I've shared this report with PriusChat.
INTRODUCTION
The "prius_technical_stuff" team has been doing some transaxle oil studies that may be helpful to folks experiencing transaxle problems. Never forget that the 2001-03 Prius, the NHW11 model, has a growing and notable list of long lived examples:
- 326,000 miles, 2002 Prius, Jesse's, MA (as of 10/13/07)
- 256,250 miles, 2001 Prius, Empress Taxi, Victoria, British Columbia
- 207,500 miles, 2001 Prius, Yellow Taxi, Vancouver, British Columbia
- 164,687 miles, 2002 Prius, Dept. of Energy fleet test
- 160,045 miles, 2002 Prius, Dept. of Energy fleet test
- 126,185 miles, 2004 Prius, Dept. of Energy fleet test
- 125,663 miles, 2004 Prius, Dept. of Energy fleet test
- 100,000+ miles, Ebay listings, varies
However, there have been some transaxle failures:
- Germany, Florian Stephan
- San Francisco, Art's repair
- Washington State, photos
- 156,250 miles (250k km) Canadian 2002
- Berkley CA, two including a salvage replacement
- Kansas City
- others
BACKGROUND
In November 2005, I became interested in Prius cold weather performance and decided to try replacing the existing transaxle oil, 53,000 miles, with a high performance synthetic. Before the change, I conducted a series of hill rolling tests in cold weather, near freezing, to benchmark the performance. I then changed the oil, taking a sample, and repeated the tests. Sure enough, after the change, the hill rolling test results were improved. At operating temperature, the ending speed down the hill was 16 miles per hour versus 13 miles per hour with the old oil. Still, I sent oil samples for testing: virgin Type T-IV; virgin synthetic; and drained Type T-IV oil.
The old oil test results came back with terrible numbers. The viscosity had fallen by more than 15%, the threshold the oil testing service PdMA reports as severe. Worse there was evidence of possible dirt/grit contamination and excessive particle counts. The old oil was beyond worn out at 53k miles even though the recommended change interval was 60k miles.
Working with others in the "prius_technical_stuff" group, we had other transaxle oil samples tested. Of all the samples, one 2001 Prius, 20k miles came in just over the 15% viscosity threshold. The other samples at 40k, 44k and 53k were all well beyond the 15% threshold and some of the results showed similar dirt/grit indicators as my first sample.
This data suggests the
2001-03 Prius transaxle oil, Type T-IV life is between 15-30k miles.
TYPE WS AND OTHER TRANSAXLE OILS
I had failed to drop the pan when I switched to a 3d party, synthetic transmission oil so I decided to treat the first change as a flush. After 7,000 miles, I dropped the pan, wiped it clean of the sludge layer, installed a vent-plug modification to minimize negative air pressure, and refilled with the synthetic. This oil was also tested and the results showed a significant loss in viscosity, 8% in just 7k miles. This was half-way to the 15% change threshold.
Some of the 2004-current Prius owners, model NHW20, decided to have their oil changed and we sent a virgin copy of Type WS as reference. So far, the NHW20 samples changed at 60k miles have come in right on or about 15% down in viscosity. Then we noticed that the starting viscosity of Type WS oil used in the NHW20 transaxle is the same as the ending viscosity of Type T-IV.
This suggested that Type WS might be an acceptable replacement for Type T-IV. Toyota's SAE paper on the changes between the models also mentioned the low friction transaxle oil along with a bearing change. Some have already started the experiment but I still had a synthetic oil under test. IMHO it is very important to test use of any non-OEM oil to determine the wear patterns.
The synthetic oil viscosity held up great after 27k miles, only a 9% loss and the 7k mile change showed an 8% loss. This suggests the viscosity loss happens pretty quickly but in the case of the synthetic, Amsoil ATF held up better than Type T-IV. But there were problems with the synthetic transmission oil.
The synthetic oil 27k sample particle count was higher than desired but the Cu levels were over the top. The data suggests the synthetic, Amsoil ATF, additives are doing something bad to the copper in the transaxle or possibly it was bringing it into solution. Regardless, such high copper levels are unacceptable in a transaxle with copper stator windings. The viscosity behavior is great but the Cu levels are unacceptable.
DO NOT USE AMSOIL ATF IN PRIUS TRANSAXLES!
Along with the pan sludge and debre analysis, I had already decided to change to Type WS. I cleaned the pan and installed a modified drain plug so I could take intermediate, mileage samples.
At 1.5k miles, the Type WS sample showed about 15-20% contamination from the earlier synthetic oil from the higher viscosity and additives. Also, there was evidence of growing levels of copper. The 5k mile sample was even worse with an unacceptable rise in both Cu and particulates. Either the Type WS or residual gunk and synthetic oil are doing something to bring Cu into solution.
OIL SAMPLE COLLECTION
All of the transaxle samples from PdMA can be found in the YahooGroup, "toyota-prius-sat2" files section. Having any test results is better than not but not all labs report the same data. Feel free to add your data to that area where anyone can pickup copies and do their own analysis.
One critical test is to get both a 40C and 100C viscosity and the resulting viscosity index that gives an indication of oil quality. The extra viscosity test adds $5 to the $15 PdMA basic oil test. Recently, I've started testing the acid/base for some of the extreme cases. This extra $10 may lead to understanding of aeration dieseling and electrical conductivity.
I've also started looking at the acid load, which increases with aeration dieseling. I only have two samples and can not draw any conclusions or trends. However, the 318k sample came in acidic and the 5k Type WS was basic. Acidic oil tends to be more conductive and is a risk factor in transformer oil.
OLDEST OIL SAMPLE
About two months ago, Jesse's 318k mile transaxle oil sample came in and was tested. The test results were "HOLY COW":
318k - 53k - element ppm
1250 - 279 - Ca
990 - 116 - Fe
570 - 67 - Cu
190 - 258 - Si
170 - 4 - Pb
100 - 17 - Al
318k - 53k - baseline viscosity
26.7 - 26.6 - 35.3 cSt @40C
5.0 - 5.2 - 7.4 cSt @100C
Jesse's transaxle was changed and the last reports are it is still running strong . . . of course it has the cross contamination with the old oil, 15-20%. The cross contamination would probably make the current version look like my 53k sample! Also, Jesse's pan debre showed evidence of old case sealant yet Jesse's Si level was significantly lower than mine. But where did all that calcium come from??
Florian's transaxle disassembly, photos from the Washington transaxle and Art's repair transaxle suggests that a unique failure mode in the Prius transaxle is stator shorting and subsequent charing of a set of coils. Once this failure mode starts, it is progressive and marked by a hum or vibration that is speed sensitive. The shorted coils draw energy from the spinning rotor and puts a significant stress on the transaxle. That 'hum' and excessive MG2 temperature is the signature of impending failure.
FOLLOW-UP
This weekend, I am dumping the current Type WS, throughly cleaning the transaxle including sucking out oil from the differential gallery. I'll put in a fresh load of Type WS and run it for about 600 miles. Then I'll dump it, keep a sample and clean all exposed surfaces and drain and restart testing with a new, Type WS test load.
In parallel, I'm starting a Type WS exposure test to transaxle sealant. There has been some speculation that the sealant may be responsible for some of the high Si readings. We should have some results in about a month. Curiously the sealant has a warning about use around copper.
With a little luck, we should have enough data by December to make data supported assertions about using Type WS in the NHW11 transaxle. If the cold weather shows up soon enough, I'll even replicate the hill rolling tests. If worse comes to worse, I may revert to Type T-IV but at least it will be for data driven reasons. Then an unusual sample with 318k miles arrived.
RECOMMENDATIONS: NHW11, NHW20
If you have not had your 2001-03 transaxle oil changed, I would strongly encourage you do it including having the pan dropped and cleaned and a new gasket used. If possible, ask the service technician to suck out the oil under the differential. The replacement OEM oil is Type T-IV.
Schedule your future transaxle oil changes to be 15-30k miles, an annual or bi-annual change. Those living in dusty areas should have it changed more frequently. Be sure the pan is dropped and all gunk and exposed surfaces wiped clean and a new gasket used.
Some of us are testing Type WS in the NHW11 but the test is not finished. If you wish to join the experiment, contact me by PM and lets discuss methodology and risks.
For good measure, save an oil sample, about a cup or 100 ml. saved in a clean, dry, polycarbonate bottle. Testing is optional but impossible if a sample is not saved. I prefer to use small, half-pint, water bottles.
If you have a 2004-current Prius with 60k miles, it is time to have the oil changed. Testing is optional but impossible if you don't have a sample collected in a clean, dry, polycarbonate bottle. Testing does not fix anything but it gives us insights to determine if more aggressive, preventative maintenance is recommended. If your sample comes out smelling like paraffin and opaque, please PM me with the age and miles.
CONCLUSION
About all we can do is minimize the risk of shorting from worn-out, contaminated oil and overheating from high speeds, +70 miles per hour. A single excursion to 100+ miles per hour won't fail the transaxle but over time, heat weakens the wiring insulation and contaminates in the oil can attack metal parts. In the meanwhile, we continue to look at what makes sense to extend the service life of our marvelous rides.
Bob Wilson
ps. Feel free to copy or share this reports in other forums. Just understand this is master copy and I may make changes to correct errors and add new information.
DRAFT: oil testing report
