The V8 Owners Forum -

My radiator doesn't have a built-in transmission cooler so I'll be fitting an air cooled one when I do the LT77 to ZF4HP conversion.
Apparently about 80-85ºC is the maximum working temperature for ATF but is there a minimum? I'm just wondering if a freezing cold day with enough speed to lock the torque converter could end up overcooling the oil?

You could buy a cooler that's fitted with a thermostat, that way it will bypass the cooler until the fluid is hot enough to need it.

I would just get the Range Rover Classic cooler if that will fit. There is no thermostat in that. If not, any little one will do.

As I understand it, most heat is generated in the torque converter when it's slipping and under load. And pulling a heavy Range Rover on a hot day.

I would think on a trike, you could get away with a very small one.

I don't believe you can over-cool one. I am sure my P38 Range also does not have a thermostat in the transmission cooler and it's pretty big on that.

Chris.

I've spent hours and hours on the internet and overheating is mentioned but overcooling isn't (I don't count slow shifts etc while warming up) so you're right, I'll just use a cooler from the gearbox donor car and keep an eye on the fluid temperature to and from the gearbox.
If it turns out to be always running cold I'll add a thermostat although I'd prefer not to, it's one more thing to go wrong. I'll check if a dead thermostat sticks open or closed first.

The worry with a thermostat is making sure there is a bypass route, so the gearbox is not starved of oil. I wouldn't be surprised that if you block it off, the transmission won't work at all! I would definitely consult with some technical documentation for it to find out before adding a thermostat.

Chris.

I was thinking of a Mocal thermostat, the one intended for an engine oil cooler that you plumb into the flexible hoses - it has an internal bypass so cold thick oil tries to take the path of least resistance back through the return hose rather than through the cooler and as the oil heats up the thermostat starts closing off the bypass so more oil is sent to the cooler (if I've understood right).
They're meant for engine oil so I'll check with them to see if they're compatible with Dexron 3.

The oil typically is the exit from the torque converter back to the sump. Small 14 row mocal oil cooler will do the job - no need to overthink it.
you wont overcool it.

You could, of course, do as the SD1 does. The transmission cooler is in the rad. So will be heated too under some circumstances.

DaveEFI wrote: ↑

Wed Oct 27, 2021 5:48 pm

You could, of course, do as the SD1 does. The transmission cooler is in the rad. So will be heated too under some circumstances.

And fill your trans up with coolant and turn your rad into strawberry milkshake when it springs a leak

Eliot wrote: ↑

Thu Nov 04, 2021 1:51 pm

DaveEFI wrote: ↑

Wed Oct 27, 2021 5:48 pm

You could, of course, do as the SD1 does. The transmission cooler is in the rad. So will be heated too under some circumstances.

And fill your trans up with coolant and turn your rad into strawberry milkshake when it springs a leak

Not something I've heard of. It is simply a pipe looped through the coolant in the rad.

I read somewhere about this being a not uncommon Lexus(?) problem where the oil pipe in the radiator corrodes and being under pressure, radiator water is forced into the gearbox oil.
I haven't heard of this being a problem with Rover/Land Rover/Range Rover boxes but I think I'll stick with an air cooler just in case...

Cross contamination due to the transmission cooler in the radiator used to be a problem, and in particular a big problem with Volvos for some reason. I did recovery work for a few transmission repair workshops and towed in lots of cars where it had happened. That's why now I'd always have a separate cooler.

harvey wrote: ↑

Fri Nov 05, 2021 1:48 pm

Cross contamination due to the transmission cooler in the radiator used to be a problem, and in particular a big problem with Volvos for some reason. I did recovery work for a few transmission repair workshops and towed in lots of cars where it had happened. That's why now I'd always have a separate cooler.

The Volvo auto is an Aisin Warner auto used by many VM's including Toyota, GM & PSA and often uses a trans oil heat exchanger with the coolant. These heat exchangers could corrode (particularly with missed coolant changes) and so auto oil and coolant mix.