For us, it's the alternator every two years... would help if we fixed the power-steering leak.

Trying to wrap my mind around how a bad battery makes the tranny shift odd. If the alternator is good, even with a bad battery, you should have around 14.2v... does the tranny pull so much current, that the alternator can't supply it, and the battery is needed to give up some of it's capacitance...?!

 

I went through the same exercise and never found the answer. Yes, with an alternator putting out 14V, one would think that everything would operate correctly, but one would be wrong, as my experience showed (as well as those other cases I linked in my post above). I think what happens is that with a weak battery, you lose too much voltage/charge getting the car to start; then, even though the alternator is putting out 14V, the rest of the system is still at say 11.5V, and that's where the various components are trying to get their power from. The alternator is busy trying to charge the battery, so all of its 14V output is going into charging the battery, and the electrical system is still operating at 11.5V.

I also think that only some cars are susceptible to this for whatever reason - most folks here have never heard of this situation. But I know that I'm not alone, and after replacing three transmissions in 3 years for this exact issue (and almost doing a fourth, until I found the real culprit), I have been on the same tranny for 130,000 miles, without a hiccup (but 5 new batteries).

 

 

Understand, just haven't experienced it. Our '98 GS4 battery light doesn't come on until about 10.5v, so when the alternator goes, we don't know until the starter is slow. Really wish it went off at 13.5v, so we have another 3v of driving in an emergency. Even when voltage got down to single digits, and we had to turn the lights off to limp home, the transmission never stopped shifting normally.

Not saying it's not a valid issue, just that we haven't experienced it, and we've seen some very low voltgages (during alternator failures).

 

I added a voltmeter near the clock to monitor voltage. It's truly fascinating - with a new battery, it's always at about 12.6V after being charged. Yet over time (months), I see it slowly dropping to 12.4, then 12.2, and eventually closer to 11.5V - it simply does not accept full charge after a couple of years. Again, this is consistent over 14 years and about 5 batteries.

Agreed, it's definitely an unusual issue that affects only some of our vehicles (like mine). But it's worth a try (for the OP).

 

2 Week UPDATE:

The issue has not returned after replacing the battery. Car fires up like a champ every time now. Odd how a weak battery could effect the car in such a manner. Thanks lyonkster, saved me from a transmission job!

 

Great news! Now you know how happy I was when I realized that a new battery just saved me the need to replace a perfectly good transmission .

 

 

Hi.

my 2000 GS 300 also when its engine is cold due to parked for 8hrs at work, it doesnt move rapidly once I crank it up and take off. The temp gauge doesnt move fast enough and gears dont work properly. Could it also be a thermostat issue?