Aveo automatic transaxles and fuel economy

[Thymeclock]

I think I now know why Aveos with automatic transmissions get such disappointing FE in city driving.

I recently found the service manual for the four speed Aisin 81-40LE. It can be found on-line at: http://www.anagabriela.ro/Manuale%20...0TRANSAXLE.pdf

The graph on page 5A2-12 shows the shift points. (The graph is stated in KMH, so I’ve converted the figures to MPH for this discussion.) The graph shows the shift points will vary somewhat according to the TPS reading. On my car the TPS reading at idle is about 16; very conservative (slow) acceleration would be about 25; normal or average acceleration would be about 32.

According to the graph, at 25 TPS the shift points should be at about 11, 20 and 27 MPH. At 32 TPS it should shift at about 12, 22, and 32 MPH. I compared what I am actually getting on my car, at about 32 TPS, which would be considered normal or moderate acceleration. I found it actually shifts at about 15, 25, and 45 MPH. In other words, the upshift into the highest gear (fourth) is too late. The delayed shifting into high gear at around 45 is a major reason for delivering poor city FE. My old Celica had a 5 speed automatic, and it would shift into fifth (overdrive) at about 38-40 MPH.

I live in suburbia. On local streets I am finding that it helps to accelerate to 15 MPH ASAP to get into second gear and then glide to the inevitable stop sign at every corner. On major roads, typically the speed limit is 40. Unfortunately, when there is traffic it is impossible to break 45 – thus the car is spending most of its time in first, second and third gears, at best.

Would anyone know what is involved in reprogramming the TCU?

[Thymeclock]

Here’s an update on my A/T & city fuel economy situation on my 2009 Aveo after taking it to a Chevy dealer. They replaced my accelerator pedal assembly. They said there was a code set in the memory of the computer indicating it was not within correct spec range. Since I got it back I test drove it and here’s what I found: It now up-shifts a bit sooner into fourth gear – before it wouldn’t up-shift until about 38 MPH, and now it shifts at about 32. The shift into fourth gear feels a bit firmer (less murky) with a more noticeable drop in RPM’s when it shifts. The TPS reading I’m now seeing on my ScanGauge is slightly lower at idle, at about 15-16 TPS instead of 16-17 seen previously. So there is a perceptible difference, and marginally a slight but noticeable improvement in the way it shifts.

A few days before they changed the pedal assembly I took it for a drive on a stretch of highway with no traffic of about 10 miles. I maintained a steady 65 MPH with the A/C on the whole time (it was unseasonably warm here), and the Scan Gauge said I got 37 MPG for that highway portion! Apparently an A/T Aveo gets excellent highway mileage once the TC is in lockup mode – but short of speeds below 45, great fuel economy can’t happen.

I strongly urge anyone with the same murky shifting symptom to take your car to a dealer if it is still under warranty. Your car may have the same problem as mine and the accelerator deficiency may be more widespread than just an isolated case.

It’s too soon to tell with certainty whether our overall MPG improves any. I don’t expect to see much improvement as I’m still feeling a pronounced shift with an RPM drop into TC lockup at about 45. In other words, this probably rectified a relatively minor problem, but not the more prevalent one of the way the shifts are programmed. Unfortunately GM programmed this transaxle to delay TC lockup and treat it like a shift into overdrive. The sad and annoying thing is that there is no overdrive gear on this transmission beyond the actual fourth gear. The programming that delays the converter lockup amounts to fudging an up-shift into lockup mode, treating it as a sort of quasi-overdrive: as such, it is too little, too late.

[99LeCouch]

Sounds like a slushbox geared for good highway mileage at the expense of city mileage. My Buick is the same way. First and second are stump-pullers, 3rd is a compromise where it can have the TCC locked, and 4th is highway where the TCC stays locked most of the time.

Check for converter lockup by an RPM drop when you're feathering the throttle in 3rd gear. It doesn't happen immediately, and it'll be dropped quickly when accelerating more than very gently. It may also have been disabled to make for a "peppier" drive around town.

[katiep]

I've found my Aveo doesn't get what was posted on my sticker. It said 30 mpg city, 35 mpg highway, fueleconomy.gov has a more realistic EPA listing, with about 26 mpg city, 30 mpg highway. I'm getting 30mpg city, 33mpg on a good highway trip, with one more mpg added to those in the summer. (But I think all that has improved since I removed the seats. We'll see!)

I've noticed my car shifting late also, just like yours, but I wasn't car-wise enough to realize this during the years my car was under warranty Y'all know how much it might be for them to replace the accelerator pedal assembly? Thymeclock, did you see an overall gain in mpg?

Do you think using the "hold" button would make a difference? I haven't had a chance to play with it, but I don't *think* it would really help with this particular problem, since it seems that you have to have already shifted up to a certain gear to "hold" it there.

What totally bites about all this is that mine is a 2004, and if yours is a 2009, it's like Chevy totally ignored this issue! Surely they were/are aware???? Surely it would've been a quick fix in the assembly process? =/

[Thymeclock]

Originally Posted by katiep

I've noticed my car shifting late also, just like yours, but I wasn't car-wise enough to realize this during the years my car was under warranty Y'all know how much it might be for them to replace the accelerator pedal assembly? Thymeclock, did you see an overall gain in mpg?

We can't assume that there is any connection between the accelerator pedal being faulty and it shifting late. Before they replaced the pedal the shifts felt murky, or ill-defined. After replacement the shifts feel more solid and well-defined, much clearer as you watch the tachometer. The shift points are determined by the TCM, much more than by the gas pedal. No, it didn't improve the MPG at all.

Do you think using the "hold" button would make a difference? I haven't had a chance to play with it, but I don't *think* it would really help with this particular problem, since it seems that you have to have already shifted up to a certain gear to "hold" it there.

No, the hold button doesn't function that way. It was designed for use in snow or mud to provide better traction. It is for when you want it to remain in first gear without revving into a higher gear; otherwise it might upshift due to your wheels spinning. Beside which the torque converter lockup is not really an actual gear. TC lockup is a more direct coupling of the transmission with less slippage than the amount that normally occurs in the lower gears. It feels and acts like a shift, but it really isn't. When the car is not in full lockup mode the transmission isn't really fully engaged - which is why we call automatic transmissions "slushboxes". All traditional automatics are programmed to fall out of TC lockup below a certain speed (in this case it seems to be at 40 MPH). The real problem is that it won't enter lockup until about 45.

What totally bites about all this is that mine is a 2004, and if yours is a 2009, it's like Chevy totally ignored this issue! Surely they were/are aware???? Surely it would've been a quick fix in the assembly process? =/

It appears they probably used the same lousy programming of the TCM for all the cars they make. When the dealer was replacing the gas pedal they provided me with a loaner car, which was a an HHR. The shift points were the same. Actually, they could redesign and change the programming of the module as it does run on software. But why should they? They don't want to improve the car you've got, they only want you to buy a new one. The only things a car manufacturer is concerned with are avoidance of lawsuits from things that could be safety related. And GM was/is bankrupt, to boot.

Actually the delayed shift points give a slight increase in acceleration. Unfortunately, it also results in reduced FE. If the shift points were lower, the car would have comparatively more sluggish performance, and most people don't want to buy a car that can't seem to get out of its own way.