Version 02.00, August 7, 2007 - Warning to not use AMSOIL ATF.
This web page has the following major sections:
Prius cold weather performance falls off significantly:
Measure cold rolling speed change
During a cold week, we measured the coasting distance down a gentle hill close
to our house.
The initial temperature was recorded from the 'outside temperature' display and
rolling stop locations recorded in the 'white' background markers.
The Amsoil stopping locations were recorded in the 'green' markers:
Warmed up, the original transaxle fluid coasting reached a maximum
speed of 15 mph and we braked going 13 mph.
After switching to Amsoil, the maximum speed was 17 mph and braked
going 16 mph.
Bringing transaxle up to operating temperature makes a
significant improvement in rolling drag for both lubricants.
The following shows the energy relationships:
95,742 j - potential energy at top of hill
32,488 j - kinetic energy @16 mph, Amsoil
21,447 j - kinetic energy @13 mph, original oil
There was no attempt to adjust for drag from air, tire and
stone-asphalt road surface.
Tires were at 42/40 at beginning of tests but have not been
measured or adjusted since.
More cold weather measurements will be made when (if)
the cold returns.
AMSOIL SYNTHETIC TRANSMISSION FLUID
I can not endorse using Amsoil ATF in the NHW11 transmission due to
excessive copper contamination found in the oil sample tests.
Amsoil has twice the calcium and phosphorus additives of the Toyota
OEM Type T-IV and Type WS oils and the results are consistent with
a chemical reaction with copper.
Amsoil ATF retained viscosity in service better than the Toyota Type T-IV
but the risk to the copper stator windings are too great.
Amsoil ATF transmission fluid technical
indicate it has significantly improved, lower, cold temperature performance.
It is listed as Type T-IV compatible, the standard for a Prius Classic (01-03).
But there are risks when changing from the vendor's recommended, Type T-IV,
Due to the improved cold viscosity:
- further distance on the cold-soak transaxle, hill roll test
- improved, cold weather mileage
Risk: Electrical Characteristics
MG1 and MG2 use the transmission fluid for direct cooling and in the
Prius Classic (01-03) that means exposure to voltages of ~273 VAC.
The transmission fluid specification does not list the breakdown voltage
or any electrical characteristics.
The electrical characteristics of used Type T-IV fluid are also unknown and
subject to considerable variability.
So far, there have been no reports of internal shorts or arcing suggesting
that used, Type T-IV fluid poses an electrical risk
(In PriusChat.com, we have a report of a 2001 Prius with 159k miles that had two coils burnt.)
We have yet to find reports on Amsoil ATF use in a Prius.
The Prius II (04+) uses twice the internal voltage and a different
fluid, "Type W".
Again, we have no technical specifications for the fluid or any electrical
These can be measured but since we only have an 03 Prius, it remains
an unanswered question.
Risk: Thermal Characteristics
The transaxle fluid transfers heat from MG1 and MG2 but we do not
have the heat transport characteristics of either the Type T-IV or
Amsoil ATF fluid.
Our expectation is there is little risk of heat stressing either
MG1 or MG2.
Risk: Use in CVT
Amsoil cautions against using their ATF oil in a Constant Velocity
Transmission, specificly the Honda CVT.
However the Prius CVT does not share the sliding cone, belt drive
of the Honda.
The Prius power split device, a motor generator and planetary gear, has
more in common with a manual transmission than the Honda CVT or even
How To Change Transaxle Lubricant
The following tools are needed in addition to the fluids:
Be sure the vehicle is safely on a secure ramp - NOT A JACK!
- turkey roaster pan - to catch the fluid, use the largest available
- break-over socket wrench - to open the plugs
- torque wrench - WARNING: to avoid damaging the aluminum case,
torque to 49 N-m or 36 ft-lbs.
- funnel with 20" extension - the transaxle fill plug is difficult to reach so use a funnel with an extension `
- sample bottles - for lab analysis, polyethelyne or polycarbonate, 100 ml.
There are three, 24 mm plugs.
The two on the right are the transaxle drain and fill plugs.
The one on the left is the inverter coolant drain.
By accident, we drained and replaced the inverter coolant, which
also has a replacement interval.
This is much easier if both the inverter coolant and transaxle fluids are
Be sure to use the torque wrench to seat the plugs to avoid damaging
the soft aluminum case.
When draining the transaxle fluid, have the clean sample bottle near and
try to capture from the stream.
If this is your first transaxle oil change, get the transaxle gasket and
throughly clean the pan and all exposed surfaces.
There is no transaxle filter so the pan and magnet, which is loose, as well as
all exposed surfaces need to be wiped clean with shop rags or paper towels.
OPTIONAL: get the "o" ring and remove the take-up pipe and clean the inside
like a barrel.
After draining the fluids, we have samples of the inverter coolant and
the transaxle fluid.
The transaxle fluid was dark, opaque and smelled of paraffin.
TRANSAXLE OIL TESTING
ROBERT WILSON TYPE T 53,000 01 Pri 02 Pri 20k-01 AMSOIL 7,000 TYPE W 61,000
Iron Fe ppm 0 69 116 64 32 2 18 1 206
Chromium Cr ppm 0 2 2 2 1 0 1 0 2
Molybdenum Mo ppm 0 0 0 0 0 0 0 1 0
Aluminum Al ppm 1 21 17 16 3 0 3 1 56
Copper Cu ppm 0 75 67 62 15 0 28 0 21
Lead Pb ppm 0 3 4 1 0 0 0 0 1
Tin Sn ppm 0 2 8 7 1 0 4 0 3
Silver Ag ppm 0 0 0 0 0 0 0 0 0
Nickel Ni ppm 0 5 15 11 1 0 1 0 7
Vanadium V ppm 0 0 0 0 0 0 0 0 0
Titanium Ti ppm 0 1 2 0 0 0 0 0 0
Manganese Mn ppm 0 2 2 1 0 0 0 0 5
Cadmium Cd ppm 0 0 0 0 0 0 0 0 0
Silicon Si ppm 5 227 258 74 37 4 45 2 153
Sodium Na ppm 0 4 12 3 6 7 0 0 0
Boron B ppm 81 43 46 64 60 48 41 59 27
Magnesium Mg ppm 1 1 2 3 1 0 3 0 0
Calcium Ca ppm 116 127 148 219 228 245 199 109 115
Barium Ba ppm 0 13 9 3 0 0 2 0 20
Phosphorus P ppm 269 255 279 247 354 415 320 245 227
Zinc Zn ppm 2 38 26 0 16 2 9 0 8
Water % vol Nil Nil Nil Nil Nil Nil Nil Nil Nil
Solids % vol 0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1
Viscosity @ 40'C cSt 35.3 26.1 26.6 26.1 30.6 37.3 33.9 24.6 21.3
Viscosity @100'C cSt 7.4 5.2 5.2 5.1 6.3 7.5 6.7 5.5 4.7
Viscosity Index 183 133 129 126 63 174 159 171 144
PARTICLE COUNT (61,000 mi., NHW20)
ISO Code 4 um 24
ISO Code 6 um 20
ISO Code 14 um 13
PARTICLE COUNT (53,000 mi., NHW11)
ISO Code 4 um 24
ISO Code 6 um 20
ISO Code 14 um 13
Check for sources of abrasives entry.
The iron, aluminum, and copper content have been flagged
for observation. Abrasive contamination is suspected based
on the silicon content. The viscosity of this sample does not
correspond to the reference lube provided. Certain particle
count values are higher than desired and have been flagged
5909-C Hampton Oaks Parkway
Tampa, Florida 33610
The "TYPE T" is a baseline sample sent to establish what unused
Toyota Type T-IV oil contains.
The "53,000" sample is the first transaxle oil change of an
This is the only sample to have a particle count
The "01 Pri" is Jerry Jorgenson's, 2001 Prius from Plano Texas.
His sample came from an NHW11 with 44k miles.
The "02 Pri" is from Woodson Moore's, 2002 Prius from Denver Colorado.
His sample is the second change from an NHW11 with 40k miles since the last change
and over 100k miles total.
The "Type W" and "61,000" mile sample are from Dr. Denenberg's 2004 Prius from Connecticut.
He provided both a Type W virgin and 61k used sample from his first change.
The "AMSOIL" is a baseline sample sent to establish what unused
Amsoil ATF oil contains.
The "7,000" sample is the early, flush change of the Amsoil ATF
This change was done at the sametime the transaxle vent plug
was replaced by a neutral pressure vent tube to the
Oil Test Results
The interesting results are comparisons of the used to new OEM Type T-IV
oil and the OEM Type T-IV to Amsoil.
We also have Type W new and used oil and this is particularly interesting
when compared to the Type T-IV.
Used to New Toyota Type W
The great news is the 100C and 40C viscosities were less than 15% under the
virgin oil sample.
The 61,000 mile change interval is right on the numbers for viscosity wear.
The particulate count was also lower than 53k sample from an NHW11.
This suggests the wear pattern is much improved in the NHW20 models.
Type W to Type T-IV
The new Type W has starting viscosities in the range of the worn-out, used
Type T-IV. It is also lower, significantly lower, suggesting lower
rolling resistance than the Type T.
Toyota reports that between the NHW11 and NHW20 models,
they used a lower viscosity transaxle oil and replaced
one bushing with a bearing.
These data suggests the changes should make a
significant reduction in transaxle rolling resistance.
Used to New Toyota Type T-IV
Obviously the used oil has significantly lower viscosity.
This strongly suggests severe wear and a loss of lubrication
Unfortunately, the oil testing service does not have a film strength
test that would directly measure the librication property.
But it is reasonable to assume that a substantial change in
viscosity would go hand-in-hand with a change in film strength.
However, the particulates in the oil would compromise the film
librication and increase drag.
New Toyota Type T-IV to Amsoil ATF
The viscosity of Amsoil ATF is slightly higher than Type T-IV.
In the operating temperature range, they are virtually identical
which would lead to nearly identical operating temperature.
The Amsoil viscosity index is lower which means a slower increase of
viscosity with lower temperatures.
Furthermore, the decrease in viscosity at higher temperatures should be less.
To the extent that viscosity and the viscosity index is an indication of
lubrication properties, Amsoil should be an improvment.
Old Type T-IV to Amsoil ATF
The hill rolling test confirms the new Amsoil
has less drag than the old Type T-IV both at operating temperature and cold.
But that has nothing to do with Amsoil performance compared to new Type T-IV.
We do not know the relative film strength of new Type T-IV and Amsoil oil.
The high partical count of the old oil would have compromised the lubricating
film that seperates moving parts.
Viscosity of Oil Samples
The new oils have virtually the same viscosity profiles.
However, the used transaxle oil had suffered significant
It will take time to figure out if Amsoil is better able to handle
the heat and mechanical stress that wore out the original.
One observation is the Amsoil cold temperature viscosity does not appear
to differ significantly from the OEM Type T-IV.
If anything, the Amsoil has slightly higher viscosity which suggests
it should have higher flow drag than the OEM or used oil.
But the hill rolling test confirmed the new, uncontaminated Amsoil has lower
rolling drag than the lower viscosity, used oil it replaced.
Flush Oil Change
The 'flush' oil change was done after 7,000 miles included this time,
removing and cleaning the pan.
This was done after installing in a
breather plug vent to the air filter.
Impressions from this change were:
The second sample came back less bad after 7,000 miles.
The silicon level was 40 ppm, higher than desirable, but understandable because
the first change did not include cleaning the pan.
The oil testing service report that the 10% viscosity reduction was OK and
they usually don't flag it until it reachs a 15% viscosity change.
This suggests the oil should be good for 10-11,000 miles.
We will find out this fall.
- flush oil was darker but clear
- flush oil smelled like the replacement oil, not parafin
- the magnet was covered by gunk that looked like a black 'bad stuff'
- the oil input tube and any horizontal surfaces had a fine layer of black 'bad stuff'
- there is about a cup or so in another gallery that needs a tube to suction out
CORRECT SOURCE OF ABRASIVES
The plan is to follow this checklist:
- Finish ATF analysis; test and calibrate drain plug
- Get replacment ATF; gaskets; cabling; tie wraps
- Put Prius on ramp
- Dump inverter coolant
- Disconnect battery safety plug in trunk and AUX battery ground
- Remove inverter
- Photo and clean area under inverter
- Photo and remove breather plug
- Clean breather and get plastic tubing and fittings
- Open air cleaner and install fitting and close
- Install breather and connect plastic tubing
- Dump and sample transaxle fluid
- Drain engine oil level to 3/4 F (sample?)
- Remove transaxle cover and clean
- Photo with ruler to 'blue print'
- Reinstall transaxle cover
- Install temperature sensor drain plug
- Refill ATF and verify level with dentist mirror / camera
- Install current sensors, additional thermisters on heat exchangers
- Install inverter
- Refill inverter coolant
- Connect AUX battery
- Bleed inverter coolant
- Install traction battery safety
- Back Prius off ramp
- Complete inverter coolant bleeding
Transaxle Temperature Probe
The thermister used has these temperature-resistance points:
- 1,870 ohms - 32 (F) / 0 (C)
- 819 ohms - 97.6 (F) / 36.4 (C)
- 411 ohms - 163 (F) / 72.7 (C)
- 266 ohm - 212 (F) / 100 (C)
The lowest resistance measured has been 511 ohms which puts this well below the 72.7 (C) / 169 (F)
This suggests that ordinary driving even at high temperatures and speeds does not heat
stress the oil.
AIR SHIELD TESTING
We have started testing the effectiveness of an air shield to
minimize excessive, cold air drag:
The initial observations:
This new approach uses a pool toy, a "water noodle," to cover the two inlet slots built into the bumper cover:
- Outside air temperature -
This shield also blocks the outside air temperature and we are seeing
10 (F) temperature rise in the space in front of the coolers.
- Warm-up time shortened -
The "cold" light goes off much sooner, ~0.7 mi., than the ~1.2 mi. before.
At lunchtime, 2-3 hours later, the engine warms up significantly sooner.
The radiators and coolers are effectively serving as a
thermal reservoir, a feature of the next generation Prius.
- Needs automatic control -
The air shield needs to be thermally operated and automatic.
When the outside temperature is low, parking should "seal it up."
When the outside temperature is high, parking should "open it up"
and may need a small, independent fan (ultracapacitor operated?)
to improve cooling.
Just cut a slit and the part will fit over the middle vane in the bumper opening.
To prevent it from 'working out,' I'm holding it with two strips of duct tape but eventually, I'll
come up with some sort of latch.