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TCC unlocking at cruising speeds, locking under load

D

dka99burb

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Location
Reidsville, NC
This is a continuation of a discussion we were having a while back in the 6.5 area about studdering at cruising speed. Turns out that I believe the TCC lockup was causing this issue as it confuses the cruise control.
Here is the data that I hope will help:
1. 1999 K2500 Suburban with 257k miles
2. Purchased this summer
3. No SES light.
4. Fluid level okay.
5. No Codes.
6. Fluid was brown and smelled terrible when I first started working on it. I have changed the fluid twice and the filter once.
7. Speedometer seems to be reading correctly. I have not verified with the GPS but it is close.
8. Transmission is shifting normal.
9. I have replaced the solonoids and wiring harness in the pan(kit from Advantage Electronics suggested in this forum) trying to fix this.
I don't think it really matters but I have replaced wastegate vac solonoid too.
10. Driving to work; 25 miles, mostly highway.
11. Typical weather. North Carolina: from 98 down to 70 recently.

My friend and I finally got together and connected his big scanner. I am not sure of the brand. We drove it and monitored. Shifting looked good, input and output sensors read as I would expect, trans temp stayed around 165, forcing the TCC with his scanner worked fine too. What is happening is the TCC locks whenever APP% is above 8% and unlocks as soon as it drops below 8%. I am only at 8% if I am climbing a hill or accelerating. Cruising speed is only around 4% APP. This was the only correlation that we were able to note. Does anyone have any experience with one doing this? Do you think it is the APP? The voltages look pretty close to what I have read they are supposed to be. Any suggestions will be appreciated.
Thanks.
 
Possible computer is not coding? Look at TCC slip via RPM's etc. as I am not sure how to read it on a 6.5. Suspect from bad fluid you have a slipping trans or TCC clutch. Possibly from a worn TCC valve. This needs to be sleeved/repaired to work properly again.

What is the trans temp running at? Going over 220 unloaded is a bad sign. A plugged from debris trans cooler is also possible. A trans shop can test pressures for you from the test port.

Again trans slips a component and the computer may drop TCC lockup. Misfires or brake switch closing also can drop TCC lockup. Monitor the brake on/off from the scanner.
 
I feel like something is up with the APP or the computer is not coding correctly. I watched the input and output speed sensors and they stayed the same while in 4th so I don't think there is slipping. The trans-temp stayed at about 165 the entire time (about 20min of testing after warm).
There are a couple reasons I suspect the APP. Please tell me if this sounds right.
1. You cannot ease into the accelerator. As soon as you touch it, it seems to jump from idle. Very difficult to hold low rpms.
2. Only about 50% of the throttle seems useful. Above that power stops increasing and smoke starts. I realize that it's a small turbo but ~50% usefulness seems low to me.
3. The TCC lockup seems to be soley based on APP%. While testing, anytime APP is above 8% TCC locks, drop below 8% and it unlocks.
4. Trying to stay at 60mph on fairly level highway requires about 3-4% APP. This seems very low to me.

I would like to hear other's thoughts before spending $450+ for a new pedal assembly.

Thanks in advance.
 
TCC slip will be engine RPM vs. input shaft speed. You can run the throttle range on the scanner to verify if it has issues. You get like 0-100%.

Smoke can be lack of air from the turbo boost falling off. Watch the boost on the scanner when the smoke starts. Is it falling off or stuck at 6PSI?
Also bad fuel or plugged fuel screens/filters. Tank sock, fuel pump screen on aftermarket units, fuel filter, last ditch screen, IP inlet screen. Bugs in the fuel caused me a good nightmare... Higher RPM with a restricted fuel system will drop boost, but no smoke. Bad fuel can cause smoke alone.

Also check that the brake switch is working properly.

I agree with PMD, but have you tried the 2000 RPM test?
 
If the APP was bad you would have a service throttle soon lamp illuminuated. It is a double fail safe sensor with 3 differrent sensors inside of it. It sends the 3 volatge readings to the PCm, and it determines everything from there. A bad PMD WILL cause the problems you're describing though, and would be the first thing I owuld check as well. There is also a chance you have a bad PCM. Did it ever go to 100% on the readout? If not your scanner may not be compatible with the 6.5 PCM as it is sort of a bastard child for data stream and many aftermarket scanners simply don't interpret the outputs from it correctly. Or if the PCM is bad it could be sending the wrong data through it and causing this as well. First thing you need to do is get a good scanner on it and see what it is actually doing and diagnose the problem as throwing parts at it will get expensive really quick.
 
I am starting to think that the PCM may be the issue from some observations I made this morning while trying a different APP, without success. I was thinking about it and the cruise control has the same issue with the senstive throttle that I have with the pedal (so the APP was a waste of time). There is no 'easing into throttle'. It is almost just a choice, accel or idle. Very difficult to hold steady. The CC even applys throttle to speed, then it goes to idle, accel, idle, accel, idle, etc. It can't hold steady without climbing a hill either. Almost like something is amplifying the input...
I can hear the turbo spooling as soon as I tough the throttle. It takes off quickly but this is not always good!

The PMD only has about 5-10k miles on it and is a Bill Heath kit so I don't think it is the problem. The previous owner did say that he had put a couple PMDs on it over the 100k miles he owned it to solve the stalling problems. He used Kennedy until he put this Heath unit on shortly before I bought it from him. He says he never had tranny issues.

I am not sure where this thread belongs now...

If anyone has suggestions on what to try please let me know. I can probably get Evan to help me with his scanner again on Monday.
Thanks!!
 
From what you are saying about the touchy throttle, I would say it iseither a PMD but most likely a failed fuel armeture inside the IP. A tell tale sign of a failed injection pump armeture screw is going from idle to WOT at a slight touch of the throttle. A PMD can do this as well, but not very likely.
 
I have consequently found a 99 C2500 suburban with a bad engine for crusher value here locally so I am thinking of grabbing it for parts. My question is: What will I not be able to use from the 2wd on my 4wd? Is the PCM the same, for example?
Thanks for all the advice!
 
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