Allison code 6600

[brainsboy]

I have been through all the connections on transmission , inside kick panel disconnected reconnected ecu, looked inside ecu no corrosion, and front of truck connections, Keep getting code D1-6600 . Down shifts are pretty hard when I let off the gas all the way. 6600 is coming up as a serial communication issue. Any other places I should check? Found a schematic but not sure which wire is for communication.

2001 WTEC3 tcu-29541151

 

[GeneralDisorder]

That means the Allison computer has problems with it's J1939 bus communication. The FIRST thing to check is the resistance between CAN Hi and CAN Lo pins on the harness. If the coms bus termination resistors are blown, damaged, or missing that will take down the bus - probably the most common thing we see with A1/A1R/A1P2 trucks. The resistance between Hi and Lo should be 60 Ohms plus or minus 1-2 Ohms. If it's 120 Ohms you have one bad resistor, and if it's got really high resistance you likely have two bad resistors. One of the resistors lives under the PDP panel, and the other lives next to the engine ECM under the cab. They are 3-pin Deutsch connectors that appear to be a wiring harness "dust cap"..... like this:

DT06-3S-P006 | Deutsch DT 3 Way J1939 120 Ohm Plug

DT06-3S-P006 Deutsch DT J1939 plug connectors in stock. Orders placed before 2pm PT ship same day. Free U.S. shipping on orders $25 or more.

www.customconnectorkits.com

Though the communication bus should have no effect on transmission operation. It doesn't get it's throttle position for shifting from the bus. That is hard wired from a dedicated ECM output.

Shift harshness is a result of the older transmission to an extent as well as the selection of motor oil for operation - but mostly it's due to the gigantic mismatch in turbine speed to engine RPM from the torque converter not being fully loaded with the gear reduction hubs. If you want smooth shifting you must get ECO hubs. Changing over to ATF can help some but ECO hubs will completely eliminate that "feature"...... which is normal operation due to the rpm mismatch when locking and unlocking the TC. You can't solve that without "fixing" the mismatch and the only way I know to do that is with ECO hubs.

 

[brainsboy]

Which wires do I test it from?

 

[GeneralDisorder]

Any J1939 bus wires going to any termination resistor or module. Easiest is probably the bus termination resistor next to the engine ECM - with that one unplugged you should see 120 ohms and the resistor itself should be 120 ohms.

Or you can front probe the databus connector on the dash. Just be careful not to spread the pins. Pins C and D.



Mis-spoke above in my first post...... The bus should be 60 Ohms with both resistors in place. 120 Ohms if one is bad, and infinity with both of them bad.

 

[GeneralDisorder]

F

 

[Ronmar]

what General said. Probably a bad or missing buss term resistor.

hard shifts are not uncommon unloaded due to the final axle gearing. Basically the mechanical advantage puts the trans clutch roll-on/roll -off PWM signaling at the end of its range. Put a ton or so in the back and it should smooth
out. Or get rid of the final 2:1 at the hubs…

The TCU bases a lot of its behavior from what it sees from your foot(uses throttle position info from the ECU ), so data buss issue could be messing with its behavior…

 

[GeneralDisorder]

what General said. Probably a bad or missing buss term resistor.

hard shifts are not uncommon unloaded due to the final axle gearing. Basically the mechanical advantage puts the trans clutch roll-on/roll -off PWM signaling at the end of its range. Put a ton or so in the back and it should smooth
out. Or get rid of the final 2:1 at the hubs…

Probably needs to be a lot more than a ton. Mine was still noticeable on the 2-3 shift even with the WTEC IV, ATF, and the 3,400 lb 1079 box on the back. Maybe 2-3 ton? IDK.

The TCU bases a lot of its behavior from what it sees from your foot(uses throttle position info from the ECU ), so data buss issue could be messing with its behavior…

The TCU gets it's TPS from a dedicated ECM output. It does not use the bus for anything other than diagnostics that I'm aware of. Same goes for all the rest of the modules on these trucks. One of the reasons I like the way they are laid out. Discrete modules that don't perform coms with any other modules. So if you lose one such as the CTIS or the ABS or the bus goes down it doesn't render the truck imobile. So if a module brings down the bus by grounding it or something everything else will keep on trucking.

 

[brainsboy]

C and D show 60.25 OHMs.. Maybe a bad TCM?? , anyone have a tcm I can swap out and return for testing? They are on ebay for 800 or so. Dont mind spending it if it fixs it, but will loose sleep over it if it doesnt fix it.

By the way the trans originally was shifting so hard it would cherp the tires on downshifting. Cant be good for the universal joints. I had to pull the trans for a torque converter leak and rebuilt the valve body during the process. After installation the trans was 100% perfect, silky smooth downshifts and great up shift timing. After a 100 miles of driving the downshifts have been harder, not as bad as originally, but I found by not letting the gas pedal return fully it will downshift nice. Thats when I pulled the codes for the transmission and found D1-66 00.

 

[GeneralDisorder]

Well the next step now that you know the bus is intact is to test for that same resistance at the TCM truck side connector CAN Hi and Lo pins. That will be at the WTEC III controller itself under the passenger side of the dash.

 

[brainsboy]

Well the next step now that you know the bus is intact is to test for that same resistance at the TCM truck side connector CAN Hi and Lo pins. That will be at the WTEC III controller itself under the passenger side of the dash.

Ok , it has 3 plugs on it blue white and black not seeing a hi lo on the schematic

 

[GeneralDisorder]

Sorry - Colloquial terminology for the CANBUS architecture - they are labeled J1939 Data Link (+) and J1939 Data Link (-)

And yeah - the trans computer is learning I'm assuming. Unfortunately these do shift/TC lock/unlock hard and the CAN communication error is highly unlikely to be related. Till you load the truck with a LOT of weight or put ECO hubs on it I'm afraid that's just how she be.

The reason it's not as hard when you keep the RPM's elevated is that the transmission isn't having to accelerate the engine to it's absurd input shaft speed because you are holding the RPM higher with your foot.

 

[Ronmar]

Probably needs to be a lot more than a ton. Mine was still noticeable on the 2-3 shift even with the WTEC IV, ATF, and the 3,400 lb 1079 box on the back. Maybe 2-3 ton? IDK.



The TCU gets it's TPS from a dedicated ECM output. It does not use the bus for anything other than diagnostics that I'm aware of. Same goes for all the rest of the modules on these trucks. One of the reasons I like the way they are laid out. Discrete modules that don't perform coms with any other modules. So if you lose one such as the CTIS or the ABS or the bus goes down it doesn't render the truck imobile. So if a module brings down the bus by grounding it or something everything else will keep on trucking.

I don’t see any way to get ECU TPS info from ECU to TCU except via the J1939 buss…

 

[GeneralDisorder]

I don’t see any way to get ECU TPS info from ECU to TCU except via the J1939 buss…

Interesting. You are right there is not. I was thinking of vehicle speed. Vehicle speed is exchanged outside the bus.....

I have witnessed more than one truck driving without the J1939 bus operational so the TCM must be switching over to the J1708/J1587 bus for that information or it has a fallback strategy using engine RPM. I know the Allison can see TPS when you monitor it with Allison DOC so it either uses the other bus or doesn't really care if it has that data or not. The Allison computer does have it's own engine speed sensor and gets it's exhaust brake control commands from outside the bus.

I'm still skeptical that having the J1939 bus not operational would cause significant drivability issues. It doesn't seem so that I have witnessed.

It would be interesting to disable both data busses and see what effect it has.

 

[brainsboy]

TCM tested on black plug #13 and #29 shows 60.32 OHMS .. going to take a guess and say check engine ECU now?

 

[Ronmar]

The Allison will operate without TPS input, it falls back to its shift-point RPM table, but that can make for harsher shifting, and it looses the ability to call for a kick-down when calling for acceleration at speed… Not sure if there is a different/separate code for TPS data issues when using bussed communications, beyond the 6600 code for general serial comms…

 

[GeneralDisorder]

TCM tested on black plug #13 and #29 shows 60.32 OHMS .. going to take a guess and say check engine ECU now?

Well - yes you could do that and probably should do that given it sounds like you don't have a diagnostic solution - the real first step and where I would go now is to hookup a laptop (or something like the Bluefire adapter and a phone/tablet) and see if the data it wants is actually on the bus. Throttle position should be displayed by any diagnostic reader in the live data. If throttle position is present then then next step would be to ensure the TCM has solid power and ground on all the appropriate power/ground pins from the harness..... assuming that checks out then you make the unenviable call that the TCM is likely damaged in terms of it's bus circuitry and will have to be replaced.

That is never a comfortable call to make. I do this for a living and making the call that the computer has failed is very uncomfortable. Triple check EVERYTHING before making that determination. If possible - try your existing TCM in another working truck first. If not...... have to bite the bullet and worst case you end up with a spare and can rule it out. They can and do fail though so having a spare wouldn't be the worst outcome possible.

 

[GeneralDisorder]

Also - just for grins - you should check the connector pin "drag" on the male/female pins for the bus on the truck side connectors - sometimes people overlook pin-fitment issues with connectors and they may have good connection to the wiring but poor grip on the TCM pins inside the connector. That can cause intermittent connection. Worth a visual inspection and maybe tightening them up. That, testing power/ground, and checking the live data with a scan tool are about all you can do without an oscilloscope (which are cheap now and you should have one of those also).

 

[hike]

I have witnessed more than one truck driving without the J1939 bus operational so the TCM must be switching over to the J1708/J1587 bus for that information or it has a fallback strategy using engine RPM. I know the Allison can see TPS when you monitor it with Allison DOC so it either uses the other bus or doesn't really care if it has that data or not. The Allison computer does have it's own engine speed sensor and gets it's exhaust brake control commands from outside the bus.

I'm still skeptical that having the J1939 bus not operational would cause significant drivability issues. It doesn't seem so that I have witnessed.

It would be interesting to disable both data busses and see what effect it has.

We confirm that with both R12 and R13 blown (J1939 down), an M1078A1 with WTECIII the Allison operates normally. We haven't seen the J1708/J1587 side down.

 

[brainsboy]

We confirm that with both R12 and R13 blown (J1939 down), an M1078A1 with WTECIII the Allison operates normally. We haven't seen the J1708/J1587 side down.

If this is the case then can I program the TCM for softer shifts or different RPMS ?

 

[GeneralDisorder]

If this is the case then can I program the TCM for softer shifts or different RPMS ?

No. The problem isn't a "shift" harshness. It is a torque converter lockup clutch engagement that you are feeling and the problem is the difference in input speed to output speed at the point of engagement - this is due to the converter slipping because it is not fully loaded, and the clutch engagement forcing the input and output speeds to match when it locks. This abruptly changes the engine RPM and is the "shock" you are feeling. Making this less harsh would only result in rapid failure of the lockup clutch.

And in any case you can't adjust any of that from within Allison DOC. Allison themselves can of course change all of that stuff but it's done by their engineering department and I'm sure they would agree that trying to soften this engagement from the software side is the wrong answer and would only result in rapid wear and failure.

The "fix" for this is to fully load the converter. Either by adding about 10,000 lbs or more of weight to the vehicle so it must work harder, or by changing the gear ratio of the final drive to again make the engine work harder and thus load the torque converter closer to it's 1.98:1 reduction. This will smooth the shifts to where you expect them to be for a modern automatic transmission experience. My truck at 370 HP / 932 Ft/lbs shifts as smooth as any passenger car - because ECO hubs load the converter properly the way Allison meant for it to be used. Our trucks are operating outside the envelope that Allison engineered the transmission for. The 7.8:1 final drive is the outlier here, not the transmission control scheme.