'02 with 489,000 code p03120

The red thermometer warning light implies something is overheating, usually the engine or the inverter. However DTC P3120 does point to a failed transaxle. If you are able to obtain the three-digit info code associated with that DTC, then it may be possible to pin down the problem further.

A few comments:

1. What is the condition of the engine cooling system? Any leaks, especially at the engine coolant pump? Can you determine engine coolant temp by using a handheld IR thermometer on the hose connected to the thermostat housing?

2. What is the condition of the inverter cooling system? Do you see turbulence in the inverter coolant reservoir? The fluid level at the very front of the reservoir should be higher than the level elsewhere. If you don't see this then the inverter coolant pump is not working.

3. Was the car in a front-end accident? Possibly a bad wiring harness might contribute to the problem.

4. Some of the info codes associated with P3120 relate to a temperature sensor malfunction within the transaxle. However you may have to replace the entire transaxle (rather than replace the sensor.)

I agree that is hard to believe that a transaxle failure is the problem, if the car will run after it is given time to cool down. Often a bad transaxle will produce unusual drivetrain noises.

 

See if Patrick agrees LOL. My wild quess is, it needs a new inverter pump. I believe pump is suppose to run all the time and you should always see turbulence.

 

The inverter coolant pump should be running at all times when the car is IG-ON (where all dashboard warning lights are on) or READY (when the car can be driven.) The pump sounds like an aquarium pump and you should not hear any air bubbles.

If you should find that the pump is not running then that should be replaced. DTC P3130 "Inverter Cooling System Malfunction" is supposed to be logged, if the inverter coolant pump is not running.

If the owner had not kept up on routine maintenance (such as replacing engine and inverter coolant at 30K mile intervals) then the inverter/transaxle coolant channels may be pretty plugged up by now.

DTC P3120, info code 250: "Motor Temperature Sensor Malfunction. Motor temperature sensor performance problem"

Maybe that code will be addressed if you replace the inverter coolant pump. However it is possible that the car still has more than one problem remaining.

If you find that the P3120 code persists after replacing the inverter coolant pump then the transaxle might have to be replaced (most likely with a salvage unit, to reduce the repair cost.)

Do you know the car's repair history? If so, what major repairs have been performed (such as traction battery, inverter, electric steering gear replacement?)

How about taking and posting a photo of the odometer? Your customer's car may be the highest mileage Prius that I've seen a post about. Further, it seems that the odometer of the 2G Prius (2004 - 2009 model years) will stop at 299,999 miles so we are probably not going to see documented evidence of 2G lasting beyond that odometer reading.

 

The dealer is correct. The symptoms, combined with the DTC and information code, are consistent with the diagnosis.

The MG2 motor winding has only one temperature sensor. If the fault does not occur in the region where the temperature sensor is located, then the heat from the fault will not set a temp code. The motor will continue to operate until it degrades enough to affect operation, or until it sets a different code.

If, however, the fault originates in close proximity to the sensor, the heat generated by the short will set a code, usually within about 7 to 10 miles of driving on a level road at highway speeds (assuming a cold start). When the temperature rises beyond a certain threshold, the code sets and motor current is limited to protect the motor. If you pull over and let the motor cool, normal operation will be restored until the motor overheats again.

A failed sensor also requires replacement of the transaxle. However, it would set a different information code.

 

It will not. The DTC and information code are set by the temperature sensor circuit for MG2. No other malfunction will trigger it.

 

Patrick is an excellent source and the inverter pump is a likely part to fail. I would typically expect inverter pump failure to induce an inverter over temperature condition. We have a thread already addressing solutions. I'm more interested in transaxle diagnosis.

What sort of scanner do you have? In particular, does it have the ability to read MG1 and MG2 temperatures? The reason is normally MG2 runs relatively cooler than MG1 which is sandwiched between the ICE and MG2. But when MG2 stator windings short out, it runs hotter than MG1. I've seen reports of temperatures in excess of 100 C.

The most frequent transaxle failure we've seen in the past is shorting of a stator winding. This short draws power from the turning rotor and the stator continues to heat and char the stator. The usual symptom is a speed related humming from the transaxle that does not go away when the car is shifted into "N" while running:

This is from a Prius that was abused in Washington State however Stephan's photos of the transaxle he disassembled showed similar charing of the stator windings.

Thanks to Vincent, we now have a way to use a ScanGauge, $150-175, to read these critical transaxle values. Another alternative is the $400 Auto Enginuity but it is pricy and their support is poor. I learned of Auto Enginuity from a salvage rebuilder last December who was dealing with some failed transaxles. You might also search the archives for Orange4boy's transaxle replacement thread.

BTW, I'd like to suggest some online training:
http://www.auto-careers.org/
Their online production is a little rough but the content is great. They are not specific to just the Prius and if you'd like to hang a 'hybrid shingle,' this would be a great place to seek training.

One other online source is the YahooGroup, "Prius Technical Stuff," whose archives date to November 2000. It typically has a high signal-to-noise ratio although recently, last year or so, we're seeing an increase in 'opinion' postings. Still the message archives includes transaxle forensics after one that failed in Germany.

Bob Wilson

 

Aw, crap. That means I'm gonna have to buy another one of these things when it hits 300K. :madgrin:

 

So you don't see any turblence in the inverter resevoir?

There have been scattered reports of a 'jelly-like' substance forming in the coolant channels. If you decide to change the coolant, I would very much like a sample for chemical analysis. Just put it in a dry, clean water bottle, a cup will be enough, and send it to:

Bob Wilson
9011 Randall Rd.
Huntsville, AL 35802

When draining, look for globs, I'm especially interested in them. I'll send it to R&B Labs in Florida who also handles my oil analysis. Yes, I'll share the results.

Is the coolant 'pinkish' or 'reddish?' The pink is distinct:
2003 Prius - Cold Weather and Transaxle

Thanks,
Bob Wilson

 

That looks like a really nice tool. Some lessons learned:

1. ISO-9141, KWP2000 - protocol to use. Don't use the CANbus because one of the OBD signals is tied to the Tc/Ts lines and this can induce false codes (I see false ABS codes if I don't use a pig-tail to prevent the spike.)
2. Transaxle data should be under the HV ECU. The transaxle is pretty much totally operated by the HV ECU. Now the Auto Enginuity calls this "enhanced powertrain" but it is the same as the HV ECU.
3. You're looking for MG1 and MG2 temperatures but you can also get the inverter temperatures. DO NOT RULE OUT BROKE OR RODENT BIT WIRES!!! One of the transaxle replacements was held up by a temperature error because after 10-15 minutes, none was reported so a temperature error was reported (search the messages.)

Here's hoping it is a quick diagnosis and easy fix!

Bob Wilson

 

A great effort, and thanks for posting the photo.

Looks like the transaxle has indeed failed, so your customer will need to decide whether to put more $$$$ into the car or not. Perhaps it would make sense to swap out both the engine and transaxle, if it is decided to repair the car.

 

If you go to the freeze frame for the information code (not the freeze frame for the DTC), you'll see the MG2 tempearutre that was recorded at the time the code set. It will probably be in excess of 300ºF.

Either scanner should work fine for this vehicle and DTC, provided that they have Asian coverage.