Author Topic: About that "MATH" function...  (Read 5353 times)

Offline Jeffro

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About that "MATH" function...
« on: October 19, 2015, 10:47:07 AM »
I would like to know a bit more about the MATH function in reference to my Honda tranny temp gauge.

I finally found the right code to use and the gauge does display but I'm not sure it's reading correctly.

the X , Y , and Z values that you pre-programmed are  0001  0001  FFD8

Which translates to ( 1 / 1 ) + (-40)

ScanGauge uses  0009 0005 FFD8  which sounds like the C to F conversion, but subtracting 40 does not make any sense either.  Shouldn't they be ADDING 32 ?

Questions:

1. What is the purpose of subtracting 40 from the data?

2. Should not the OUT format of  10  take care of the C to F conversion?

3. Is the MATH performed BEFORE or AFTER the OUT conversion from C to F?

Offline UltraGauge Admin

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Re: About that "MATH" function...
« Reply #1 on: October 19, 2015, 12:12:05 PM »
The -40 is what is common in the OBDII standard when dealing with Celsius temperatures.
Since the output of the ECU is from 0 to some number....the way they achieve negative temperature values, is to always subtract 40 from the value.  This gives a range of -40 to 215C

All vehicle parameters are in metric.
Hence all temperatures are output with Celsius units.
Math is performed first

-40C and -40F are the same temperature.
0C and 0F are not the same temperature

Offline Jeffro

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Re: About that "MATH" function...
« Reply #2 on: October 19, 2015, 01:54:55 PM »
Awesome, thanks!

I've got it set for 0001 0001 FFD8 on the Math and OUT is 10.

I'm reading about 125F around town, and that seems reasonable I think.

What I found a bit strange was when I had it at 0001 0001 0000 and OUT 10 I was seeing about 195F under same conditions.

I guess I need to do some math of my own... after subtracting the FFDE why there was a 70F change, but I guess that's part of the C to F conversion.   Yeah, 1.8 X 40 = 72 ...

Think I'm good, you agree?

Offline jfoj

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Re: About that "MATH" function...
« Reply #3 on: October 22, 2015, 02:27:50 AM »
125F for an automatic transmission temperature sounds very low.

I think the 195F sounds correct.

Most automatic transmission temperature run very close to the engine coolant temperature except under severe conditions where the automatic transmission temperatures may spike above the engine coolant temperature.

This assumes we are dealing with a typically automatic transmission that has a cooler loop built into the vehicle radiator and may also have an additional cooler look as well.

195F sounds more correct than 125F.


Offline Jeffro

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Re: About that "MATH" function...
« Reply #4 on: October 28, 2015, 07:39:43 PM »
Hi jfof,

I thought so too... but that temp was after a leisurely drive of about 7 miles, and on a cool day.

Since I posted, I've seen it up as high as about 155F on longer trips.

When the engine and trans are COLD, they both read the same temp, so I'm inclined to think that what I've got now may be correct.

The car is an 08 Honda Element... Not a lumbering hulk that taxes the tranny much.  Yes, there's a cooler in the radiator, but no additional cooler. 

Thanks for replying!