To check the auto gearbox oil level you need the engine running, and to cycle the gearbox through all the gears (I usually pause for a few seconds in each gear) to get the oil fully circulated throughout the gearbox, all its valves and oil ways. Then put it into park or neutral and check the level, the two different levels are for whether your checking the level with cold or hot oil.
Something is definitely not happy if your getting the over temp light up in Wales, the system should be good for much higher temperatures and altitudes than can be experienced there. I have two auto 4wd's both of which I have used for a number of years in high load, high temperature and high altitude situations, one is a LR Discovery TDI auto, there other my LC TD 80 series. In the early days with the discovery the over temp light did come on a couple of times, I soon learned that when in very high load situations I needed to manage the auto gearbox heat build up early on.
When driving on a flat road with light load the gearbox has no trouble managing the heat as its often 'locked up', but when the vehicle is climbing up a mountainside hairpin road, the gearbox doesn't have the opportunity to 'lock up' and is slipping all the time. Imagine as you negotiate the hairpin bend the box is in first, it then generates heat as it builds its way up and changes into second gear, and the same again into third, then you slow down and negotiate the next hairpin. The vehicle is at altitude, the air is thin, air flow is low and the ambient temperature may be high, the auto gearbox has a really hard time shedding its heat. Its in these situations that you have to start manual shifting the auto gearbox to help limit the heat build up, the gearbox doesn't know the terrain that you're driving, but you do! So the driver can interpret the conditions ahead and shift the gearbox accordingly to keep the auto gearbox oil temperatures under control.
When driving steep terrain I manually select a gear, say second, so that the gearbox will only shift between first and second, once into second I let the engine rev and take the strain. By limiting the gearbox to second you stop the box attempting to change up into third and generating lots of heat in the process. If the distance between hairpins increases and I need third, I just manually shift up and the box responds quickly with minimal slip and therefore minimal build up.
I was once laid a challenge to get to the top of a long hairpin climb with my discovery, whilst keeping up with a LC 120 series (manual), with careful manual shifting of the auto gearbox I made it all the way up in one go and kept up with the 120 in front. Apparently 4x4's with auto gearboxes normally have to stop part way as the over temp light comes on.
Driving with the auto gearbox temps in mind I have managed to get the Discovery everywhere I've wanted to it to without over heating the box. The 80 doesn't have to work as hard and I have never had the over temp light come on. Both vehicles have standard cooling setups. My FZJ105 in Oz is also an auto, and we really used her full capacity when off roading during the summer high temperatures at altitude, and sometimes trying to keep up with Ben! Again with careful control of the auto box, we didn't over temp the gearbox, and this was under the perfect conditions to overload a box. The 105 was fully loaded, at altitude, climbing for long periods at ambient temperatures of 45 degrees celsius.
If I was to add an aux cooler, I'd set it up as Toyota have on the 100, through the aux cooler first then into the rad, as under certain circumstances the cooling system will help to keep the ATF temperature within an acceptable range. We have to think of the all circumstances and weather conditions that our vehicles will be subjected to, mine have driven through snow in mountains and sand on a sunny beach in the same day. Plus the aux cooler will reduce the heat loading placed on the engines cooling system by the ATF.
The FZJ105 has a separate auto gearbox oil cooler which seems to work very well in my experience, I think FZJ80's have the same set up, a possible source of aux coolers? A word of warning, I did once know someone who bought a secondhand aux cooler for a vehicle and installed it only to find that it was still contaminated with the contents of a previous failed gearbox, this then lunched his. So if condition is unknown, always flush the cooler first.
I have no firsthand experience of 90/95 series, but if it was mine I'd be tempted to use both the OEM and aux coolers, aux first then into the OEM rad cooler. As others are doing, I'd change the rad regularly with a genuine Toyota one to minimise the risk of an age related cooler failure.
The current location of the aux cooler may be too far to one side, and out of the airflow generated by the viscous fan. You wouldn't really notice it when driving under normal conditions as the loading on the auto gearbox would be light, and the airflow through the cooler from the moving vehicle would be unto the job of removing the ATF heat. Its the low speed, high load circumstances that find the weaknesses.
I personally haven't seen rads/coolers mounted hard up against each other, I would expect that if an aux cooler was mounted in contact with another rad, it would lead to vibration and rubbing between the two and possible failure as a result. On the FZJ105 there is a gap between the auto cooler and AC condenser, and then a strip of foam between the condenser and engine rad, to eliminate rubbing.
The auto gearbox slipping and not picking up straight away concerns me, I'd be flushing the ATF a couple of times and changing the filter, even though the oil still looks good. Its seen high temperatures and has experienced slip, I'd change it and see how it responds.
All this sounds like doom and gloom but it isn't, I still prefer an auto for my style of driving and usage, they are so smooth and controllable that 9 times out of 10 they are a joy to drive.
(Apologise if there are any mistakes above, I'm still recovering from a stag do at the weekend!).
Happy spannering Steven, you love it!!!
