Auto Transmission Problem

[BONZER]

[bigsad]My Discovery 3 has a transmission problem. The car has done 120 000 kms and has had a transmission service done at 100 000 kms. at that time I had the pan conversion done as well.
My problem is that I have a 2 tonne caravan and the car won't tow it. After a short distance towing, say up to 100kms, a big F shows up in the transmission window, the car loses power, and then settles into a lower gear that limits me to 80kms/hr @3000 rpm. This makes for a very slow trip. During these episodes the car does not change up or down. I am locked out of manual and sport modes.
I drove to Queensland like this. Unhooked the caravan and everything went back to normal, until I had to move on. Then the first hill killed it again. I left the van in storage and drove back to Melbourne without incident. I have since done another 15000kms and no problems have shown up.
This is my second D3. I love the cars, my family reckon I'm an idiot for staying with the brand. My first caught fire and burnt to the ground.
Any suggestions???
John

[shanegtr]

Do you have any way of reading fault codes?

[BONZER]

Do you have any way of reading fault codes?
I have just bought an iCarsoft i930. Just thought someone else may have had a similar experience. Problem I now have is to generate the fault. The pre existing fault has been cleared from the car.
John

[discorevy]

Which oil was used in the trans service , and is yours possibly an 08 / 09 model?

[BONZER]

Genuine ZF oil was used in the transmission and yes the car is a 2008 model
John

[shanegtr]

Just thinking are you able to monitor the trans oil temp while towing? Could be getting too hot while your towing and causing the fault?

[mr_squiggle]

Perhaps putting the vehicle on a dyno & loading it up would replicate it?

[TDV6]

Before I became educated about the "Filled for Life" furphy and also towing a three ton van, on hot days going up hill sometimes I would get transmission "F" etc where I would limp to a pull off then idle engine for approx ten minutes and clear the faults then continue on as per normal, since then I had changed the oil twice them 60k ago replaced the gearbox, and now I have all the gauges to warn me about whats going on with the car

I have an "Engine Watchdog" gauge on my dash which tells me the transmission oil temp plus radiator coolant temp of the fluid entering the motor, I have a Scangauge to tell me what the internal engine temp is plus this gauge gives me volts, MAP (Manifold Air Pressure) which I use to monitor economical touring and digital RPM which indicates if there is more than normal rpm fluctuation for the same speed/load.

This is the link to a previous post that shows the setup of my dash.

D3 extra gauges on the dash (https://www.aulro.com/afvb/l319-discovery-3-and-4-a/231993-d3-extra-gauges-dash.html)

Ryall