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  #1  
Old 11-12-2001, 01:47 PM
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In-car temp sensor (thermistor)

Greetings,

Just wondering if anyone knows the specs on the in-car temp sensor or thermistor as it's called. Mine on the '80 300TD seems to have gone bad and am wanting to see if it's possible to replace it with perhaps a thermistor from Radio Shack. The unit doesn't show a spec on it but if you happen to be an electrical genius perhaps you know.

Thanks,

Charles
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  #2  
Old 11-12-2001, 03:01 PM
LarryBible
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Are you sure that the rubber tube behind the glove box is not rotted? I replaced mine with 3/4" foam pipe insulation. It now works great.

Good luck,
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  #3  
Old 11-12-2001, 04:23 PM
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Greetings Larry,

The in between part that I guess was foam was missing on mine, probably for quite a while. Seemed like everything worked fine until a week ago. With the thermistor removed from the speaker housing and hooked up I still get no heat in the max setting, but remove the thermistor from the plug and it turns hot then. I seem to be getting a varying ohm reading from the sensor so it does operate. Do you think the temp wheel needs adjustment? I have read about ohming it out, but don't have the special tool they show to adjust the shaft to the wheel temp. Will anything else work for that? Maybe I need to look elsewhere for the problem.


Charles
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  #4  
Old 11-12-2001, 04:51 PM
LarryBible
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I don't have experience at this level, but I might be able to take some readings from mine so you can compare. Do you have a late monovalve system or an early servo system?

I think the '80 is the servo system. If so comparing to mine won't do any good. I have a bulletin at home that may be helpful. If you will email me privately with a fax number, I can fax this information to you.

BTW, you have replaced the tubing behind the glove box, right?

Good luck,
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  #5  
Old 11-12-2001, 05:53 PM
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Can-do,

Whenever the system sees O ohms in the temperature sensor loop it causes the servo to go to full heat (the good news is at least you know your servo is working). Try putting the sensor in the freezer -when you take it out wrap it with a cold pack and try plugging it back into the system to see if that triggers it to serve up some heat. This test will show if the thermistor is working, but just out of range. This is not a definitive diagnosis of your problem as your amplifier may be out of whack.

If you do need a new sensor, RadioShack will not have a thermistor that matches the temperature/resistance curve. The reason I know this is because several years ago I attempted to do the same thing. I obtained the chart that shows the resistance vs. temperature curve on the sensor -- unfortunately I was not able to substitute it from another source.

RTH
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  #6  
Old 11-12-2001, 08:50 PM
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RTH zero ohms also means no adjustment

Greetings RTH,

I kind of figured the sensor being unplugged defaulted the valve into the heat mode only as there is no temp control at all with it unplugged, only heat. You're right, at least I know my heating valve is working. Now if I can just figure out what isn't I'll be fine. I think I'll ohm out the ambiant temp sensor next and then if that pans out alright I'll try the temp dial to ensure it's set correctly. If you have any other easier ways, let me know.


Charles
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  #7  
Old 11-12-2001, 08:59 PM
LarryBible
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Charles,

I have the troubleshooting bulletin I talked about here in front of me. It says that the in car sensor between pins 4 and 5 should be:

2000-2400 at 65 F
1590-1970 at 75 F
1170-1560 at 85F

These measurements are taken at these pins on the 11 pin connector. If there are two wires to the sensor itself, I expect that is the same thing. Sorry I'm not familiar with the early climate control.

I will do my best to remember to take this to the office tomorrow, if you need me to fax this procedure to you, just email me your fax number. This is a good article on troubleshooting the servo type system, not as good for the monovalve, later system.

Good luck,
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  #8  
Old 11-12-2001, 09:04 PM
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Thanks, But I Have the Troubleshooting pages

Greetings Larry,

I have the cd manual on the ohms / temp guide for all the related sensors for the heating input to the brain of the system, so called. I am on the way out to make sure the ambient temp sensor reads according to the sheet.

Thanks,

Charles
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  #9  
Old 11-12-2001, 09:50 PM
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Cold makes the heater work

Greetings,

I ohmed out the outside temp sensor, but with the ohm meter I am using it measures in K so it's hard to tell if it worked accurately. Outside temp about 60 F and the ambient temp sensor out of the range showed about 200 ohms. The inside temp sensor when placed on a bag of ice caused normal heating operation in the vehicle, center vents closed, heat to floor and side vents. I would imagine the ice kept the sensor at 40 F or so. Measuring the sensor now in the house at house temp of 78 F the measurement is about 1600 ohms which falls within the operating range of the sensor. Could it be the temp wheel needs to be resynced as mentioned in the manual? Has anyone ever done this without the shaft locking tool, and if so how?


Charles
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  #10  
Old 11-13-2001, 12:13 AM
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Can make the tool from flashing

Greetings,

to keep from being defeated by a tool I didn't have I cut a strip of aluminum flashing off a roll I had and cut teeth in one end of it. Seems to be the right thiness to get between the dial and the housing, and it actually worked. I managed to adjust the wheel where heat once again flows through the vents with the thermistor hooked up. I haven't ohmed it out for accuracy yet but I must be headed in the right direction at least, and hopefully problem solved. Take this one to the bank with a no cost repair, just a lot of head scratching wondering what the heck has happened thinking. Will keep you posted on my progress.


Charles
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  #11  
Old 11-13-2001, 12:22 AM
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Can-Do:


Try to get you your hands on a volt/ohm meter that you can trust to give you an accurate reading. If the sensors are within specification, look elsewhere as your servo IS operating properly (be thankful for that!) as well as the vacuum elements.

It seems unlikely that your temperature wheel is that far out of sync unless it was moved previously. If the potentiometer were dirty, it would increase resistance resulting in a greater trend towards heating, so again not very likely. The temperature wheel can be moved on the shaft without the special tool -- I happened to have had some very thin wrenches that I think were meant for bicycle hub bearing adjustments (at least I think that's what I used in the past, not sure though). Bear in mind that any adjustments made were minimal, a few degrees at most -- just enough to get the temperature wheel recalibrated to reflect actual cabin temperatures.

I have fixed all sorts of problems with these early climate control systems, including actually rebuilding the servo itself. I hesitate to make a definitive diagnosis, but I would take a good hard look at the amplifier. If you can get your hands on another one to try out temporarily, go for it.

Good luck!

RTH
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  #12  
Old 11-13-2001, 12:36 AM
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Can-Do:

I just read your post after (slowly, painfully slowly ) typing out the last response.

You can't argue with success -- if it works properly in both heating and cooling modes, then pat yourself on the back & leave your wallet in your pocket!

By rotating the temperature wheel on the shaft you are rebalancing the resistance chain comprised of the two thermistors & the potentiometer. As you may already know, the amplifier reads this resistance and supplies voltage to drive the servo until another potentiometer within the servo matches the resistance; this is how the servo ‘ knows’ where to stop within its internal travel. By adjusting the wheel, you may be making up for deficiencies elsewhere -- but like I said if it works, W. T. H.!

Congratulations!

RTH
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  #13  
Old 11-13-2001, 09:04 AM
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Unhappy 300E WITH THE SAME TYPE PROBLEM: HELP

If have a 1991 300E with nearly the same problem with my climate control system. The unit was replaced about a year ago - but I had not noticed that the temperature control was not accurately reflecting the cabin temperature.

The main problem is not enough heat (unless you put the wheel all the way to max.) If I put the wheel on 75 degrees - it remain only about 65 in the cabin. So, the temprature management part is out of whack! Also the blower fan always seems to be on at a medium level (no matter how long the system is on). HELP!

I performed air intake test on the sensor in the sunroof control (using as small piece of paper to show the suction).

Well, I think the problem is either the wheel, out of adjustment, Or the sensor (in the sunroof control) is not providing the proper ohms.

ANY thought on what might be causing my temperature problem?

If it's in the sunroof control - how do I pull it down to get at the climate sensor?

THANKS SO MUCH!!

David
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  #14  
Old 11-13-2001, 09:09 AM
LarryBible
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Charles,

I faxed you the troubleshooting bulletin. If you didn't receive it, let me know.

Have a great day,
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  #15  
Old 11-13-2001, 12:06 PM
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Thanks a Bunch Larry

Thanks Larry,

Sorry you had to call twice, that early in the morning the kids pick up the phone thinking it's a school friend calling.
The article you send was great and actually a quite reader friendly document. Don't have to assume a lot when it gives all the details of the system checks. Like I mention in an earlier post I wasn't gonna let the system beat me so I constructed a tool and seemed to have gotten the temp wheel back in sync with the amp and sensors. I'll ohm it out this morning to fine tune it to spec. Your article will come in handy because my wifes car has the other climate control it mentions and what to look at if it starts to act up. Was this a magazine or manual that the pages came from? I'd be interested in any releases that go into good detail as this.
David, after reading Larry's article I would suspect your mono valve as being your culprit. Would be more than happy to scan a copy of this article on the newer climate control and email it to you if you like.


Charles
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