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4L80E Trans Problem


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My 4L80E tranny started acting up in overdrive a month ago. I had it rebuilt and it did not fix the problem. It's a 1992 GMC 3500HD with the 7.4 ltr. After a test drive with a computer hooked up to it, the transmission tech said something was telling the computer to dis-engage the torque converter clutch. The trans torque converter clutch keeps shifting in and out when runnung between 60 to 80 mph. It only does it when running level and on slight uphill situations. Hit a big hill and it acts fine. It has a new torque converter as well as the updated electronics. All new sensors and selenoids. Replaced the brake switch since GM said that was your most likely problem. Well it wasn't. The local GM dealer is puzzled as am I. Any ideas as to what might be causing this problem?

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i have a similar problem with my rebuilt 93 4l80e with all new sensors ,i heard maybe a worn distributor shaft or ignition switch, but by the time im done with this so called heavy duty transmission i could have bought a new truck. you pretty screwed because i posted this on a lot of forums and i have only found more confused people with this type of problem but no fixes.

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I assume that the tech looking at the scanner was able to see the TCC being commanded off and on by the PCM. This would mean to me that the problem was with inputs to the PCM. If you can see it is commanded on and you are loosing lock-up, then it would be a lock-up solenoid wiring, solenoid, hydraulic or mechanical failure.

 

Assuming it is an input, I would look to the speed sensors first. In a 4L80E, the output speed sensor and the input speed sensor regulate toque converter clutch operation. These have to be producing a good clean signal to the PCM. Also the engine temp sensor affects TCC operation so you want be sure that the PCM is getting good information from it.

 

Good Luck,

Doug

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i have a similar problem with my rebuilt 93 4l80e with all new sensors ,i heard maybe a worn distributor shaft or ignition switch, but by the time im done with this so called heavy duty transmission i could have bought a new truck. you pretty screwed because i posted this on a lot of forums and i have only found more confused people with this type of problem but no fixes.

 

 

 

 

I saw one of your earlier posts concerning this problem. I think it may come down to just replacing parts until I hit the right one that's messing up. I did tune it up last week but it did no good. Today however I put 100 miles on it and it didn't act up once. I shouldn't have said that cause now I just put a curse on myself.

Thanks, campspringjohn.

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thanks dougmac, the transmission will not act up when the tech has the computer on it and he cant waste that much time driving for hours untill it does it ,is there a way i can tap into the command wire with a test light or volt meter to see the command? and do i understand this correctly, the pcm send the command to the tcc to lock and if it does not lock it will drop to third gear,and when it does this how will i tell if the pcm is telling the tcc to unlock becuase of the sensors telling it to or becuase the the clutch pack is slipping and pcm is commanding third. dam i think im confused

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This may not be it but I had a 94 chevy 1/2 ton with the 4l60e and it shifted awkwardly. I don't remember the exact problems but a chevy service tech did a tear down of the tranny. replaced some bands and something. That did not fix it. After another tear down and no idea they finally called tech support and found out it was an issue with the pcm. It required a wire to be soldered in because the trace on the board could not handle the amperage on the circuit. It required a heavier trace. That fixed it and ran fine for 3 more years before trading.

 

Brian1

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thanks dougmac, the transmission will not act up when the tech has the computer on it and he cant waste that much time driving for hours untill it does it ,is there a way i can tap into the command wire with a test light or volt meter to see the command?  and do i understand this correctly, the pcm send the command to the tcc to lock and if it does not lock it will drop to third gear,and when it does this how will i tell if the pcm is telling the tcc to unlock becuase of the sensors telling it to or becuase the the clutch pack is slipping and pcm is commanding third.  dam i think im confused

 

 

 

Hi todster, dougmac and Brian,

Thanks for your input but I don't think any suggestions here will help so far. When taken for a test drive with a GM Tech in the truck and his computer hooked up, when it did act up, the truck computer was telling the torque converter clutch to disengage for a brief second. The problem is, there are no codes coming up indicating a problem. It's not right. The tech said it's not right. But no one has an answer. I said yesterday it didn't act up, well, today it did. I'm leaving in about 7 hrs for a job in Virginia. We'll see what happens. Perhaps I'll be coming home in 2nd gear! Drive her till she blows!

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When taken for a test drive with a GM Tech in the truck and his computer hooked up, when it did act up, the truck computer was telling the torque converter clutch to disengage for a brief second. The problem is, there are no codes coming up indicating a problem.

 

It sounds like the PCM is commanding the torque converter clutch off. There are a number of coded faults that will cause this. In your case you have no trouble codes so I would conclude that the PCM is receiving faulty information from a sensor.

 

The information is probably still in range so it is not flagging it as a fault. For example, the TCC is disabled below 130 degrees of coolant temperature. If coolant temperature sensor had a corroded connection and was intermittently reading 120 degrees the PCM would shut off the TCC and not set a code. The same would happen if it was getting erratic or false information from the speed sensors.

 

The best approach to this problem is have a scan tool hooked up to the truck and do a snapshot of the incoming data at the time that the disengagement occurs to see what is causing the problem. Unfortunately you are going to have to find a fairly sharp technician to pull this off. A lot of technicians run out of “talent” when they see the problem and no codes.

 

Good luck, this is going to be a tough one. The problem may progress from an intermittent to hard failure. They are much easier to diagnose at that point.

 

Doug

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actually there is a code 68 i think but its just a general code that the pcm saw a slip or somthing like that,its at a new transmission place now and there saying its the input output sensors ,which are new ,so they replaced them anyway but now it may be something else and there looking into it deeper, sounds like some more bullsh!@ coming my way.

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When taken for a test drive with a GM Tech in the truck and his computer hooked up, when it did act up, the truck computer was telling the torque converter clutch to disengage for a brief second. The problem is, there are no codes coming up indicating a problem.

 

It sounds like the PCM is commanding the torque converter clutch off. There are a number of coded faults that will cause this. In your case you have no trouble codes so I would conclude that the PCM is receiving faulty information from a sensor.

 

The information is probably still in range so it is not flagging it as a fault. For example, the TCC is disabled below 130 degrees of coolant temperature. If coolant temperature sensor had a corroded connection and was intermittently reading 120 degrees the PCM would shut off the TCC and not set a code. The same would happen if it was getting erratic or false information from the speed sensors.

 

The best approach to this problem is have a scan tool hooked up to the truck and do a snapshot of the incoming data at the time that the disengagement occurs to see what is causing the problem. Unfortunately you are going to have to find a fairly sharp technician to pull this off. A lot of technicians run out of “talent” when they see the problem and no codes.

 

Good luck, this is going to be a tough one. The problem may progress from an intermittent to hard failure. They are much easier to diagnose at that point.

 

Doug

 

 

 

 

Monday while on a service call, I was in a hurry to get there and get back home. I pushed the truck harder than I normally do, running 80 to 85 mph. At that point, it was acting up a lot more than it does at 65 or so. It rarely acts up below 65mph. Given this info, I'm lead to think it's in the throttle sensor, if it has one. It does act up at 55 to 60 mph if I have the transmission in drive.

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Monday while on a service call, I was in a hurry to get there and get back home. I pushed the truck harder than I normally do, running 80 to 85 mph. At that point, it was acting up a lot more than it does at 65 or so. It rarely acts up below 65mph. Given this info, I'm lead to think it's in the throttle sensor, if it has one. It does act up at 55 to 60 mph if I have the transmission in drive.

 

 

 

The TPS could cause this. Perhaps a faulty input or output speed sensor might be the problem. These are just guesses. I would be inclined to think if it was speed related one of the speed sensor could be suspect.

 

The only real way to know is to have a scan took hooked up when it happens. Then examine the parameters that could be out of whack. I wish I could be of more help.

 

Doug

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I have been having problems with my tranny on my 1998 GMC 3500 4x4 6.5L Dually.

 

After searching through this site, I believe I have a bad vehicle speed sensor.

 

My problem:

 

Speedometer basically stops at around 60mph. The truck is going faster but, speedo stops around 60. It will occasionally go higher when I begin to slow from a higher speed.

 

The transmission unlocks if heavy throttle is applied for a hill or passing. This always happens if the speedo is stopped at the magical 60mph mark. The transmission shifts fine at any speed below 60 even towing heavy load (35ft 5th wheel).

 

The ABS also acts up at slow speeds (below 5mph) resulting in zero stopping power and a few close calls. I unplugged the dc power to the ABS to avoid an accident.

 

My question:

 

Is the speed sensor located on the tranny or the transfer case?

 

While searching under the truck yesterday, I noticed something on each that could be the VSS. I am just a backyard mechanic and don't know what they look like.

The one on the transfer case is easier to replace. I removed it for inspection. It looked clean - looked like a magnetic sensor.

 

I called the parts store and was told that the were several listings that make up the VSS. He asked for "internal, input, or output" - I could not answer.

 

Being on a tight budget, I would rather not replace anything that is not needed.

 

Any help is appreciated.

 

Thanks,

Mark

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Thanks!

 

Replaced it today.

 

Speedo working above 60 and tranny shifting normal again.

 

The ABS may still be an issue not related to this sensor. I have read that the wheel sensors probably need cleaning. I will put that off until warmer weather and just leave the power disconnected to the ABS (I don't care for abs anyhow).

 

Mark

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