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Hey guys! Wondering if you guys can help me out, I have a 97’ Eclipse RS so it does have the 420a motor. Car runs and turns on fine, but this engine light is on..
It’s throwing a code for “battery temperature sensor” and when the code was going to be cleared it would just come back. I’ve read that the ECU’s aren’t compatible with early models (1995?) or that they need to be the exact models, years.
I also don’t want to spend $400 to reprogram it at a dealer just for something that isn’t necessary. If someone knows who can program these let me know!
Couple things I’ve done:
• It is another ECU from another car (same model, trim, engine)
• new battery
• coolant temperature sensor already changed 3 times, and still the engine light persists.
• I’ve ran the car for it to learn for 20 minutes, people said that it needs to communicate with the TCM, to measure engine behavior
• idle bounces, from 300-900rpm, I’ve cleaned the IAC, tps, throttle bottle, maybe because of ECU learning?
Thanks you all!!
 

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95 is the only 2G that has OBD1. Perhaps some very late year 95s may have OBD2 as Mitsu started transitioning in 95. My 95 has an OBD2 port under dash but it does nothing and theres definitely an OBD1 ECM under the hood. Most aftermarket OBD2 ECU scanners wont connect. 95s read engine codes by key on/ off 3 times then read the check engine flashes. Now that you know about 95s...curious why are you asking when you have a 97?

97 Definitely has OBD2 and can be read with most aftermarket OBD2 scanners. You should also be able to reset the CEL using a scanner. Cheaper ones only reset basic codes. For ABS reset you need a higher end code reader and for Mitsu SRS code you need to see the dealer. If the light CEL keeps coming on (and I'm assuming you just disconnected the battery for 30mins or so to clear codes) there could be a problem with the system/part indicated. IF it shows a bad battery temp sensor Ive read thats inside the ECU. As temperature changes...the resistance in the sensor changes and it varys how much charge the battery receives based on extremely cold or hot conditions. If the batt sensor is bad it would require ECU replacement. A code P1493 could be battery temp sensor or indicate other issues with the charging system.

I'd check the alternator as well to make sure its good and not causing your issue. Hook up a digital volt meter to the battery and start the car. See if the voltage increases with the engine running. Load it down with headlights and the AC on max cool and fan high. If it maintains around the same voltage with the engine running and accessories on (13.5-14.3 give or take) then its charging ok. If the voltage is dropping with accessories on and its not charging even if you increase engine rpm then you need to start by checking the alternator itself to rule that out. Only way is to remove it from the car and bench test it at a local auto parts store. (Unless you take it to a garage that has an alternator testor that can check it while its still on the car but disconnected from the system...and they WILL charge nicely for that I'm sure)! So remove it then bench test at the parts store. If bad, replace it. If good then you can look at the ECU as the the possible issue. In addition to the battery temp sensor, the alternator voltage regulator is inside the ECU as well (on 95s anyway...and likely 96-99 too but I cant confirm).

I assume the reason you say the car is surging is due to relearning after the battery was disconnected? My OBD1 has never taken that long and never had that idle surging after a battery disconnect. It starts and runs. The car should idle normally after starting... and driving around town a bit should be all the re-learning it needs to do for air/fuel ratios. Perhaps removal of the IAC motor is why you're getting that surging. You didnt screw the motor shaft in or out by hand did you? I've taken mine off to clean the carbon off then put it back on. As long as you don't change the position of the motor by hand while cleaning it or turn the key on while the sensor is removed (or connected but not seated in the throttle body) it shouldnt change the last positioning the computer set it for. Again you DONT want to do those things. When you removed the IAC motor did you clean the seat in the throttle body real good so theres no gunk on it. Gets nasty in there and if its not clean the IAC cone wont seal the openings and it will high idle or surge. If I recall you should be able to push open the throttle blade on the throttle body to see the small opening were the IAC feeds air to the engine. The other small hole should be in front of the throttle blade. Thats where it picks up the air. I always push a rag in the throttle body to cover both of those holes before spreying inside the IAC motor empty hole from the outside for 2 reasons. 1 you dont spray junk from the IAC seat into the throttle body. And 2... it wont spray out If there's carbon build up the IAC cone wont seat in the opening and may cause surging or high idle. It gets nasty in there. Leave the MAP sensor on the back side of the manifold alone. Dont spray it with anything or it could be damaged.
 

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My bad... you didnt remove ECU to clear codes. I forgot you said you'd replaced with one from another car. Long as it was the same year and engine/transmission combo it shouldn't need reprogramming. I also checked alternators at Advanceauto online and the 95 and 97 are the same part number meaning the regulator is external and likely inside the ECU like my 95.
 

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I always push a rag in the throttle body to cover both of those holes before spreying inside the IAC motor empty hole from the outside for 2 reasons. 1 you dont spray junk from the IAC seat into the throttle body. And 2... it wont spray out If there's carbon build up the IAC cone wont seat in the opening and may cause surging or high idle. It gets nasty in there. Leave the MAP sensor on the back side of the manifold alone. Dont spray it with anything or it could be damaged.
Yeah this doesnt make a whole lot of sense. I started typing then my browser froze up as I was changing some things and It saved a jumbled up mess. WHAT I WAS TRYING TO SAY WAS.... " I push a rag in past the throttle blade on the throttle body so it covers the IAC in and out holes so 1. when I spray throttle body cleaner in the IAC seat from the outside it doesnt push junk from the seat through those holes and into the throttle body or intake manifold ...and 2. MORE IMPORTANTLY ...it doesnt spray out those holes and right INTO THE EYES! Thats never a good time! Goggles are a good idea when working with spray cleaners!
 

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The 2g n/t ecu is not progamable, at least not by anyone less than a dealership or even an engineer from Chrysler.
 
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