Aisin 6 speed automatic - Limp mode - p2728, p2729 dtc - sharing some diagnosis notes

[Sven L]

Hello all,


I just thought I'd share some information about French cars, or indeed other makes of cars fitted with the aisin 6 speed/am6/tf80sc transmissions.

In short; if the gearbox is going into limp mode, and has the above fault codes, p2728 or p2729; it's probably the transmission control unit (TCU); a little computer that sits on top of the transmission.

Please read on if you'd like some more information on how I came to this conclusion.

I bought a lovely Pug 308 wagon that was hard shifting into reverse, and hard shifting from 2nd to 3rd. Every time the car would hard shift, it would immediately go into limp mode.


I scanned the transmission control module; found codes p2728, p2729; problems with the pressure control solenoid E.



I tested the wiring harness from the TCU connector to the solenoids; all good.



Checked the resistance of solenoid E; all good.



I replaced the solenoid E anyway; no effect on the problem.



Checked all relevant earth wires, opened up and physically inspected the TCU; all looked good.



Carried out audible solenoid tests (clicking test) with an advanced scan tool; all solenoids sounded good.



The only possible remaining component that could cause the car to electrically go into limp mode was the TCU itself. It must not have been able to exert control over the shift solenoid properly, the transmission was hard shifting in all gears related to control circuit E, and the TCU was placing itself into limp mode.



I found a wrecker selling a TCU with – and this is important – both part numbers matching on the casing matching the original TCU. The tcu has two part numbers on it; they both have to match.

I know this because I took a punt and purchased as TCU from a similar diesel Peugeot ( a 2009 Peugeot 307 2.0hdi), and it didn’t work, same casing, same physical shape, same chip set and PCB, but it just doesn’t work.



I installed the second hand TCU, with matching part numbers, and the car now changes gear silky smooth.



I’ve been taking bits of information to help myself out off of forums like this for many years now, and I just hope that this little contribution will help someone else out one day, as many other people who had contributed in the past have helped me.

Please find attached a picture of the offending TCU; and I've circled the part numbers that must be matched.

 

[Sven L]

Just thought I'd also share this as a possible repair option;

One might also want to consider the repair of the TCU itself by a computer hardware specialist.
I did briefly engage with a company in Perth - Cyber Dynamics - and they were friendly and willing to take a look at my old TCU. It never eventuated because I was able to find a replacement part.
But this company, and others similar, is able to test chips, boards, PWM output signals etc etc. All the stuff on the near micro-chip level

 

[dmccurtayne]

Did you reset the auto adaptives after replacing the valve.

 

[Sven L]

Did you reset the auto adaptives after replacing the valve.

No.

The old valve tested good so I ended up popping it back into the body. That is what the car is currently running with at this moment. The only new part in the system, aside from transmission oil, is the TCU.

 

[dmccurtayne]

Just be careful a new ecu can hide a issue until it relearns the positions and pressures I’m not saying your wrong but sticking 2-3 shift valves that test ok are common.

 

[FiveoEight]

Just be careful a new ecu can hide a issue until it relearns the positions and pressures I’m not saying your wrong but sticking 2-3 shift valves that test ok are common.

Hello dmccurtayne - I replaced a control valve on my 508 several years back, but never reset the auto adaptives, in the ECU. It was good for 3 -4 months but reverted back to its old habits. hard shifting especially once engine the oil temp exceeds 90deg. Is there workshop in the Newcastle area that could reset the auto adaptives ?