Offroad-Express

A friend has a 1kz surf, it has a bigger oil cooler fitted but when towing a 1500kg load up steep hills the at oil temp light is coming on.

What temp does it come on at? Is it a sign that the trans is on the way out? It drives and shifts fine tho

HI

Towing with that transmission requires you to use it as a manual or it will overheat.
The torque converter will not stay locked up and that is where the issue is.
I have a temp Guage fitted to mine and tow a caravan over the Arthurs pass with no issues. I do control which gear it is in and will change down early.

It is being locked in 2nd, sometimes 1st depending on how steep it gets.

probably more likely its a crap cooler or poorly installed.
i'm used to the aussie guys with overheating surfs and they don't have to much problem even in their 40+ heat.
a few guys on here have had issues and its been the coolers.

post up some pics of it.

I use low range. You can climb under the truck and on the gear box around where the levers are is a couple of plugs. I just slipped one apart and that allows 2wd low range, the button has to be pressed to engage 4wd now. The plug allows automatic engagement of 4wd when it detects the low range selector moved. Really simple mod to do and awesome. I tow my S3 LR and trailer (near on 2.5t) up steep hills like it. much much better than high range.

The trans temp warning light comes on around 120 C.
Typical normal running temp is about 55 to 60C but this varies depending on load etc etc.
The biggest heat generator is the torque converter... So if you dont have the converter locked and its doing its slipping thing it will generate a heap of heat.
There are mods you can do to improve the flow in the converter circuit but this will require the valve body to be sent away to someone who knows how to tweak it.
You can do a mod to manually lock the converter.
Also a good cooler helps.
The aussies have developed some good stuff with the Hilux / Prados for towing. Better converters and valve bodies.
Also kits to do the lock up trick.

The manual lockup is good. I just installed a switch into a wire (providing power) and it locks it up at the touch of a button. Just have to be careful not leaving it locked when stopping all of a sudden or slow speeds.