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Old 07-04-2022, 02:38 PM   #1
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Allison 2000 P0768

I didnít come across any other posts with these symptoms. Hereís what I have going on. I have a 2003 Thomas bus with the AD2000. Everything seems to be running fine until the transmission is pushed hard at highway speeds. I just got back from a 4 hour highway trip. I kept the speed around 60mph the whole drive until the last 30 miles or so. I decided Iíd see if I could induce the light because up till this point it was a newer problem I was still trying to figure out. Sure enough, after about 10 minutes of full speed (itís governed at 68), the light came on. When it comes on, the transmission makes a different sound. Sort of like itís running faster, but different from a regular downshift sound (which I realize is more engine sound than anything else). When the light is on, if I stop and try to start driving again the engine revs and the bus slowly accelerates and will make it to highway speed eventually so it doesnít seem to be in limp mode. It feels to me like excessive torque converter slippage. When I get the light, I can shut the bus off and turn it back on. The light is extinguished and the bus drives normally. The code that comes up is P0768. I just changed shift solenoid D and put in a new wire harness. I also have done 2 pan fluid swaps with Transynd and have changed the internal and external filters. Iím starting to suspect it could be a torque converter issue but internet research doesnít seem to tie P0768 to a TQ issue. I have also noticed a little drivetrain vibration when accelerating which also makes me wonder if thatís a clue pointing to the TQ.
Iíd really appreciate any insight any of you may have. Thanks!
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Old 07-05-2022, 10:42 AM   #2
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It sounds like your torque converter isn't locking up for some reason. Maybe the temp gets high, and the TCU does it??? Do you have a trans temp gauge?
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Old 07-06-2022, 10:33 AM   #3
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Thanks for the response unplanned tourist! That is exactly what it seems like. I don't have a temp gauge. It's not throwing a high temp code, which it should if there's an actual overheat. So you think maybe it's not reaching an actual overheat but maybe reaching some other temp that's causing the TCU to command a TQ unlock? That description completely fits the symptoms. Do you think that's a programmable parameter by a mechanic with the right equipment or is it maybe better fixed with a new TCU?
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Old 07-06-2022, 10:56 AM   #4
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Only code info I have for the 2000 series is for 2006 GEN IV controls and P0768 isn't listed .

I'll say if your running full throttle the trans controller could be unlocking the TC to allow the engine to develop more power by increasing RPM.
Without the lock up trans temps do increase , but as mentioned , why no trans temp light ?
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Old 07-06-2022, 11:08 PM   #5
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Thanks Skip! I'd be curious to know if the TCM would do that. It seems to me a downshift (to get the necessary RPM for more power) with the TQ still locked would be what it would do. But then again, I'm always learning!
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Old 07-06-2022, 11:10 PM   #6
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I'm thinking a TCM replacement may be my next move. Any recommendations based on experience? I'm going to start with Google right now.
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Old 07-06-2022, 11:29 PM   #7
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Quote:
Originally Posted by Tommywass View Post
Thanks Skip! I'd be curious to know if the TCM would do that. It seems to me a downshift (to get the necessary RPM for more power) with the TQ still locked would be what it would do. But then again, I'm always learning!
A downshift to a lower gear ( W TQ locked up ) would only occur, if the lower gear RPM was within engine operation limits .

I doubt that 68 MPH can be achieved in a lower gear .
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Old 07-06-2022, 11:41 PM   #8
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JMHO : I think a trans temp gauge, and a tach would be a better investments than a TCM.

Tach will tell you if TQ is unlocking at WOT .

Your description of ; starting from a stop with the light on , sounds exactly like trans fluid , foaming at the boiling point .

How heavy is the rig and how big is the aux trans cooler ( oil to air ) if the rig has only a cooler in the radiator ( oil to water ) then an aux cooler is in order too.
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