Replacement automatic TCU without the exact same part number

[Zebedee]

This has had me wondering for a while now.
I've read that you need the exact same part number on a replacement TCU for it to work properly, but why? I've searched online and not come up with a definitive answer. Hell, even my local vw main dealer have no idea.

What is it that would stop a replacement automatic transmission control unit (TCU) from working properly if one without the exact part number was used?
I assume one from a diesel wouldn't work on a petrol van due to differences in inputs from the engine or different engine characteristics.
So let's say the engine code was the same (AAF 2.5 petrol as an example).

Would it matter if the final drive ratio in the transmission was different as there seems to 3 different ratios available for the same year and engine all with TCUs with different part numbers. I assume it would give change points at different revs. But would it still work?

What about actual different gear ratios, as it looks like the pre 1993 transmissions used a different 2nd and 4th gear ratio?

Also pre 1993 vehicles have the sport mode switch on the dash with later ones having the TCU that controlled the driving modes through adaptive learning.
I assume that would confuse the TCU too.

I'm struggling to work out which TCU will actually work on my van as the original part number 098927731G just doesn't exist.
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[67rustavenger]

As a suggestion, reach out to German Transaxle of America for TCU replacement information. Ask the receptionist to connect you with Ken.
https://www.germantransaxle.com/

When I had my 2003 EV there for a trans rebuild, one of the services offered, if required was, in house rebuild of the TCU. Luckily mine was ok.

Maybe they can help you with a few examples of compatible TCU's.
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[EuroTec]

[Zebedee]

[Zebedee]

Looks like i'm back to the stage i was a couple of months ago.
The T suffix TCU i got turned out to be a unit from a 1991 passat with a part number starting 095 and won't work properly.

I found a slightly earlier TCU in the UK (it was new from VW Classic and has only been used for 20 miles).
Comparing the wiring diagrams, the early TCU (in a van with the Sport switch) just has a single wire from terminal 36 that goes to the sport switch, then theres two wires from that switch to the PNP relay's terminals 5 and 7.
Terminal 5 on the PNP relay is also linked into the multifunction switch on the gearbox (and the fuse for reverse lights).

In the later (01/1993>) TCU the wire from 36 just goes to the 10-Pin Connector, somewhere behind instrument panel, with the wire not going to anything from there. Its the same connector the OBD socket and other stuff is wired to.
Shouldn't be too difficult to just add a new terminal to that plug, though I honestly can't see there being any problem with not having the switch as everything else is wired the same.
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If you do things right, people won't be sure you've done anything at all.

[Zebedee]

Seems the reason the earlier (pre 1993) TCU really doesn't work properly with later (post 1993) gearbox is that VW changed the valve body and solenoids slightly at the same time and the later TCU can't supply the correct signals to the early solenoids.
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If you do things right, people won't be sure you've done anything at all.