The Nissan Path

I did the Radiator bypass about 2,000 miles ago and it has not been an issue. I have had a trans temp gauge hooked up for about 50k miles (plugged into the side of the trans case). Before rad bypass Temp was almost always 120 or less. It would creep toward 180 when towing in the heat but it never went up other than that.

A couple weeks ago, I was in Phoenix and stuck in moderate traffic. it was about 80 degrees out and I was minding my own bidness, looked down and saw 180 on trans temp. Moderate traffic=0-45MPH, for about 30 mins, a few stop lights here and there. I know in the grand scheme of things, 180 is not that big of a deal, but at 80 outside degrees, not towing, and my normal temp is 120, this (60 degrees hotter than normal) was concerning. This is with nice fresh Valvoline Max life (did the trans fluid change at the same time as trans bypass).

Anyone else notice this on their rides? I am thinking I should add a fan to my cooler and/or get a bigger/better cooler. I live in Boise and we see 110+ often. I offroad in 110+... Summer is coming and my trans puking scares me more than anything!

The reason it's happening is because the ECM doesn't consider the transmission temperature when deciding when and how fast to run the aux electric fan. It only looks at engine temp and A/C pressure. With the heat exchanger in the transmission cooling loop, the trans can't get much warmer than the radiator outlet tank, typically 140~150F in a healthy cooling system. With the exchanger removed, the trans becomes totally dependent on airflow to cool itself, which is fine except when you're stuck in traffic and the ECM is only concerned with keeping the engine cool. Removing the exchanger essentially severs the thermal masses of the engine and transmission from one another, so you're bound to see discrepancies.

That said, 180F is actually a good temperature for a conventional A/T, and anything up to around 200F isn't generally a problem. Above that you may start to oxidize the fluid, but if 180F is all you're seeing, it isn't really a cause for concern.

PS - If you do see higher temperatures in your offroad sojourns, the right answer is to get a later model "safe" radiator and not a bigger fan. Having the thermal masses of the engine and tranny linked together obviates a lot of thermal issues, including the tranny running too cold and wearing out prematurely.