Trans shifting and RPMs?

[14erclimber]

Hi all. Great to see .info going strong.

My VX now has 7200 miles on it. Here is what is going on. I tried to search for a previous post describing my problem to no avail.

Mountain pass road driving.

The scenario:

Cruising along.... pushing down the gas. VX shifts to second from third. The RPMs will jump up to whatever they need to... lets call this 3750.

It will hold the RPM for a half second, then lose about 250 RPM. It happens less often when going from D to third, but it still does it. It upshifts to whatever RPM it needs, then drops a consistent 250.

After two days this is making me numb considering I am @180 miles from a Isuzu dealer.

I have not checked the trans fluid, but I will be doing that this week.

Anyone have this strange RPM problem while shifting? Any ideas?

Thanks,

Hate to drive to Denver with this problem, and would hate to have to do it many times to get it fixed as well. :(

[andy454]

Are you sure it is not the torque converter clutch locking up. What happens if you tap the brake? Does it raise 250 rpm and then drop? if so this is the tcc locking up and this is normal. Just a thought.

[14erclimber]

It shifts, the RPMs stick, then immediately drop 250 before going up yet again as usual. This is not under a braking situation, this is climbing a mountain pass while under acceleration.... WOT and less. It is like a "ghost" gear 250 RPMs below where it should be. It is like a "mini" shift. It is particularly cruel going from third to second. I can't keep driving the vehicle like this.

It is also like this in the city now. I will check the fluid this week, AND change it. I fear this to be a horrible problem. I hope I am wrong. :confused:

[ScubaSteve]

dude don't sweat it... search for a post by me called "Isuzu Tranny?"... it should put your concerns to rest. is this your first 4WD vehicle?

[14erclimber]

Thanks for the reply Steve.

I had already read your thread. That really is not my problem at all. I have had many 4x4 vehicles of many types and ages. My last 2WD car was over 15 years ago.

I will be changing the fluid and then getting the vehicle towed to the dealer if it does not help.

Thanks again!

[Jolly Roger VX'er]

Originally posted by andy454
Are you sure it is not the torque converter clutch locking up. What happens if you tap the brake? Does it raise 250 rpm and then drop? if so this is the tcc locking up and this is normal. Just a thought.

He is absolutely right...I don't think you understand what he is saying here. He isn't talking about braking uphill...he means
that you should tap your brake pedal after you notice the drop in rpms... if your rpms raise about 250 and then drop when you release the brake pedal then that is normal....as he says above.

I used to have a Trans Am that did this but it was when they first came out with tcc lockup and it functioned quirky at best...so I disabled it. Vehicles today with it seem to function much better.

[14erclimber]

OK... so to test this I would need to tap the brake a bit with my left foot, while the gas is held?

I am just trying to make sure I am on the same page. Does the brake electronically control the tcc? So tapping it will allow me to sort of test it as I described?

Thanks and sorry for the lack of clarity on my part.

[Jolly Roger VX'er]

cars have 1 or 2 switches mounted on the brake pedal that activate when you depress the pedal to both light the brake lights and uncouple the torque converter from direct mechanical lockup (similar to clutch/pressure plate in manual transmission) via a solenoid controlled by ECM to a conventional "fluid driven" automatic transmission. This was done around early 1980's on GM cars so that the rpm's would drop around 300 rpm's so as to get slightly better economy and less emissions.

So your description of a half shift is dead-on with what is felt when your transmission goes from fluid driven to mechanically driven and your rpm's fall about 250.

To test this...you need only drive your VX until you feel the shifting settle to the lower rpm's..then lightly tap/ride the brake pedal and you should see/feel the rpm's raise 250 and as you let off the brake pedal they should drop 250. This is normal as your ECM is getting the signal from your brake pedal to alternate from mechanical driven to fluid driven...then when you let off brake the signal is no longer sent to the ECM so it re-engages the Mechanical Torque Converter Lockup.

On my '82 Pontiac Trans Am I simply unplugged the harness at the transmission and drove it strictly as a fluid driven transmission. I did it only because the TCL was very distracting during city driving and engaging and disengaging with the least little provocation...not smooth operation like today's trannies.

Hope this helps. Someone please chime in if they can explain it better.

[14erclimber]

Sounds like a pretty good explanation to me. Is there any reason that it just started doing this (to my knowledge)? I would know if it had been doing it (in my infinate wisdom..hehe). I have been driving over Independence Pass a lot lately for climbing. Yesterday it was doing it, but I did notice it the day before.

I do feel better, thanks. Being very far from a dealer is sort of rough. Having you all to mellow out my irrational fears is a comfort to say the least.

[Jolly Roger VX'er]

Wouldn't hurt to have somebody check the fluid level in transmission. As you probably know there is no dipstick. Suggest you take it to lube place or to someone knowledgeable (doesn't have to be the dealer).


* check the level by pulling the upper fill plug on transmission to verify fluid level.

[14erclimber]

OK. After a few tests.... I am convinced... that I was certainly ignorant to the process of the tcc. I did as suggested and applied the brake pedal under acceleration. Imagine this?! It did EXACTLY what you all said it would do. It was precisely what it was doing on it's own on the hill at speed. I could almost feel the change between fluid and mechanics today....hehe.

(for clarity sake. It was not exactly what it was doing. It was exactly opposite. It rose 250 RPMs with the brake depressed like suggested, and then dropped after brake release.)

Well... needless to say I am very happy and somewhat sheepish...heh

It is still something new. I wonder if at 7000 miles it made some kind of computer adjustment. It did start right at about 7000. With 7200+ on it now it seems to fit in to the last mountain pass drive. It was about 150 miles. Just a thought.

Thanks again everyone including Tone!:D

[AREA 51]

Try keeping it in 3rd for the mountain pass driving. this may prevent the trans from hunting for a gear so often.

[14erclimber]

Yea, I'm with ya there. I often keep the VX in third for that reason.

I also disable power mode on the highway if I use the cruise control. This stops the VX from shifting back and forth all the time from drive to third just to maintain the set speed more perfectly.

Automatic transmissions scare me. I just hate them. ALL of them.... hehe.

[AREA 51]

Don't hate all GM auto trans's::)

Powerglide- 2 speed goodness and practically indestructable , when built right.

TH-400- another tough one, just leave it in 1st and hold W.O.T. until it shifts itself into 2nd CHIRRRP!

4L60-E - electronic version of TH-400, programmable from in the car with hypertech programmer.