Help diagnose my trans/converter problem *vids*

[phoonTy]

I thought about posting this in George's forum, but figured the more input the better. Here's what's up.

When driving my truck, under acceleration, the transmission shifts fine through all gears. I've been taking it easy since putting it back together and haven't driven very far or fast, so I can't tell you how firm the 1-2 shift still is, but for all intents and purposes, all the gears are there and it goes into reverse just fine. (I'm getting it registered and I'll get some DM runs next week some time.) There doesn't seem to be any issue accelerating... Like I said, all gears seem to work fine.

It's decelerating where I have the problem. Say you give it some throttle to 3k rpm and hold it for a bit to get up to speed, as soon as you let off the throttle, the rpms will drop down to 1k, bounce (I say bounce cause that's what the tach looks like) back up to about 1500 rpm and then steady out. For lack of a better description, it feels like something is "free wheeling" and then "catches" again. I really don't know how better to explain it, so I have video. (my apologies for the shakiness, my wife did as best she could holding the digital camera... thanks babe! )

Please, Right Click Save As. I don't have the bandwidth for streaming. :tup:

Video 1

Video 2

Note: Watch the tach increase rpm, suddenly drop, then seemingly bounce back. I don't touch the gas at all after the initial let off... the tach is on it's own, and it feels transmission/coverter related.

So... what's the damage?

 

[Tooky]

Re: Help diagnose my trans/converter problem *vids*

Good call on identifying it as a trans/converter problem. Do you have a non-stock, high stall converter in there? If so, I'm 99% sure your problem is a result of the little-known fact that one of the many compromises of high stall converters is the reduced engine braking capability. If your chip is still programmed for a stock converter, it won't be a correct tune for a high stall due to this (and several other issues.)

My truck and another local Syclone both exhibited this behavior after switching to high stalls (2600, 2700, and 3000 converters all did it).

I did a quick google search and found this excellent explanation:

Fuel cut-off. The torque converter of the automatic transmission is designed for transmitting power from the engine to the wheels. Its ability to transmit power in the reverse direction is limited. During deceleration, if the torque converter's rotation drops beneath its stall speed, the momentum of the car can no longer turn the engine, requiring the engine to be idled.

The solution is changes to your chip's DFCO (Decel Fuel Cut Off) and other areas. In the meantime you can tap the gas pedal slightly, just enough to prevent it from occuring. Annoying, I know.

The other possibility (if your converter is stock), and this happened to me when I had the stock converter, is that your overrun clutches wore out or your overrun blew, both of which will result in zero engine braking.

 

[phoonTy]

Re: Help diagnose my trans/converter problem *vids*

Good call on identifying it as a trans/converter problem. Do you have a non-stock, high stall converter in there? If so, I'm 99% sure your problem is a result of the little-known fact that one of the many compromises of high stall converters is the reduced engine braking capability. If your chip is still programmed for a stock converter, it won't be a correct tune for a high stall due to this (and several other issues.)

My truck and another local Syclone both exhibited this behavior after switching to high stalls (2600, 2700, and 3000 converters all did it).

I did a quick google search and found this excellent explanation:



The solution is changes to your chip's DFCO (Decel Fuel Cut Off) and other areas. In the meantime you can tap the gas pedal slightly, just enough to prevent it from occuring. Annoying, I know.

The other possibility (if your converter is stock), and this happened to me when I had the stock converter, is that your overrun clutches wore out or your overrun blew, both of which will result in zero engine braking.

The story on my truck is that it's almost as stock as you can get. Only aftermarket is 3" dp & exhaust w/high flow cat and boost controller. Stock 2bar, stock ecm, stock stock stock! :lol:

I bought the truck with 102,000 on the odo, and was told it had a transmission with only 20K on it... which makes it about 50k now. So as far as I know it's stock rebuilt trans with converter... so who know's what I have. All I know is that something isn't right. :lol:

 

[Tooky]

Re: Help diagnose my trans/converter problem *vids*

Do you get engine braking? When travelling 35 MPH, if you grab the shifter down into 2nd, does the truck do engine braking and shoot the RPMs up? If it just free wheels then your overrun in the tranny is toast.

 

[turbodig]

Re: Help diagnose my trans/converter problem *vids*

So... what's the damage?

You assume there is some...

Converter clutch lock/unlock goofyness?

Get some data of it. Also look at IAC counts when it happens.

On a firm trans, you will feel downshifts. I don't consider that abnormal.

 

[phoonTy]

Re: Help diagnose my trans/converter problem *vids*

Do you get engine braking? When travelling 35 MPH, if you grab the shifter down into 2nd, does the truck do engine braking and shoot the RPMs up? If it just free wheels then your overrun in the tranny is toast.

No, I don't think it engine brakes anymore.... But like I said, it's been a year since I've "Driven" it... I'm getting it registered again on Monday, so after I do, I'll go datalog it and post the files for you guys.

Dig, I'm really hoping something is goofy and there really isn't any damage... but it is a Typhoon :lol:

 

[George Blake]

Re: Help diagnose my trans/converter problem *vids*

Don't do a 3-2 on a trans with an unknown problem.

Go into 4th. Steady speed of 50 MPH. Tap break pedal with left foot while not changing the throttle. Watch RPM. It should bump up a couple hundred RPM. If it doesn't, no lockup.
Going along in 4th. 50 MPH steady. Release throttle completely. See how it feels?
Now.
Going along in 4th. 50 mph steady. Manually downshift from 4th to 3rd. Stay at steady throttle. RPMs will go up. Normal. Now, release throttle completely. Don't worry about the tach. FEEL to see if you have any kind of engine breaking. If not, overruns are done. Busted seal or burnt clutches.

 

[phoonTy]

Re: Help diagnose my trans/converter problem *vids*

Don't do a 3-2 on a trans with an unknown problem.

Go into 4th. Steady speed of 50 MPH. Tap break pedal with left foot while not changing the throttle. Watch RPM. It should bump up a couple hundred RPM. If it doesn't, no lockup.
Going along in 4th. 50 MPH steady. Release throttle completely. See how it feels?
Now.
Going along in 4th. 50 mph steady. Manually downshift from 4th to 3rd. Stay at steady throttle. RPMs will go up. Normal. Now, release throttle completely. Don't worry about the tach. FEEL to see if you have any kind of engine breaking. If not, overruns are done. Busted seal or burnt clutches.

Thanks a ton George. I'll try this out on Monday just as soon as I get it registered. :tup: You guys are awesome!