charge air temp mystery

1196 Views 3 Replies 3 Participants Last post by risen, May 1, 2008

acannell

Discussion Starter · Apr 16, 2008

I'm trying to determine the resistance vs. temperature relationship for the charge air temperature sensor.

I found this table in another thread (thanks Naj!):

(charge air temp sensor temperature vs. voltage)

0 deg = 4.70 volts
20 deg = 4.47
40 deg = 4.11
60 deg = 3.67
80 deg = 3.08
100 deg = 2.51
120 deg = 1.97
140 deg = 1.52
160 deg = 1.17
180 deg = .86
200 deg = .65
220 deg =.48
240 deg = .35
260 deg = .28

This table is somewhat misleading, as the sensor itself is a thermistor, and does not output a voltage on its own. These voltages are what you would measure at the 5V reference side of the thermistor while it is plugged in.

The output impedance of the 5V source was measured and determined to be 10k by placing a known resistance across the sensor pigtail with the pigtail unplugged, measuring the voltage drop across the known resistance, and then calculating the series impedance of the 5V source using ohms law.

Based on the table above, the 5V source, and the 10k output impedance of the 5V source, the following table can be generated:

Temp (F) V Rt Rt
0 4.7 156667
20 4.47 84340
40 4.11 46180
60 3.67 27594
80 3.08 16042
100 2.51 10080
120 1.97 6502
140 1.52 4368
160 1.17 3055
180 0.86 2077
200 0.65 1494
220 0.48 1062
240 0.35 753
260 0.28 593

The first two columns are the same as the first table. The third column is the calcuated resistance of the thermistor.

These values contradict the values on thedodgegarage.com which lists the following:

32F Degrees = 29,330-35,990 Ohms
77F Degrees = 9,120- 10,880 Ohms
212F Degrees = 640-720 Ohms

Anyone know why this might be?

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risen

· #2 · Apr 30, 2008

I was actually looking into this for a piece of datalog software I'm working on. Below is a thread with some info about the resistor in the charge temp circut, and it says that it's bias resistor is 5k ohms, not 10k. So perhaps that's the problem with your final table?

http://www.turbododge.com/forums/f4/f60/79453-charge-temp-coolant-temp.html

Anyways, I just used easytherm to generate a table with an adc -> temp map based upon the 3 ohm ranges in another post here on td.com. I've pasted the header with the calculated coefficents. I can paste the whole map, but you can easily reproduce it with easytherm, and it's pretty big. The adc table does look to be accurate compared to all the other info I've seen.

; Generated using EasyTherm4.exe
;
; Coolant Thermistor Comment Field
;
; ***** CAUTION - NON STD BIAS RESISTOR REQUIRED *****
; File generated for use with 5620 ohm resistor at R7
;
; Steinhart-Hart coefficients: A= 1.134887E-03 B=2.32812E-04 C= 9.580155E-08
;
; Input Data: Temp, degF Resistance
; 32 32500
; 77 10000
; 212 680
;

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xrattiracer

· #3 · May 1, 2008

a different value bias resistor is applied over a certain temperature range too, in an attempt to make the scale a bit more linear. i dont remember exactly what the value and switch point are, but its documented in the code for at least one of the computer families.

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risen

· #4 · May 1, 2008 (Edited)

xrattiracer said:
a different value bias resistor is applied over a certain temperature range too, in an attempt to make the scale a bit more linear. i dont remember exactly what the value and switch point are, but its documented in the code for at least one of the computer families.

I have been looking @ the 87 T2 code over the past few days and in the MM_DetermineCoolantTemp_v subroutine it handles the switchover and calc of temp from sensor. From the post I linked to above it looks like it's 10k ohms for cold (< 125F) and 1k ohms for hot. The constant in the function for the switchover is #$dc, in case anyone's interested.

That is for the coolant temp sensor though. I've looked at most of the code where the RawAirTemp value is referenced and I don't see any of the ioPortxxx values being written to. I don't think that the charge sensor gets switched. Am I missing something?

I did calculate the curve for the sensor from the easythrem output and it best matches a cubic with negative coefficents. It has a rather sharp range in the middle and flatness twards the ends. It looks like a reverse narrowband graph, sorta. Since the data seems to make sense, I'm going to take the table easytherm generated and use that as a lookup for the RawAirTemp. I'm planning on doing some confirmation by checking the value of the coolant and the air temp on various temperature days before the car has been started so everything is the same temp.

Now, does anyone know how to convert the battery temp sensor's value in the '87 cars to a real value? If not, no biggie, I'm going to try and curve fit that one too.

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