This makes no sense, and has me and my 2UZ guru buddies grasping for straws.

Background: I've had what has felt like a shudder when shifting from 1 to 2, even after several trans flushes. Other weirdnesses are that downshifts may be sluggish occasionally, and when approaching the 2 to 1 threshold the trans will get "stuck in between gears" if I hit the throttle again (then it revs for a second or two before a gear grabs).

...then we get a nice day yesterday, so I turn off the climate control and NO MORE TRANS ISSUES! I mean zero. The thing drives perfectly. What gives??? There are no weird noises when the climate control is on. The only hint that something needs attention is a little engine flutter/shudder when the A/C is turned on that I was attributing to a worn engine mount.

Anyone got a clue?

-Z

 

Reading what you have written above, I become in a mood to try to check the voltage regulation of the 5V Vcc line in the ECU. Because the rough AT shifting is one of typical ECU problems caused by that.

As I dealt with ECUs just for my curiosity at different shops and dealers, I found some 1UZ VVTi engine's ECUs tend to cause rough AT shifting problem prior to other poor engine performance issues. The major reason to cause this is the poor regulation of the 5V Vcc voltage regulator in the ECU. This is often caused by failing QAS capacitors. Some others are caused by the increased resistance of a resistor R862. The photo below shows semi-burnt R862. It is a 2.7 ohm resistor but the resistance is increased some. The higher resistance causes a poor voltage regulation problem. This is a part of the ECU board for my 98 Celsior and the actual resistor is indicated by an yellow arrow (R862) at lower part of the photo.

See R021 which is indicated by a purple arrow located at upper part of the photo, the color is totally different, as you see. Both R862 and R021 are oxidized metal film resistors and should be the same color. Those type of resistors are strong against heat but when used for a very long time with hot condition, the heat damages the metal film a little by little and the film becomes thinner and the resistance increases. Thus there happen problems of insufficient power supply. When total power consumption of the car is big/much. I mean when you turn head lights, A/C, seat heaters, fog lights and others ON, the 12V B+ line is dropped some, and the problem tends to start due to the voltage drop and the insufficient power.

Check your car's 12V line voltage regulation using something like this. How much does it drop when the A/C is on? If it goes down 1V or more, you need to check the reason why so and also the ECU checking the resistance of R862 and the impedance of QAS capacitors. Sometimes a weak alternator causes the problem too.

 

I had this issue a couple of years ago. No cel, no check vsc light or any indication of a bad charging system. No pending codes indicating an issue elsewhere. I let it go a month or so (car only driven roughly 10 miles in that time) and each time it was driven the shifting would be a little off, hanging in between 1 and 2. HVAC on, it was substantially worse. Voltage drop was substantial enough to warrant an alternator and battery change to freshen everything up (can't remember the difference between battery and alternator at this point. It's been a while). After battery and alternator replacement, the neutral safety switch set a code that wouldn't clear. It was immediate. Wouldn't go back into park, hanging between gears was worse, downshifts seemed to last forever. Since the contacts on the NSS are serviceable, I cleaned them and haven't had an issue since and the code vanished. It's possible you have an underlying issue that won't pop up until proper voltage is applied.

After rereading your post, the shifting issue actually started one day when we got a heavy rain. The next clear day, no issue whatsoever. Then it rained for a week straight and the shifting issue was persistent after that. No codes though, which lead me to check voltage everywhere and warranted the battery and alternator change. It was cold out so keeping the defrost on (warm air but ac on) exacerbated the problem for the first little while.

I doubt this will help you but it was my experience with a very similar issue.

 

Echoing both answers above, as there seems to be a pretty common problem with some of these old Lexi ...

Soft alternator failures, probably caused initially by an oil or PSF leak, lead to insufficient system voltage, especially at low RPM. A "soft failure" means something has not failed completely (in this case, the charging system still keeps things mostly alive) but is not working properly (in this case, not maintaining system voltage as loads come and go).

Step one in addressing any problem that could possibly be related to this (which is almost everything that is electrically controlled) is to monitor your system voltage. A $2 eBay digital voltage gauge stuck in your 12V cigar lighter outlet will do that for you. You'll see any dips, etc.

And as WhiteUCF reported above, sometimes a weak system voltage may be hiding another problem that becomes more apparent once the voltage is solid.

When debugging, it is important to know (and let us know) what year your car is. I know the earliest cars, like my '91, do not have the ECU involved in alternator control. It is a relatively simple, standard circuit. But some time later in the 90's they got the ECU involved controlling things, and the debugging and potential for problems expands in complexity.

You will read on here how the early alternators suck and need to be upgraded. Well I can tell you that if they are in good shape, an old alternator will do just fine. As a habit, I just leave my voltage gauge in all the time, and always have a rock solid ~14V at idle when warm/cold, lights on/off, etc.

When my old alternator was weak due to worn brushes and slip rings (due to PSF leak), it was a different story, and the fan speed would change when I'd come to a stop sign, etc. Common knowledge on here was that this behavior was normal; truth is that it is very common, but indicates a problem with the charging system, and when fixed, it will no longer occur. Can you imagine if when Lexus designed and released this car back in MY1990 that they would make something that did that by design? Of course they did not.

Voltage seems relevant to this problem since when the AC turns on, it produces a big electrical load, as well as a load on the engine and tension on the belt, which may affect the alternator's ability to maintain voltage if it is marginal. But if you have a newer car, with the ECU involved in AC and Alternator control, it could be more complex than that.

 

The negarive battery cable terminus is AC compressor ...

Probably time to replace the negative battery cable.

https://www.clublexus.com/forums/ls-...-issues-2.html


Check all other ground points.