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Discussion Starter · #1 ·
Hi,

I recently had my radiator replaced and my mechanic noted that my 2003 IS300's old radiator didn't have the AT cooling section, instead it utilizes this heat exchanger on the engine block.
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Is this an OEM solution, or did one of the previous owners install this for whatever reason?
Do you think it could be good enough? I've driven about 20k kilometers with it without issues, until yesterday.

Yesterday my IS stalled on me on a highway, giving me error code P0715 and not being able to carry on, so I had it towed. Obviously I will get the sensor and wiring this error code is about checked and/or replaced, but I can't help to wonder if perhaps this could be caused by the AT overheating? It was after a 2+ hours highway drive on stable 130-ish km/h.

Another possibility that I googled was related to fuel pump functionality. Do you think it is worth investigating as well? At the time it happened, I had over half tank left.

Also, I don't use the manual mode too often, but yesterday I tried it on a curvy road and noticed that the AT made a downshift when I didn't. Uphill, 80-ish km/h, 4th gear, full throttle, AT downshifted to 3rd but kept showing 4th, then at better revs shifted back up. Is that normal behavior? I haven't driven with manual mode enough to evaluate myself.

Currently, the car doesn't run at all, upon starting, it idles for several seconds and then the revs slowly drop until stall. Can faulty input speed sensor really cause this?

Thank you!
 

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input speed sensor usually will fuck with shift points and speedometer, but I've never seen one cause a no run condition but its possible. I have worked on at least 15 auto jz cars and never seen auto cooler lines end there. all radiators come with the ports for cooler lines unless buying one specifically for a manual (dont even think those are sold anymore unless going koyo or mishimoto). When in manual mode you are only telling the car what maximum gear it can shift to but it will auto down shift or upshift if needed.

I would start by replacing the speed sensor.

note that my experience is with lhd usdm is300/gs300s so if you're elsewhere this comment might be irrelevant
 

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There was one I saw on here a few years back with someone asking the same question about the same gizmo, i had never seen or heard of one either. Personally i cant imagine those would be as good at cooling the trans oil as going to the radiator, from what i can see you are relying on the cast iron block to conduct the heat to the cooling system and then to the radiator instead of it just going direct.

Only thing i can think of is that depending on the temperature in the country of its origin perhaps the radiator made it run too cold or hot, so maybe this was a stop gap for that country?
 

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Mr. Roo
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Only thing i can think of is that depending on the temperature in the country of its origin perhaps the radiator made it run too cold or hot, so maybe this was a stop gap for that country?
This is what I was thinking too. In a colder climate/country, you wouldn't need or want the coolant getting so cold and freezing out in a radiator all the time. I've never heard of any sort of thermostat for the transmission fluid. So perhaps this mechanism is something similar to that (I can see it has 3 lines connected to it) that allows it to either be heated or not, rather than constantly cooled.
 

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Discussion Starter · #5 ·
Thank you guys!
I am in middle-eastern Europe, we don't have too harsh winters nor summers here. Since I had the radiator replaced for the OEM one with the AT in/out, I may reroute the AT cooling there, just for safety. However since none of you raised a major red flag over it, I'll suppose that it's not that big issue.
phock.it also answered my downshift question - thanks, that explains it and it makes sense.

However, I'm concerned about the no run condition with the speed sensor. While googling around, I saw some posts about P0715 mentioning that while it is a speed sensor code, it may be triggered by fuel issues. However, these threads had it specifically mentioned that the cars affected had somehow low fuel. Do you think it is worth investigating this way for me as well, even though I had over half tank? Could the pump be going? The car has less than 170k km (100k miles).
 

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Most searching shows it to be an input speed sensor error in the trans, that has nothing to do with fuel levels as far as i know unless the extra weight at the back of the car when the tank is fill makes the sensor make contact and not when the tank is empty. I would start with that sensor and go from there, its the most obvious answer.
 

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Discussion Starter · #7 ·
I definitely will start with the sensor (it is also the cheaper thing to start with) however I would like to have as many possible causes checked as possible since the car is already at the mechanic.

This is the thread on clublexus which made me wonder about the fuel stuff: Car stalled on highway - P0715 code - ClubLexus - Lexus Forum Discussion, namely posts #7, #18 (2nd point on the bottom of the longish post) and #25. Thoughts about this?
 

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Discussion Starter · #8 ·
A follow up.

So my mechanic checked the sensor and its wiring, measured its resistance etc. and found that it is not faulty.

He then followed the other trail that I suggested - fuel issue, and this did lead to solution. Previous owner did a bad fix of fuel pump relay which worked for a while but failed me now, so he re-wired the relay and now it works.
I don't understand the topic thoroughly, but from his explanation I believe that as the wiring failed, resistance in it was too big and the pump couldn't work at full efficiency, fuel-starving the engine.

Apparently the error code was generated in a similar way as in the post on clublexus I mentioned above - ECU had no indication of a fuel-related issue so it expected fuel in the engine, but didn't get it (or enough of it) and generated P0175.

Perhaps this will come useful to another unfortunate soul in the future. :)
 

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A follow up.

So my mechanic checked the sensor and its wiring, measured its resistance etc. and found that it is not faulty.

He then followed the other trail that I suggested - fuel issue, and this did lead to solution. Previous owner did a bad fix of fuel pump relay which worked for a while but failed me now, so he re-wired the relay and now it works.
I don't understand the topic thoroughly, but from his explanation I believe that as the wiring failed, resistance in it was too big and the pump couldn't work at full efficiency, fuel-starving the engine.

Apparently the error code was generated in a similar way as in the post on clublexus I mentioned above - ECU had no indication of a fuel-related issue so it expected fuel in the engine, but didn't get it (or enough of it) and generated P0175.

Perhaps this will come useful to another unfortunate soul in the future. :)
Glad you found the solution, where ever it ended up. Nothing beats good troubleshooting with the subject in front of you ;)
 
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