salimshah

Technically, why would you need transmission cooler for RX450H? Although it uses transmission fluid, but it is purely for lubrication purpose. There is no slush box that heats up the fluid when there is intended/un-intended slip between the engine and the gearbox. I am still perplexed as to why Lexus/Toyota opted for transmission fluid instead of gear-box oil.

Now if there was torque convertor involved, yes additional cooler would help as there would be more slip due to additional weight being pulled.

Salim

mcomer

Here is the options on the VIN listed. It has Tow Prep and the Spec Page shows
how it differs from those without Tow Prep. Others correctly mention that RX's
are rated for 2000 lbs without /TP and 3500 lbs with.

tus

The problem with the list is it includes a Heavy Duty Alternator as a part of the Tow Prep Package. The RX 450h does not have an alternator,

As salimshah pointed out, I don't know of a technical reason for putting a transmission cooler on the PSD, which fits what I have observed on the limited sample of 450h's I've looked at.

I would guess the Tow Prep Package list is for what is added to an RX 350. This is probably where the confusion at the dealerships is coming from. Someone can just look at the list and quote from it, while someone else who knows more about the vehicle would look at it and see that it does not make any sense to have these parts on the hybrid.

Guess what I really want to know is if by just adding the receiver and wiring pieces to a 450h you can safely tow 3500lbs?, or are additional parts, which would appear to just be a heavy duty radiator, that should be added? (,or is the heavy duty radiator a part of the hybrid build sheet because of the nature of how a hybrid works and you need nothing?)

BTW, since the oil cooler is not mentioned as a part of the tow package I would guess that it is now a standard component and would be seen on all later 3rd gen RX's, which is what I've seen so far.

Htony

Whether it is RX350 or RX450h, idea of towing something at max. rating is absurd. I'd give at least 10% safety margin. Why don't you consult with a reputable hitch shop and seek their opinion. I used to own 450h, IMO
it is a vehicle not meant to do serious towing.

tempestv8

I do think that the ATF in the hybrid drive is subject to significant amounts of heat in the normal operation of the vehicle, let alone towing.

The source of the heat is not from a torque converter like what's found in a conventional automatic transmission (ala RX350). I believe that the heat comes from the hybrid drive's inverter. The inverter, approximately the size of a large shoe box, has a thick aluminum case that is packed with high power transistors, capacitors, microprocessors. This thing gets gets very hot, hence why the hybrid drivetrain gets its own dedicated cooling system, separate from the engine's cooling system.

The inverter is able to handle more power than often runs through a household wiring system. Its power conversion occurs in a series of transistors that turn on and off thousands of times per second. The inverter boosts the voltage from the traction battery to very high voltage level required by the traction motor(s). During this process, heat is generated as a by product of the conversion. Conversely, when the driver applies the brakes to slow down the vehicle, the process is reversed and power is returned to the battery. This process also generates heat inside the hybrid drive.

Voltage conversions result in heat - plain and simple. It's most prevalent during the acceleration and deceleration phases. When cruising at a steady speed on the highway, there is significantly less heat generated, as it is the ICE that is doing most of the work to maintain the vehicle's speed.

On the topic of the fluid, this is why when the hybrid drive is drained, the ATF often comes out looking very dark. This is because it's been subjected to significant amounts of heat from the inverter. The fluid from the rear motor (in the case of a RX450h AWD version) comes out looking near new because it doesn't get heated up, as there is no inverter in the rear. No inverter in the rear differential = no significant heat to the fluid = rear diff fluid doesn't darken in colour.

This is why I will periodically change the ATF from my hybrid drive, every 75,000 kms. The fluid's lubricity is impacted by the amount of heat subjected to it. It's quite telling, just by looking at the colour of the fluid.

The fluid inside a conventional auto (RX350) can remain quite fresh looking for a long time, compared to an RX450h, driven in the same conditions. I know because I've serviced both transmissions personally and can see the condition of the fluid that comes out.

If I were towing regularly with an RX450h, changing the ATF out of the hybrid drive will need to be a more regular maintenance activity. That's my personal opinion and how I would maintain my own vehicle.

I would also maintain the freshness of the coolant in the hybrid drive. Colour is not a good indication - the coolant colour doesn't change much so it needs to be changed at regular intervals, as specified by Toyota.

I wouldn't be using aftermarket products for either the ATF or the coolant, but that's just to keep everything as Toyota has intended.

tus

You do realize the 10% you are adding is on top of what I would guess to be the 50-150% that the engineers and lawyers have already put in place for towing numbers because of all the idiots out there

Prime example of this would be my idiot neighbor who tows a 25 ft dual axle car hauler, with a car on it, with his first generation Highlander. Just the trailer would put him over capacity but it does work.