Strange RPM issue. Tranny or engine?

I just had this start happening:

I'll be on the freeway going over 60. I am at approximately 2000-2500RPM. When I give it some more gas to pass someone or go up a hill while maintaining speed, the tranny will downshift. The RPMs jump to around 3000, which is normal. A second later, the RPMs quickly slide up another 500RPM, then slide back down to around 3000. It is not another downshift; I cannot feel the tranny shift when this happens. Only the engine RPMs appear to be affected.

What the heck could this be? I changed out the tranny fluid when this happened the first time. It was brown, but not burnt with no metal particles. Even with the new fluid, this still happens.

 

Tranny could he slipping. Is the fluid definitely at the correct level? Letting it warm up before checking the level?

 

Converter lockup.

 

Checked while running and warm.

Manual states to add 3 quarts to start with. It seemed a little high to me after that.

I had the damn tranny rebuilt about 4-5 years ago, too.

No other issues shifting on the road except for this.

 

I don't know but I'd install new spark plugs and see what happens....and check the driveshaft u-joints for play. Just guesses, here.

 

Could you elaborate on why you think that could make a difference?

 

Sounds like classic torque converter lockup for cruising, it may not be staying in lockup for some other reason though.

 

I've heard that some "tranny" problems can maybe be related to ignition and or dying batteries but ya, it's probably not your problem. Carry on.

 

When you let your foot off the gas the converter unlocks.

 

So if the converter is locking up, what is the solution?

Honestly, I'm so sick of putting money into this truck (it's totaled out, too BTW), unless this will kill it in less than a year or two, I'm happy to ignore it. It's purely for commuting at this point.

 

The Torque converter clutch comes on to save you fuel by locking the engine directly to the transmission .
When you are at highway speeds and plan to pass someone , two things have to happen inside the transmission in order for you to get past the vehicle and this all depends on how hard you are on the gas pedal as you do it .
First , the torque converter should unlock
Second , the transmission will downshift from 4th to 3rd .
As you pass the vehicle ,the transmission then upshifts back into 4th gear .Once the computer in your truck sees 4th gear , it will then tell the torque converter clutch to lock the engine directly to the transmission .
Judging by your explanation ,it sounds like there is a timing issue . Its taking too long for the torque converter to unlock .
Since we are working with a 5 year old computer controlled transmission ,the first thing you need to do is scan for codes since you already checked the fluid.
There is some info here that may help .
http://www.tacomaworld.com/forum/te...warner-340-series-transmissions-write-up.html

 

Are these not the type of codes that show up through the OBDII port? (I have a ScanGauge).

Lots of info on that link. Thanks!

 

Yes ,a scan guage should give you the codes if there is any.
You are following the proper diagnostic breakdown by scanning for codes next .
There are many solenoids and sensors that come into play to acheive converter clutch lock up and unlock . By scanning for codes ,you are eliminating a lot of guess work .

 

The only code I have is 0420, which is a cat efficiency code. It has been there for a couple of years and never goes away.

No tranny or other codes.

 

OK great , well not great but you eliminated a bunch of additional diagnostics by scanning the codes.With this said ,we can rule out anything computer controlled such as the many solenoids and sensors that control your torque converter.
I am fairly certain your valve body has a sticky or worn out bore at the lock up relay valve and its quite common on high mileage 340 series transmissions.

The computer commands the lock up solenoid in the valve body to shut the oil pressure off to the torque converter clutch so you can pass the car in front of you . The solenoid functions correctly and shuts oil off to the relay valve but the relay valve is worn or sticky . Let me have a look ...be right back .

 

Here is an example of what I beleive is your problem .
http://www.sonnax.com/parts/2747

 

Interesting.

The instructions on the link don't really clear up where this item resides. Is this inside the transmission, or is this something I could do myself? I have no problems following a Haynes manual.

Where could this kit be purchased? It doesn't appear to be sold on that website, but I'm on a phone now, and many not be able to see all of the options on the site.

Thanks for all the help, BTW!

 

Any transmission shop could get you those parts or you could purchase on line http://www.transmissionpartsusa.com/A340_Converter_Lock_Up_Relay_Valve_Sleeve_p/400-09785527k.htm.
That spool valve is inside the valve body but I highly doubt the Haynes manual will have any information about a valve body overhaul .
Here is the overhaul manual for the transmission http://kai.supramania.com/Supra/A340 Series Repair Manual (RM216).pdf
Whats the mileage on your truck since you had it rebuilt . There is a high possibility other areas of the valve body need to be looked at as well . If it were me , I would also order up a reprogram kit .