Trans axle repair

we have a 2002 Toyota Prius with 144,000miles. Recently it started having a slight vibration when first rolling forward from a start. After checking the brakes and the tires, our mechanic thinks it may be a trans axle problem. What possible problems can go on with the trans axle, and (in a ballpark sense) how expensive can this be?

HSD engine with electric motor and generator

The mechanical gearing design of the system allows the mechanical power from the ICE engine to be split three ways: extra torque at the wheels (under constant rotation speed), extra rotation speed at the wheels (under constant torque), and power for an electric generator. A computer program running appropriate actuators controls the systems and directs the power flow from the different engine + motor sources. This power split achieves the benefits of a continuously variable transmission (CVT), except that the torque/speed conversion uses an electric motor rather than a direct mechanical gear train connection. An HSD car cannot operate without the computer, power electronics, battery pack and motor-generators, though in principle it could operate while missing the internal combustion engine. (See: Plug-in hybrid) In practice, HSD equipped cars can be driven a mile or two without gasoline, as an emergency measure to reach a gas station.

An HSD transaxle contains a planetary gear set that adjusts and blends the amount of torque from the engine and motor(s) as it?s needed by the front wheels. It is a sophisticated and complicated combination of gearing, electrical motor-generators and computer controlled electronic controls. "

You need a better mechanice to actually determine what is wrong. A good transmission shop can also do that.

Yes, I find it absolutely bizarre to go from “slight vibration” to “oh, you just need a new transmission.” Another look at this by someone who knows the Prius is in order.

I would agree with the other posters…this transmission is heavy-ily influenced by the computer motor controller and sensors. Mechanically, it is quite simple and so I would expect it to be mechanically reliable; Electrically, its control is quite complex.

Assuming there isn’t any information to be had from the computer (DTC’s), I’d suggest isolating the problem as either an engine side or a transmission side first.

Since this car can have the 2 motor/generators and the gas engine starting, stopping, and changing velocities at any time the car is moving (or isn’t moving); a vibration may be due to any of these parts either mechanically being damaged or otherwise operating out of specification.

I’d suggest working with a Toyota dealer in this case; not that they are going to be necessarily better than an independent shop, but they may be more familiar with some of these symptoms. Your experience may ultimately be based on how talented the technician is at isolating the actual problem.

Price is difficult to pinpoint. At worst case, assuming there is no warranty coverage; the problem may involve the entire transmission half being replaced. I’m sure a dealer could give this lump sum. Of course, this problem may not be worth THAT much to you to be fixed.

In deference to this design, there are no clutch packs to slip, no servos to stick, no bands to slip, and no real gear changes as with most other transmissions.

The motor windings, like any motor can develop shorts in the insulation as a result of defect or heat, or stalling conditions. The inverter and corresponding computer comprises the control mechanism for the transaxle. I’m not sure if there are practically any serviceable parts inside.