2002 transaxle noise diagnosis

[joetho]

My 2002 Prius needed tires about 10,000 miles ago and I knew that. The rear ones were kind of scalloped and started making a wierd hum and I replaced em and rotated front to back. Hum went away.

Now I have fairly new tires on the front, and the ones in back aren't new but aren't bad.

There is a hum in the front that just didn't seem like road noise.

I replaced the old trans fluid with Toyota ATF WS fluid, cleaned the pan etc. The old fluid wasn't too bad looking and there was not all that much sludge. I bought this Prius at 180,000 and it is currently at 216,000. It had been very well maintained but I know enough to be aggressive with trans fluid changing.

The hum is still there, seems to be in front, but get this: -at highway speed, the hum is certainly noticeable, but changes when I swerve a little or go around a curve. This would indicate to me either a tire, or maybe a wheel bearing, maaayyyybe a halfshaft (don't even know if my Prius has halfshafts, <G>), but if the transaxle motor windings (?) are humming prior to failure, the hum seems unlikely to react this way to curves and gentle swerves.

The only engine code is the cat converter, and I have thought about just cutting the cat convertor out and replacing it with a straight pipe to the muffler. But that is a whole 'nuther thread!

Any thoughts?

-Joe

[joetho]

Originally Posted by joetho

The hum is still there, seems to be in front, but get this: -at highway speed, the hum is certainly noticeable, but changes when I swerve a little or go around a curve. This would indicate to me either a tire, or maybe a wheel bearing, maaayyyybe a halfshaft (don't even know if my Prius has halfshafts, <G>), but if the transaxle motor windings (?) are humming prior to failure, the hum seems unlikely to react this way to curves and gentle swerves.

I can't decide if I should worry or not. Is there anything else I can do? I suppose I could put new tires on again and see if it goes away. I really hope my transaxle lasts forever but I know it's just a mechanical device so after 216,000 miles I shouldn't be surprised...

[bwilson4web]

The death hum of a transaxle doesn't care if you are going left or right, powered or not. That it changes with your driving suggests wheel alignment or mechanical part. The death hum varies directly with the speed and is associated with higher transaxle temperatures.

Do you feel comfortable using a VOM to measure the voltage from the transaxle temperature leads?

Have you had a 4-wheel alignment check done? Perhaps you might post the numbers? Detecting scalloped tires early can save putting on another pair and getting bad results. Scalloped tires easily cause a hum.

I'm using EZ-SHIM for my rear wheels and got the camber correction bolt to make my front right tire caster closer to neutral (my preference.)

Bob Wilson

[joetho]

Thanks Bob,

I am very comfortable measuring voltage. I have done it before with needles through the insulation then a little silicone over the hole when I'm done. But I don't know where these sensors are on the transaxle, or what the normal voltage is. I will look for info about that.

I would feel a whole lot better to know that the temps were normal or at least within limits.

Thanks, Joe

[bwilson4web]

Originally Posted by joetho

I am very comfortable measuring voltage. I have done it before with needles through the insulation then a little silicone over the hole when I'm done. But I don't know where these sensors are on the transaxle, or what the normal voltage is. I will look for info about that.

I would feel a whole lot better to know that the temps were normal or at least within limits.

Not a problem!

I've got the maintenance manuals at the house and can look the connectors. Then I'll measure them on my NHW11, document the procedure and you should be able to replicate the result.

Bob Wilson

[orange4boy]

I have a similar hum at low speeds. ( I think at high too. I'll check) I replaced my transaxle with a low mileage one (15,000 ish) after the last owner let it run dry from a slow leak. While I had it out I looked at the inboard CV joints and found wear depressions in one and not the other. My maintenance records showed a drivers side replacement at some point and they used a rebuilt unit. I can tell because the replacement is rusty from being sandblasted whereas the original is still painted and nice looking. I bet it's that CV joint.

Worn CV joints can make that noise and yours sounds exactly like mine. Gets louder/quieter when you turn slightly. It freaked me out at first because I thought it might be the new tranny, but my temperatures are fine.

I wouldn't worry too much.

[tochatihu]

Good info from orange4boy. Joe, have you given the CV boots careful inspection?

[joetho]

Originally Posted by tochatihu

Good info from orange4boy. Joe, have you given the CV boots careful inspection?

I looked at both cv boots on the wheels, but not the inside ones. In my experience, the inner boots last soooo much longer than the outer ones, and when the outer boots fail, the usual response is to replace the entire half-shaft.

Wheel bearings are on the short list, but after an hour or so at highway speeds, all 4 wheel hubs feel the same temperature to my hand. I know, an inaccurate cowboy test for wheel bearings, but I have used that cowboy test before with good results.

Next: lift it up and give those halfshafts a good shake. Then find an empty parkinglot and give them the hard turn test and see if I feel anything wacky.

And I am still planning to check the transaxle temps as soon as I can find the info. And if I can't find any info, I will poke around and just guess which sensors they are, and measure the warmed-up voltage from them. Even if I don't know the normal reading, if one is way higher than the other, why, that would be a bad sign, no?

You guys are the best!

[bwilson4web]

Hi,

I believe the signals you want are on the HVC ECU located under the passenger side, firewall-floor area:

• GMTG(MG1) - pin 8, connector H10, green-white stripe
• GMT(MG1) - pin 1, connector H10, brown-red stripe
• MMTG(MG2) - pin 9, connector H10, red-white stripe
• MMT(MG2) - pin 2, connector H10, gray

The H10 connector:

• latch side row - 1,2,3,4,5,key,6,7
• middle row - 8,9,10,11,12,13,14,15
• bottom row - 16,17,key,18,19,20,21,22

I don't have the voltage-to-temp calibration data but generally speaking, the resistance and apparent voltage goes down as the units warm-up. It may use the ICE coolant curve:

(thanks to Hobbit who made this plot)

What you want is:

• ICE temp > MG1 temp > MG2 temp

Historically, MG2 fails with a stator short. Over time, MG2 gets hotter and hotter and eventually the HVC ECU throws a code. But swapping the transaxle, other than being one heavy SOB, can be handled fairly straight forward ... thanks to OrangeBoy's photos.

Bob Wilson

[orange4boy]

I tried, after the fact to, see if I could feel any play in the inboard CV but I couldn't. Mine wore a small indentation in the outer housing which, I presume the bearings ride in and out of, creating a hum when you drive.

I priced that half shaft from Toyota... sit down... $560. Your results may vary. Rebuilt or salvage is a much cheaper option. It is pretty easy to get to and replace just the inboard CV if you can get one on it's own. I can help talk you through it if you determine that's the issue.

The wheel bearings have a howl to them that resonates through the whole car when it gets bad.

The outer wheel bearing is relatively easy to get to AFAIK. That, I have not done yet on this particular car.