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4L80E TCC Unlocking defies logic

I tried adjusting the optic to correct but the IP must have been dying because I couldn't get it worked out. It is working good now with the new IP.:)
 
The TCC should be unlocking when you release your foot from the pedal. Are you having to actually use the pedal to keep speed going downhill? Seems like a steep grade should not need any pedal but the brake and the TCC should stay unlocked. The issue here may be, why do you have to use any pedal? Is there too much drag, from brakes or wind?

The PCM may be going back and forth from no throttle to throttle requested, which it would unlock with none to little throttle, then lock back up with some throttle requested.

It drops TCC lock when you get down below like 10% throttle, and will lock back up when you go back over it if you are going the necessary speed in the program. So its probably teedering the throttle too much. This could be from a bad IP, poor fuel pressure to the IP (clogged last ditch screen, IP inlet screen), or from your previous description of what happened at the dealership, they may have shocked the PMD into failure just as they burned up the glow plugs. Have you tried a different PMD?

It could be from an optic bump like stated before, because that mod confuses the PCM and it will teeder around 0 commanded fuel and then back up to keep the cylinders balanced, or simply require commanding fuel to get it up over the minimum coast fuel rates. So it would be a good idea to check what your warm idle fuel rate is on a scanner, should be about 7-9mm^3. If its lower then you might just need to unbump the OS.
 
dka99burb said:
I tried adjusting the optic to correct but the IP must have been dying because I couldn't get it worked out. It is working good now with the new IP.:)
Click to expand...

I've got one of them nascar pumps here at the house if you eevr want to go back to it. Mine was just starting to do this when I yanked the engine out for head gaskets and decided to do the engine swap.
 
buddy said:
The TCC should be unlocking when you release your foot from the pedal. Are you having to actually use the pedal to keep speed going downhill? Seems like a steep grade should not need any pedal but the brake and the TCC should stay unlocked. The issue here may be, why do you have to use any pedal? Is there too much drag, from brakes or wind?

The PCM may be going back and forth from no throttle to throttle requested, which it would unlock with none to little throttle, then lock back up with some throttle requested.

It drops TCC lock when you get down below like 10% throttle, and will lock back up when you go back over it if you are going the necessary speed in the program. So its probably teedering the throttle too much. This could be from a bad IP, poor fuel pressure to the IP (clogged last ditch screen, IP inlet screen), or from your previous description of what happened at the dealership, they may have shocked the PMD into failure just as they burned up the glow plugs. Have you tried a different PMD?

It could be from an optic bump like stated before, because that mod confuses the PCM and it will teeder around 0 commanded fuel and then back up to keep the cylinders balanced, or simply require commanding fuel to get it up over the minimum coast fuel rates. So it would be a good idea to check what your warm idle fuel rate is on a scanner, should be about 7-9mm^3. If its lower then you might just need to unbump the OS.
Click to expand...

We've got it figured out, Buddy. PCM was replaced over a year ago with a D-Tech on a remote sink, after the Stanadyne on the IP failed. The IP is original as far as can be ascertained, and now has 205K on it. The tranny was scanned while driving and checked out OK, the ECM was reflashed with the latest GM tune for the Burb. The IP is worn out, and that is causing the irratic fueling causing the TCC to unlock then relock repeatedly when coasting downhill under part throttle. My 94 has a new IP and rebuilt 4L80E in it and has none of these symptoms. Ergo, it's the IP, which still needs to be replaced just due to its age, that is causing this. dka99burb confirmed this as his was doing the same thing until he replaced his worn out IP.

Again, any recommendations on a good, reliable, low cost source to get a rebuilt or to rebuild an IP? Pensacola Diesel? Penensular?
 
Husker6.5 said:
We've got it figured out, Buddy. PCM was replaced over a year ago with a D-Tech on a remote sink, after the Stanadyne on the IP failed. The IP is original as far as can be ascertained, and now has 205K on it. The tranny was scanned while driving and checked out OK, the ECM was reflashed with the latest GM tune for the Burb. The IP is worn out, and that is causing the irratic fueling causing the TCC to unlock then relock repeatedly when coasting downhill under part throttle. My 94 has a new IP and rebuilt 4L80E in it and has none of these symptoms. Ergo, it's the IP, which still needs to be replaced just due to its age, that is causing this. dka99burb confirmed this as his was doing the same thing until he replaced his worn out IP.

Again, any recommendations on a good, reliable, low cost source to get a rebuilt or to rebuild an IP? Pensacola Diesel? Penensular?
Click to expand...

I would stay as far away from pensacola as possible if it was me. WAY to many problems from there products over the last 4 years to trust something from them. I know we have a site vendor who sells new ones, but they are expensive. For remans, make sure it is a stanadyne shop or a shop with access to stanadyne parts that you buy it from. To many chinese knock off parts here lately failing prematurely to chance by buying a cheap pump.
 
If you get the chance, I would check what the warm idle fuel rate is, as it may not be a totally worn IP, but a miscalibrated one. If the idle fuel rate is below 5mm^3, or cycling to 0 at all then you may just need to bump the OS towards the drivers side a tad. I have seen a few fresh IP rebuilds even have this issue, the fueling thing. Maybe also see what the idle timing is reporting as.
 
Husker6.5 said:
Any other brains out there wanting to be picked on this subject?:mad2:
Click to expand...

i have had theese symtoms for some monts now, it was cycling in and out of TCC lock under light load in OD only.
Talked to a a couple of tranny rebuilders, one suggested it could be a bad speed sensor or misfire, the other said bad speed sensor\sensors.
i ordered the front speed sensor and replaced it before my 2nd testrun with the tune installed last weekend.
The problem was gone, its a low cost fix to.
My front speed sensor had internal leakage, when i removed the connector it was dripping AT fluid...
 
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