be why the display is wrong though. 
I’ve not found anything suggesting what ohm they are. What year was your car with the resistor? I guess I knew that thermistors worked off ohms, but it didn’t click. That’s what I’ll do. I’ll take a couple readings are various temps and find a thermistor that matches those. I didn’t see a resistor on mine but I’ll take a closer look. 
Mike W. wrote: Feb 25, 2025 1:06 PM There was a thread long ago on this, someone might even have been making replacements.  IIRC it was a thermisistor, but I don't recall the specs.  How much have you searched?
I did see that, but no specs anywhere. I was hesitant to make a thread because that, but seeing it was dead(years since it was last updated iirc) and I saw no specs anywhere, went for it.  
Mike W.
Posts: 27513
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Mike W. »

Freight wrote: Feb 25, 2025 3:36 PM

I did see that, but no specs anywhere. I was hesitant to make a thread because that, but seeing it was dead(years since it was last updated iirc) and I saw no specs anywhere, went for it.  
No worries, better a new thread than to reactivate an old zombie one.
gadget73
Posts: 1308
Joined: Nov 22, 2017 10:30 PM
Location: New Jersey

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by gadget73 »

I’ve not found anything suggesting what ohm they are. What year was your car with the resistor? I guess I knew that thermistors worked off ohms, but it didn’t click. That’s what I’ll do. I’ll take a couple readings are various temps and find a thermistor that matches those. I didn’t see a resistor on mine but I’ll take a closer look. 
 
This was stuff for my 84 Continental's climate control.  The sensor changed somewhere around 1988 when the control unit was changed, and mine has a later unit installed. The Ford manuals have a handy degrees v ohms chart for troubleshooting the system.  The two sensors have a different part number but when I pulled the thing out to confirm dimensions on the thermistor I found the resistor soldered across it.  A little math of the fixed resistor in parallel with the value of the sensor that my late control unit required gave me values dead-on the 1984 chart so I simply removed the resistor. 



anyway the problem with measuring your existing one if the sensor is wrong, you'll just end up with a new sensor that gives the same wrong readings.  Thats why I was asking about factory manual specs.  Also you can just use those to compare expected vs actual readings. 
Freight
Posts: 95
Joined: Dec 26, 2024 3:19 PM
Location: NY

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Freight »

gadget73 wrote: Feb 26, 2025 8:34 AM
I’ve not found anything suggesting what ohm they are. What year was your car with the resistor? I guess I knew that thermistors worked off ohms, but it didn’t click. That’s what I’ll do. I’ll take a couple readings are various temps and find a thermistor that matches those. I didn’t see a resistor on mine but I’ll take a closer look. 

This was stuff for my 84 Continental's climate control.  The sensor changed somewhere around 1988 when the control unit was changed, and mine has a later unit installed. The Ford manuals have a handy degrees v ohms chart for troubleshooting the system.  The two sensors have a different part number but when I pulled the thing out to confirm dimensions on the thermistor I found the resistor soldered across it.  A little math of the fixed resistor in parallel with the value of the sensor that my late control unit required gave me values dead-on the 1984 chart so I simply removed the resistor. 



anyway the problem with measuring your existing one if the sensor is wrong, you'll just end up with a new sensor that gives the same wrong readings.  Thats why I was asking about factory manual specs.  Also you can just use those to compare expected vs actual readings. 
Ah, yeah I didn’t see a resistor on mine so it being for another car makes sense.

yeah I realize it’ll get me started with one that’s off, but thermistors are dirt cheap so if I start with some that matches the ones I’ve got, I have a sound baseline to begin purchasing alternatives. Otherwise it’s just totally shooting in the dark trying to find one that gets close, just to build off of.

i looked a bit last night and didn’t see any ohm values anywhere, but I’m going to take a second look at the Bentley for good measure
db9
Posts: 113
Joined: Jun 30, 2019 8:27 AM
Location: Mississauga, Ontario

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by db9 »

Not sure if this is of any help. This is from the e34 HVAC stuff...

  Image


  Image


  Image

  Image




 
Freight
Posts: 95
Joined: Dec 26, 2024 3:19 PM
Location: NY

Re: Ambient temp sensor, DIY replacement with generic thermistor possible?

Post by Freight »

db9 wrote: Feb 28, 2025 1:52 PM Not sure if this is of any help. This is from the e34 HVAC stuff...

  Image


  Image


  Image

  Image





 
I’ll check realoem to see if the sensor part number is the name but if so, the sheet for the exterior sensor should be what’s needed
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