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  #16  
Old 03-19-2009, 10:36 AM
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No. Series would Increase RT.
The resistor bridge is in parallel, and that lowers the CTS Resistance Total . [ parallel circuit RT is always less than smallest R in circuit], thereby fooling the ACC panels cut-in spec. The sensor decreases R as the heat increases [ Negative Coefficient Thermistor].
The problem with the bridge is if guys go to exteme R value for bridge, this same sensor is used for a/c compressor cut-out...so, I always use a R that will give me a 100C cut-in, never any lower.
The other concern is if one uses my unplugging the sensor for default aux fan test, the test will not work b/c the ACC still sees R factor [ bridge] even though the sensor is unplugged, thereby eliminating the princible of the tests..test requires the ACC to see OPEN circuit at CTS..a safety factor built into the system so that is a CTS or wiring fail, you still have an energency/default aux fan to ward off overheat condition that could blow the engine. Good thinking on Benz part..........without this default design, one could easily overheat the engine b/c of a bad $20 CTS sensor
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Last edited by Arthur Dalton; 03-19-2009 at 10:57 AM.
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  #17  
Old 03-19-2009, 10:57 AM
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Sorry Arthur, Business Administration major here; we're a tad weak on principles of electricity. I do understand that leaving the bridge in place defeats the quick aux fan test b/c it leaves resistance in place. I guess I'm not ready to picture how two thermistors connected one to the other create a parallel condition. Seems counter-intuitive, since it seems that the resistance created from the first one then goes through the second one with a similar effect, thus increasing resistance. Is it b/c of the negative coefficient business that it actually goes down? I'll just have to spend some time with a basic electric concepts text one of these days. Thanks again.
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Past Mercedes-Benz:
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  #18  
Old 03-19-2009, 11:05 AM
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The concept you are having a problem with is simple...Series circuit resistors INCREASE RT ..Parallel resistor circuits DECREASE RT. .And b/c the sensor decreases as it heats, by lowering its R with another parallel R, the R factor triggers the fan EARLIER than normal temp spec. If the CTS was not a NEG type sensor, the R bridging would not work. You are lowering the RT of the sensor before its normal heat/R relationship...in other words , you are taking it out of spec to the low side. The ACC panel triggers the fan when it sees a DECREASE in R, not an Increase. And b/c you have lowered the RT, it reaches the cut-in R spec earlier [ @ a lower temp], thereby fooling the ACC into thinking the temp is 105C, when it is really NOW a lower temp than that [ depending on R factor of bridge R used]. You have not changed the ACC panels trigger spec..you have changed the sensors temp/resistance relationship value...ie, taken it OUT of Factory spec. ya broke it to your liking ......
There is a Factory spec. chart for Temp/Resistance values for the CTS ...with the bridging R you have, your sensor would actually Fail the factory value test...but that is why we do it...and with a little math, we can change the sensors value to come on at about any temp we want.
Remember, the bridging R is NOT another thermistor..it is a set value Resistor.


http://physics.bu.edu/py106/notes/Circuits.html
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Last edited by Arthur Dalton; 03-19-2009 at 11:48 AM.
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  #19  
Old 03-19-2009, 07:32 PM
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Originally Posted by 04 Diesel View Post
I have a 1986 300E, so I am good to go. Thanks.
Ok did the jumper test and nothing happened. So I found the plug from the resistor, unpluged it and put jumper wirer to the red wirer on the fan and it came on. So does that mean that my resistor is bad?
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  #20  
Old 03-19-2009, 07:53 PM
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WHICH jumper tests. ????????? .there are TWO..one is high pressure sw and the other is bat pos jumper to resistor.
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  #21  
Old 03-19-2009, 07:59 PM
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WHICH jumper tests. ????????? .there are TWO..one is high pressure sw and the other is bat pos jumper to resistor.

The jumper to the resistor from the battery
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  #22  
Old 03-19-2009, 08:05 PM
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The resistor has 2 terminals...you jumper from bat pos to one side and you should have LOW fan..you then jumper to the other side and you should have HIGH fan.
If YES, then fans are OK and R. is OK. If only High fan, but no low fan, then Resistor is open [ or burnt connections.]

If NO fans , then you have a bad wire in between R terminals and fan motors
or bad fans
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  #23  
Old 03-19-2009, 08:16 PM
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[quote=Arthur Dalton;2144627]The resistor has 2 terminals...you jumper from bat pos to one side and you should have LOW fan..you then jumper to the other side and you should have HIGH fan.
If YES, then fans are OK and R. is OK. If only High fan, but no low fan, then Resistor is open [ or burnt connections.]

If NO fans , then you have a bad wire in between R terminals and fan motors
or bad fans[/quo

Tested both sides of the resistor and nothing on both sides, so bad resistor?
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  #24  
Old 03-19-2009, 08:22 PM
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>

No, read my post.
If you bring power to both sides of the R and get NO fan, you are past the R and you have a bad wire between the R and fan motor or a bad fan.

I suspect you have corroded terminals on R or you are not making a good connection

The only time R is BAD is when you have NO LOW FAN.
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  #25  
Old 03-19-2009, 08:25 PM
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Quote:
Originally Posted by Arthur Dalton View Post
>

No, read my post.
If you bring power to both sides of the R and get NO fan, you are past the R and you have a bad wire between the R and fan motor or a bad fan.

I suspect you have corroded terminals on R or you are not making a good connection

The only time R is BAD is when you have NO LOW FAN.
I did not get a low fan. Maybe I should take connections off and clean them and see if that helps?
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  #26  
Old 03-19-2009, 08:29 PM
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Do you get a high fan??

The R has a single wire on one side and a double wire on the other...if you bring 12v to the single side , you should have low fan..if you bring 12v to the 2 wire side , you should have high fan. That's it.

If NO, then check all connections
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  #27  
Old 03-19-2009, 08:32 PM
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Originally Posted by Arthur Dalton View Post
Do you get a high fan??

The R has a single wire on one side and a double wire on the other...if you bring 12v to the single side , you should have low fan..if you bring 12v to the 2 wire side , you should have high fan. That's it.

If NO, then check all connections
I got nothing on both sides, so I will clean them up. thanks.
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  #28  
Old 03-19-2009, 08:36 PM
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OK

Clean up that fan plug that is between the R and the fan too...that could be your problem.. that is the only other connection between R and motor.

If you get a spark when jumpering to the R terminals , then give the fan a twist or a rap while holding that bat wire on R terminal...if No spark at all, then you have an open circuit .
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  #29  
Old 03-19-2009, 08:39 PM
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OK

Clean up that fan plug that is between the R and the fan too...that could be your problem.. that is the only other connection between R and motor.

How would I check the fan plug?
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  #30  
Old 03-19-2009, 08:43 PM
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I thought you already stated that you jumpered to the plug and the fan ran ???
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