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Old 07-08-2007, 02:42 PM   #1
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Default Incorrect coolant temperature reading

I have recently learned that my coolant temp reads very low. I am using the stock mazda clt and even flashing my MS with the correct values I am reading temps still very low. I let my car idle and warm up today for about 15minutes and it only got up to 165degrees fahrenheit. Driving around I was at 135degrees fahrenheit. The values I used in easytherm for the CLT are COOLANT TEMP. RESISTANCE (OHMS)
-20C (-4F) 16150
40C (104F) 1150
80C (176F) 320

I have included a datalog of me driving. Ignore everything but the coolant temps because MS is not controlling anyhting currently. The 165F temp was taken after the car was idling. Once I started driving it drops as you can see. Does anyone have any explanation for somehting I can change to get the correct reading. These temps jsut seem to low to me.

http://mason.gmu.edu/~lsnead/poscoolant.xls
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Old 07-08-2007, 03:08 PM   #2
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Try the default RX-7 settings (Easytherm -> File -> RX-7). If you still have the stock ECU powering and driving the sensors, you need to find out what the stock ECU bias resistors are - and remove the bias resistors from the MS (R4 and R7 IIRC).

Jim
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Old 07-08-2007, 05:49 PM   #3
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I tried the RX-7 settings and put in the GM settings for the IAT, since I am using a gm IAT form DIY. Same thing happens. Could you give me more information on the removing the bias resistors and such.
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Old 07-08-2007, 07:17 PM   #4
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For settings other than the GM CLT sensor ones to work you need to remove R4 & R7 otherwise you'll end up with askew readings. Atleast that's how it was impressed upon me. The megaManual talks about this a little more as well.
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Old 07-08-2007, 08:03 PM   #5
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Originally Posted by cjernigan View Post
For settings other than the GM CLT sensor ones to work you need to remove R4 & R7 otherwise you'll end up with askew readings. Atleast that's how it was impressed upon me. The megaManual talks about this a little more as well.
Thank you so much. This fixed my problem right away. If I am ever in Tennessee or Alaska I will buy you a beer or 2 or untill we are both drunk.
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Old 07-08-2007, 08:15 PM   #6
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So it's good now? you kept the bias resistance of 2490ohms in EasyTherm? I think Arkmage saw 1250ohm on his '97. What's the temps you are getting now? maybe you'll stop overheating :gay:
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Old 07-08-2007, 09:10 PM   #7
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I left the resitance of 2490phms in easytherm. I was getting 190 idling and my fans came on correctly. I will be going out and driving in a bit and will see how those temps are while driving.
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Old 07-08-2007, 09:23 PM   #8
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yay! I'm glad we were able to figure that **** out....so annoying!
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Old 07-08-2007, 09:32 PM   #9
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So, all you have to do is just remove R4 and R7, leave them open, then set it up in easy therm?
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Old 07-08-2007, 11:07 PM   #10
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Originally Posted by karter74 View Post
So, all you have to do is just remove R4 and R7, leave them open, then set it up in easy therm?
Thats what I did. I will be datalogging and tuning tomorow. I will see how far off my tune I have been using was.
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Old 07-08-2007, 11:29 PM   #11
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Quote:
Originally Posted by karter74 View Post
So, all you have to do is just remove R4 and R7, leave them open, then set it up in easy therm?
R4 & R7 are the bias resistors... since you are sharing the signal with the stock ECU which already has bias resistors you don't want them in there. Measure your stock bias resistors if you have any doubts... you can watch the resistance drop when you pull R4/7
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Old 07-09-2007, 02:08 AM   #12
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This may be an extremely dumb question, but how do I measure the stock bias resistors. I understand how to get the readings from the stock resistors (ice water, boiling water, etc.) Where do I measure to find the bias resistance?
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Old 07-09-2007, 09:59 AM   #13
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Quote:
Originally Posted by Arkmage View Post
nope... but it was enough that my stock computer didn't see 210* (fan on temp) until my gauge said 256*. I removed the bias resistors in the MS today and calculated the stock ecu bias. For future reference '97 miatas have a stock bias of 1.2Kohm, I suspect that '96 will be the same but I can't promise it.



To determine you stock bias resistor you will need to:

1) unplug the 8 pin harness above the fuel rail

2) measure the resistance between the blue with white strip and the black with blue strip wires which are directly across from each other on the female connector (this is the sensor resistance)

3) Reconnect connectors

4) Key on the car, but don't start it... check the voltage at your wire tap for the MS (MS disconnected).



Your bias resistor will equal:



R=R1 * [(5-V)/V)]

where R1 = measured sensor resistance from above

and V=your measured voltage at the wire tap



I calculated 1205 ohms, or 1.2k-ohms.



This value is then input to easytherm in the bias resistor box. After doing this my cooling fan is working again and megatune is reading accurately.

here.
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Old 07-09-2007, 06:43 PM   #14
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Also for future reference, the 2001+ models have a CLT bias of 1550Ohm and an IAT bias of 2700Ohm. I used the RX-7 defaults and it matches to my OBD-II readings 99.9%.

Jim
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Old 07-09-2007, 07:24 PM   #15
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So based off this information on my 97, if I put a 1.2kohm resistor in place of the normal coolant bias resistor, and leave the normal 2.49k resistor for the IAT (using the GM sensor), then I shouldn't have to do anything in easy therm? Or even with the replaced resistor, I would have to change easy therm values.
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Old 07-09-2007, 08:07 PM   #16
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I have a new question about these resistors, Which one is the bias resistor for which thing? I removed both of them and now my IAT is at 60degreesF all the time. I believe this is because one of the bias resistors controls the IAT. I am using the GM IAT so I should leave that resistor in place. Could someone please tell me if r4 or r7 is for the IAT
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Old 07-09-2007, 10:20 PM   #17
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and another question... 1.6L OEM CLT reads 30 colder, correct values in easytherm. R4 and R7 installed 2.9K I believe. GM IAT reads correct.

btw, lars:
R4 - AIT Sensor
R7 - Coolant Sensor


Should I just check the resistance at R7 and then plug that into easy therm and try it again?

Last edited by Braineack; 07-09-2007 at 10:55 PM.
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Old 07-09-2007, 11:09 PM   #18
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I would remove r7 Scott and see how that works. Also dont jsut clip it out. Try to unsolder it so you can put it back in if you need to. I clipped mine out and not have to go source new ones.
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Old 07-09-2007, 11:51 PM   #19
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Quote:
Originally Posted by shuiend View Post
I would remove r7 Scott and see how that works. Also dont jsut clip it out. Try to unsolder it so you can put it back in if you need to. I clipped mine out and not have to go source new ones.
Depending on how good you are with a soldering iron that might be for the better... resistors are surprisingly easy to damage with too much heat.
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Old 07-10-2007, 02:42 AM   #20
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Quote:
Originally Posted by shuiend View Post
I would remove r7 Scott and see how that works. Also dont jsut clip it out. Try to unsolder it so you can put it back in if you need to. I clipped mine out and not have to go source new ones.
Better yet, jumper it on the backside. Resistence goes to ~0 and easy to remove.
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