Randomly shifting?

AnthraXFan · #1 05-28-2007, 12:06 PM

Maybe not so random, but it sure feels like it. Here's what's going on with the car:

While on cruise control, highway speeds around 75mph, shifter in drive, the car will sometimes go from 2500 rpm, and act like it's shifting down off of overdrive, and down a gear. After about 2 seconds, it will shift back..this happens at least 10 times during a 30 min commute...it's been going on though for about the past 4 years.
Lately though, it will do this, and occassionally drop into what feels like 3rd gear, RPMs up around 4000, then go to drive, then back to 2500 rpm. Doesn't feel like I'm losing speed, but just that the engine thinks it needs to shift.

At slower speeds, driving in "3", it will dive into 2nd gear, then back to 3rd. Car is also taking a while to shift from 1st to 2nd under normal acceleration. I've tried using the "manual" button lately, no change. Also, I've never seen the "power" light come on in the 5+ years that I've owned the car...bought it with 55K miles on it.

So this all doesn't sound like it's just a single gear problem. Lately it's been happening more when I have the air conditioning on, or when going up hills. Any thoughts?

I've read a little on the brake band, maybe just needs a tune-up? Car has never had a tune up, and it's at 138,000 miles. I had the tranny flushed at 78,000 miles. Any words of what you might be thinking are most appreciated.

Manarius · #2 05-28-2007, 06:36 PM

Throttle Position Sensor? That might be the cause. The brake band usually causes really bad slipping in shifts from 2-3.

Trevor · #3 05-28-2007, 06:55 PM

Quote:

Originally Posted by Manarius

Throttle Position Sensor? That might be the cause. The brake band usually causes really bad slipping in shifts from 2-3.

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Jason,

I am with you 100%. The TPS would be the first option to check, and details are in the How Too stuff. However I do not recommend the complicated suggested procedure, trying to prove some kind of calibration.

AnthraXFan · #4 06-12-2007, 09:31 PM

Thanks much for the information on the TPS... For someone that hasn't done more than change his air filter before on the SVX, getting that TPS unit out was quite the task. I was able to get it out, and ran some CRC cleaner through it, threw it back on, but the random shifting is still going on. I've noticed it does this at partial throttle, like when maintaining speeds between 40 and 80. If it starts to downshift, if I then hit the gas, the throttle knows to shift back.

I'm looking to see if maybe the voltage is off if I can find a quality meter.
If that doesn't work, off to the dealership to get it cleaned/tested/replaced.

Earthworm · #5 06-13-2007, 08:32 PM

You can also check for used ones, however, the life you'll get out of one is never guaranteed. New ones are expensive though.

Trevor · #6 06-15-2007, 09:36 PM

Quote:

Originally Posted by AnthraXFan

Thanks much for the information on the TPS... For someone that hasn't done more than change his air filter before on the SVX, getting that TPS unit out was quite the task. I was able to get it out, and ran some CRC cleaner through it, threw it back on, but the random shifting is still going on. I've noticed it does this at partial throttle, like when maintaining speeds between 40 and 80. If it starts to downshift, if I then hit the gas, the throttle knows to shift back.

I'm looking to see if maybe the voltage is off if I can find a quality meter.
If that doesn't work, off to the dealership to get it cleaned/tested/replaced.

Your efforts were pointless. It is very difficult to get cleaning fluid to penetrate to the internal plastic resistance elements and it essential that smooth contact is confirmed.

Any cheap meter set to ohms will do this job as calibration is of no consequence. What is more the cheap multi meters now available are surprisingly accurate and using one to set the static voltage, is sure to be within tolerance. What you advise would indicate that the setting could be anywhere.

Forget the dealership in respect of testing the TPS as a component. They can not, do not, test or fix components. There knowledge is confined to simply replacing what could be faulty, limited to the smallest assembly in the parts book.

i.e. yanking things off and "throwing things back on."