Thursday, Oct 04, 2007 at 22:33
Here is some guidance I posted on another
forum in the last 12 months that may be of assistance.
Quote:
The durability of automatic transmissions operating under heavy loads or in duty cycles at the limit of their design capability has always been a challenge for drivers to accept there are some things that must be done to provide a trouble free life.
The problem is that there is not much in the way of published guidelines on what to look for and how to respond to the warning signs.
For those who may be interested I may be able to provide some guidance here that you could find useful.
Looking at operating temperatures, there are a couple of things here.
A stable bulk oil running temperature is a must. If the temperature rises on uphill or generally hilly running or in headwinds then the transmission oil cooling system needs to be boosted.
Bulk oil temperatures less than 110 to 120 celsius should be targeted. (You should see about 90 celsius in your duty cycle. )
Above 120 celsius, there is a real impact on oil oxidation [degradation due to heat] and above 140 celsius there is a doubling of the rate of oxidation for every 10 degree rise in temperature. So the message here is get the oil temperature stable and keep it below 120 degrees.
Now looking at mineral oil based transmission fluids versus synthetics. We’ll use the Castrol products as an example. The mineral oil GM Dexron brand is TQ Dexron 3, and the synthetic product suitable for the Dexron specification is TRANSMAX Z. Some of you have chosen to use Castrol Transynd, similar comments as for TRANSMAX Z apply.
We’ve already talked about bulk oil temperatures, however looking inside the transmission at some of the working areas there are some much higher temperatures generated.
In the clutch pack, the friction plates that make up the clutch are designed to operate within a defined temperature range, usually between 300 and 400 degrees. These clutches are oil flooded and the transmission fluid is used to dissipate the heat generated and therefore control the temperature of the clutch operation, so the transmission fluid has to withstand temperatures much higher than you see in the transmission sump. Elsewhere in the transmission the oil is undergoing much higher temperatures for example in the planetary gear set as gear teeth contact one another [momentary 600 celsius] , and in the torque converter.
It is in these conditions that the mineral oil versus synthetic fluids performance must be compared.
Firstly the effect of heat on the base fluid. The temperature at which mineral oil begins to oxidise is about 140 C, while synthetic fluids of the type used to formulate TRANSMAX Z don’t begin to oxidise until
well into the 200’s. So the synthetic is more stable until much higher temperatures are reached.
Next, the rate of the oxidation or degradation is much slower in synthetics, but antioxidants can be added to both mineral oils and synthetics to slow the rate down and thereby prolong the fluids life. Suffice to say the synthetic has a head start here.
Looking at the effect of heat on the viscosity of these transmission fluids shows very little difference between them, they both have similar Viscosity Indexes. They therefore both perform very efficiently as hydraulic fluids.
However the effect of pressure on the fluids shows a different picture. Under the extreme pressures encountered during gear tooth contact the synthetic has a higher viscosity than the mineral oil. The synthetic has a higher pressure-viscosity coefficient. What this means is the synthetics viscosity rises much faster under the high pressure of approaching metal contact this providing a thicker cushion of lubricating oil. It is this property that gives rise to the claim that the synthetics run cooler than mineral oil. They are simply able to keep the metal parts separated for longer thus reducing the time for friction induced heat build-up. With the appropriately calibrated temperature gauge you can see up to 10 degrees difference in transmission bulk oil temperatures. This is the result of the gear set being some 50 degrees cooler in operation on the synthetic.
So considering these differences, there is a real incentive to use synthetic transmission fluids as they are significantly more durable than mineral oils and in turn impart better durability to the transmission.
So to the question of oil drain interval.
My view is this….If the manufacturers recommendation is 40,000 km under normal service and you load up the vehicle for long distances by towing a load nearing its GCM, then you must halve the drain interval to ensure transmission durability.
If you use products like TRANSMAX Z, and maintain the cooling regime as described you will see a return to the normal oil drain service interval and still retain transmission durability.
If you want to extend the drain interval, I recommend you have the transmission fluid tested every 20,000 km using an oil analysis system. These can be purchased from Castrol in kits that have the sample sent to an Analytical Laboratory with the results returned within the week advising on the condition of the oil and its suitability for further service.
If you don’t want to have the oil analysis done think about the cost of synthetics and drain interval in this way.
Let us assume the synthetic oil and transmission service costs $800. You travel 30,000 km each year and have the service carried out each year. It would take 10 years of servicing before you reached the cost of properly rebuilding a failed transmission.
After 10 years [and 300,000km] you could take the transmission out and replace it with a new one, of course you might also just get a new truck too!!!
As to gauges.
The Autometer range is very good and would be available through Auto Transmission service centres or locally at P&A suppliers.
Cheers
AnswerID:
265096