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Tuesday, April 23, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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April 12, 2005

More Tranny Troubleshooting, Oil Service

About two weeks after the new transmission was installed, I was on my way back from lunch one Saturday when the car started pendulum shifting again. I suddenly got that sinking feeling that I'd just spent $3700 on a new transmission for no good reason. It shifted strangely a total of three times before I got pissed enough to punch the throttle. As expected, this returned the transmission to normal operation and I went about my business.

Figuring that this might have something to do with the downshift trigger switch beneath the accelerator pedal, I managed to come up with an experiment to see if intermittent actuation of that switch was causing the car to downshift even if the pedal and throttle position did not agree with it. I accelerated to about 45 MPH and held a steady 2000 RPM with my right foot, while I carefully reached behind the pedal with my left foot to depress the switch. Nothing happened. I tried again several times to no avail. At that point I knew that switch wasn't the cause of the erratic shifting.

The following Monday I dropped by my dealer to give my mechanic the bad news. I told him of my experiment and he told me that he expected as much. He noted that on some cars that switch doesn't actually do anything -- it's there for "feel" only. The way to tell whether the switch is "real" vs. "fake" is to look for a single wire coming out of the switch (closing the switch pulls the wire's potential to chassis ground, so only one wire is necessary). It so happens that my car has the "real" switch, but my mechanic added that in such cars the switch works in concert with the throttle position sensor to determine if the driver is really asking for full throttle and therefore needs the transmission to downshift for maximum acceleration. If the throttle position is not "full", depressing the switch should indeed have no effect aside from perhaps triggering a fault code if done enough times. Oh well...back to the drawing board.

After some additional discussion, we figured it was time to replace the throttle position sensor. And yes, the irony of that moment did not escape me, as replacement of the sensor was my original suggestion. Of course, hindsight being what it is, I cannot and will not fault my mechanic. Fortunately, he said he happened to have a brand new sensor in his parts bin'o fun, and as long as the car continued to perform normally, it would be okay to simply wait until the next oil service before swapping out the sensor. I agreed.

Today, I brought the car in to replace the sensor and do a few other things:

Oil Service and Brakes/Tires Status

The Oil Service (a mid-cycle this time) went off without a hitch as usual. After replacing the throttle position sensor my mechanic took the car out for a long drive to sample the result. When I came to pick the car up, he refrained from giving me a glowing thumbs up, and instead asked me to drive it for a bit to see if I noticed any change.

He also told me that the brakes have about 10K miles left and that the front and rear are about equal in terms of wear state at this point.. That means I probably won't need to do brakes until September or so, but when I do, it will be all four wheels with pads and rotors...and that means another $1000 including a fluid flush (note to self: learn how to do brakes).

I asked him to diagnose what sounded to me like tire noise (in the form of low-frequency, rhythmic vibrations) coming from the front of the vehicle, just to eliminate the possibility of a bad wheel bearing or something similar. He concurred with my opinion that the noise is due to cupping of the front tires. I had hoped to stretch the tires until the point at which I did brakes, but given that the rears are pretty much DOT-legal racing slicks at this point and the fronts are making noise, I will likely replace them earlier -- perhaps next month. That would translate into a wear rate of about 18 months and perhaps 30K miles, so while I can't say they have lasted any longer than the Dunlop SP8000's installed previously, I can easily say that they track as true as the day they were installed -- and that's a far cry from my experience with the Dunlops.

While I was at it I asked if there was anything in the front end (tie-rod ends, bushings, etc.) that should be replaced on a car of 115K miles to preserve handling or tire wear. He pointed out a couple of bushings on the lower control arms that handle side-to-side loads and, using a sample he had hanging around, showed me how they typically fail. They're easy to replace, so I'll likely have that done while the wheels are off to replace the tires.

At the end of the day I had to ask myself whether replacing the transmission made sense, and at this time I think "yes". Except for the 1->2 shift, the transmission is definitely better behaved. The occasional "clunk" I used to feel while maneuvering around a particular corner (strong deceleration followed by quick acceleration) no longer occurs. Then there's the sanity I now have in knowing that the tranny should last until I sell the car (whenever that is) or at least 250K miles, and that's worth the coin (well, almost...$3700 is still better in my pocket than BMWs, but you get the idea). I maintain that transmissions don't last forever.

And speaking of transmissions and shifting, here's some trivia I learned during this visit: if you've ever wondered how the E46 cars manage to shift so smoothly and why during high rpm shifts the engine sounds like it momentarily dies at the top of every shift, that's because it DOES. The ECM actually shuts off fuel to the cylinders at the instant the transmission shifts. This is intentional to reduce stress on the transmission. Next time you drive an E46, punch it and carefully listen to it go through the gears. Now that I understand what is happening, it's easy to recognize. In fact, it's so obvious that I'm surprised I didn't notice it earlier.

Parts $44, Labor $38, Total $96. Total Mileage 114813.