House call . . . 2002 Prius

A local fellow bought a high mileage, 250,000 mile, 2002 Prius a week ago. A check engine light (CEL) came one but disappeared, probably the three start clear of the CEL. However, he reported a rumble. We met after work and like a typical, old fart, I abused this as an opportunity to teach instead of learn.

He only got one key and fob so I pointed out his first priority should be to get a second key and fob. We discussed using eBay to get the blank and how there is a door and button protocol to teach the key to the car, the car "learns" up to three keys and fobs. Having a spare avoids the risk of a hundred dollar repair if the one and key and fob are ever lost. I forgot to mention the door and button protocol requires having at least one working key and fob.

I brought Vol 1, the schematics, two OBD scanners, a laptop, and an old HV ECU. We first went over the basics of how the Prius is a bunch of control computers that take our inputs to operate the car. So we walked through the HV ECU schematic showing how the accelerator is a pair of pots feeding the HV ECU, the shifter and transmission interfaces driven by the HV EUC. We briefly covered that the OBD is really for emissions testing and a Prius-aware scanner uses it talk to all the computers. A standard OBD scanner won't know there is a battery control computer to communicate with but a Prius-aware scanner will. Then we did some applied driving in my 2003 Prius. His interest was to find out if the noise was common to all Prius (he only had one example for a week.)

We first walked around my car with the hood and trunk opened. I pointed out and opened the three fuse and relay compartments under the hood. He reported that both the steering and inverter pump changes had been done. I also showed him my inverter setup and the 3d party, 12V battery I'm using. So it was time to do a 'road test' with me driving first.

I pointed out that I had parked so I could see approaching traffic and keep the engine off until there was a break. So there was a gap, I started the engine and were off.

In the first 30 seconds, I was able to point to the MPG effect of the catalytic converter warm-up. It wasn't clear if he understood as he was a passenger in my somewhat grimy 2003 being driven by a fat, bald headed man who apparently has a 'motor mouth' (need to work on that!)

Once on the highway, he was able to compare the indicate mph vs GPS mph and see the effect of the over-sized tires, in effect over-drive. We took a ramp at 60 mph, not the posted 30 mph to show how the low CG gives the Prius the ability to take a turn. Then we headed over to the 'loop' that allows test driving without having to deal with traffic.

I drove some laps so we could go over efficient driving which is the application of energy management using cruise control and shifting. After a few laps, I pulled into a parking lot and at a speed of about 25 mph, shifted into "R". Nothing happened until at about 7-8 mph when MG2 began to provide reverse power. The point being the Prius has no reverse gear, just MG2 that provides all reverse power. This stunt was just to re-enforce the Prius is a computer operated car. Then we switch drivers.

So after the 'newness' wore off, he did a reasonable job of learning how to drive on cruise control and operate the shifter. There were a few 'fubs' but having seen me shift into "R", he understood it was nothing. This was more confidence building as well getting a feel for how the car handles. His last lap was pretty good as the indicated MPG went from 70 to 77 MPG. Then we drove back to his car. Once he was back in familiar traffic patterns, his past driving style came through which is what a decade of driving existing cars teaches.

He plugged in the miniVCI into his car and I fired up the Techstream. The left over codes were P0301, P0303, and P0420. I pointed out these could be from sitting on the lot and just needing to be driven. Regardless, we cleared the codes and drove around. The codes did not return on this trip but we didn't have a cold start. Alabama has no emissions testing so this is not a big deal.

In his car, we could clearly hear the rumble. Checking the HV ECU, the MG2 temperature was less than MG1 temperature, no problem with the transmission. The battery voltages looked good with less than 0.15V difference between min and max pair and the minimum voltage block cycled between at least three pairs. Traction battery looks good.

As for the rumble, I do not remember the sound changing with hard left or right turns or hard braking or acceleration. But knowing the transmission is not involved, it isn't clear that handling tests were needed in this case. Just I wished I'd remembered to ask to do a complete test drive. Regardless, we soon got back to where my 2003 was parked.

I asked him to 'feel the hubs' while I retrieved a contact-less thermometer. Three of the hubs measured 107F and the driver side front measured 150F. My diagnosis, driver side, front bearing is bad, possibly the CVT, but definitely that is the immediate problem. There could be a brake involvement for the heat and I noticed the wheel spokes had some discoloration but I didn't take a close look. Later I wondered if the driver side wheel might have met a road hazard . . .

We talked about independent area shops as neither of us think much of the local Toyota dealer. If he has access to a press, say the do-it-yourself auto shop on Redstone, it can be a do-it-yourself job. Sad to say, this is his primary ride which makes this a rough repair. Regardless, he now has a reasonable diagnosis and can work the problem.

He does monitor PriusChat so hopefully he'll add any details I've missed.

Bob Wilson

 

Hi Adam,

It probably makes sense to do a through inspection of the driver-side hub. You want to be able to put some serious pressure on the parts so chock the rear wheels, use a jack stand, and even put the wheel under the front. You want to know the car is secure and won't fall on you.

You'll want to look for slop, rattle or any mechanical issues with the brakes and bearings. In "P", there is a parking paw engaged so it should have little or no slop. Curiously, if both front wheels are raised, the differential will let one tire turn in the opposite direction of the other. Check the brake pads. If the seller put new pads on that might explain some of the heat. Take lots of photos so you'll have something to look and share later.

Your mileage is definitely low but given the high heat, 150F, found on the driver-side hub, understandable. Heat is the signature of energy loss. Regardless of the source of the drag, this is the same as driving around with the brakes partially applied.

BTW, you may remember the loop showed my car reaching 28-29 mph (indicated) in "N" using oversized tires. You might repeat the test and see what your downgrade speed is in "N".

Bob Wilson