Auto transmission rpm's pulse in OD

My 4wd automatic transmission will jump about 150rpms (or less) in OD, usually around 2700 rpms. I took it to a specialist who's diagnosis was 'low power'. He explained that the 3 and OD gears both have a 'fuel saving gear', 1 to 1 to the engine. OD and 3rd really have 2 gear options. Because the Van is 4 cylinders, it doesn't have enough power for the transmission to function properly, but, it works and isn't being damaged (he said). He seemed to be familiar with the transmission and claimed it was used by multiple car companies including Volvo. He said it was a good transmission, functioning as well as possible under the circumstances.

When the rpms are pulsing, I can slow down or speed up and it stops. The rpms tend to drop first, not rise.

I'm posting this here because I believe the 2wd transmissions are similar?

If this is 'normal', there would be a thread about it. I couldn't find one. Has anyone experienced this?

Is my van a 6 speed? HUH?

Quote:

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Originally Posted by wavegobbler

Because the Van is 4 cylinders, it doesn't have enough power for the transmission to function properly, but, it works and isn't being damaged (he said). He seemed to be familiar with the transmission and claimed it was used by multiple car companies including Volvo. He said it was a good transmission, functioning as well as possible under the circumstances.

Is my van a 6 speed?

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Your van has a 4 speed transmission, the 4th gear being the overdrive gear.

I'm no expert, but the statement from your "specialist" about the transmission not having enough power to function properly sounds like he's making stuff up to sucker you in for a major repair bill, which you might not need. Did you check the tranny fluid level? And the statement that the same tranny being used by multiple car companies sounds totally bogus.

I would like TimsRV and other more qualified people to chime in here before you go spending $ Willy-Nilly.

That being said I don't want you to make the same mistake I did when I first got my van: Relying on the first Auto Shop/Mechanic that you encounter who say they "know" what they are doing or who claim they have experience working on these vans. Most mechanics HATE working on these vans and as a result, they will dive into your engine bay replacing parts on a trial and error basis while you start scrounging for a bank loan to pay for it all.

Be very careful going about this and who works on your van.
Ask around town and on this forum for info on an experienced Toyota mechanic who knows these vans.

Thanks for the response Trestlehed!
To clarify; By 'OD', I also mean 4th gear. The mechanic explained that within the gear, the transmission has two options, a fuel saving, 1 to 1 with the engine option, and the usual gear. He claimed that 3 and 4th(OD), both have it.

Thats news to me, don't see anything in the manual about that.

He also gave the transmission a clean bill of health, no repair necessary. He believed that with a more powerful engine, the transmission would not be hesitating between the two options within the gear.

It sounds like a flaky solenoid somewhere in the transmission. Maybe the one for the torque converter lock-up? That'd probably cause a change in RPMs while still in the same gear.

Thanks Moverbaugh,
'Flacky solenoid' sounds like something to pursue. The rpm change is subtle but can't be right. The torque converter lock up sounds similar to the '1 to 1' condition he was referring to. I'm a long way from really understanding how the auto transmission works.

Torque converters are basically a sort of fluidic clutch attaching the engine to the transmission. By their nature, they have a certain amount of 'slip' between the input and output. So to improve efficiency, modern transmissions have a friction-plate style clutch inside the torque converter that can be engaged by the transmission controller in higher gears to eliminate the slip.

Thats a good description of two different types of 'clutching' possible in one gear. Thanks! I imagine the solenoid is buried pretty deep? Maybe I should examine the exterior power source of the solenoid? I will do some digging in the manual.

Maybe I was reading what I wanted to hear in Moverbaugh post. I think he is describing the possibility of the transmission being able 'engage' or 'slip' in a gear. Am I reading that correctly? Could the transmission, bumping back and forth between these two things, describe my problem?

I can't find a solenoid for the torque converter. There are a bunch of valves, some pistons (and a million other sundry) buried in the transmission. The OD Solenoid seems to work fine. It's actually the only Solenoid I can find related to the transmission. Maybe I have an early symptom of this part failing? Of course, theres switches, sensors, relays and even a computer.

Sooo many parts!

If I'm not mistaken, the torque converter clutch is not operated by an electric solenoid. It is operated by either fluid pressure or centrifugal force. On my 1990 F350 the torque converter has a noticeable delay and will lock in 3rd and 4th gear giving the illusion of a six gears rather than four. The van locks much faster. I can also disengage the lockup converter at certain speeds in my truck by tapping the brake pedal because the electric clutch solenoid is signaled by the brake switch. Years ago I looked into doing something similar with my van by installing an electric switch to activate or deactivate the converter clutch but in researching discovered it's not an electric clutch. A simple test would be to drive at a lower speed in second gear and then give the van a little throttle while watching the tach but not enough to make it downshift. You'll notice the engine speed increase immediately and the van slowly follows. Do this same thing at freeway speed in fourth gear and the reaction will be the same as a manual transmission, no immediate change in engine speed until the van speeds up. This is because the torque converter clutch creates a mechanical link between the engine and transmission instead of the usual fluid coupling which allows slippage. This mechanical link may be what your mechanic was referring to as "1 to 1 to the engine" and yes, lock up converters do produce better fuel mileage. Here's a good explanation on their operation. http://m.youtube.com/watch?v=z5G2zQ_3xTc As far as the applications of these trannies? Indeed they or variations of them were used in many different vehicles including Volvos. https://en.m.wikipedia.org/wiki/Toyota_A_transmission

Thanks Originalwyjibo. Interesting to learn about the fluid coupling, cool youtube animation. I am curious to do your road testing, watch for the mechanical coupling verse fluid coupling when throttling up. I think I'll go back to my transmission guy, see if he can do anything to improve the transition. It appears to be a function which is buried in the transmission. I'll report back if there is any repair option short of just waiting for it to fail and rebuilding.

The auto trans used in the vans is manufactured by Aisin Warner, and similar units were used in Volvos, and other make automobiles. The lock up converter functions on the 4th [overdrive] gear.

Thanks for all the great info!
To rephrase what I think I've learned:

In 4th gear (overdrive), the Aisin Warner auto transmission has a 'lock up converter' function that effectively makes the connection to the engine mechanical. In 4th gear, the transmission will sometimes also use a 'fluid coupling'. I'm not clear on under what condition the two different couplings are used and which is dominate in 4th gear.

In 1st, 2nd and 3rd gear, the transmission uses only a 'fluid coupling'.

These functions are happening inside the 'torque converter' and use centrifugal force to function.

So, back to 4th gear; what would make the transmission pulse between the 'lock up converter' and a 'fluid coupling'? (if that's my symptom)

My only theory; centrifugal force is pulsing, causing the problem. A fluid blockage could do that. Additive to clean blockages?

Sorry if I'm butchering all the solid info people are providing!

Thanks again!