How hot is too hot?

[punchn]

I tow a 3.5t boat with my 3.9L auto disco, it tows it well, with the exception that the temperature guage climbs upto 3/4, is this to hot? I did notice once after turning the engine off the fan continued to spin, is this an easy fix or should i look at an electric fan, also should i be looking at a transmission cooler? the disco is purely a tow vehicle. Thanks in advance.
cheers

[p38arover]

I did notice once after turning the engine off the fan continued to spin, is this an easy fix

Sounds like your fan's viscous coupling has called it a day.

See http://www.aulro.com/afvb/good-oil/129254-how-test-viscous-fan.html

[Grumbles]

I agree withp38arover. Plus this - I found that when the viscous needed attention so too did the radiator need rodding - one seems to go with the other.

[Discomark]

I'd definitely look at a bigger transmission cooler if you are towing 3.5 ton. Especially if its the same piddly 4 row one they put on the TDI. It seems the cooler from the Range Rover P38 is the best option and the subject is well covered on this forum if you search for "P38 cooler". I have just got one from the UK after cooking my gearbox and a 4k rebuild, I was only towing 1200kg but its the hills that kill them as the clutches generate a lot of heat when not in lock-up and my TDI does not have the grunt to get to 80KMH on a hill to benefit from lock-up. Iv'e fitted a temp monitor on the cooler line and its amazing how quickly the temp drops when lock-up is reached, about 1 degree a second and settles about 20c lower. One of the considerations is the ZF box operates at full pressure through the cooler lines in lock-up so the cooler must be able to handle at least 200psi and many after market ones don't cut it.
A ZF specialist told me these boxe's (4HP-22) aren't really suited for towing and the same box is used in BMW cars etc. Some specialist fit parts from a 4HP-24 as they have stronger larger clutches etc.
Synthetic fluid is also a good idea as it can handle higher temps and seems to run a bit cooler, in my experience anyway.