A3 <--> A4 parts swap/interchangeability (i.e., MKIII <--> MKIV)

Here is a list of parts that are, or may be, interchangeable between an A3 (i.e., MKIII) and an A4 (i.e., MKIV). Use the list at your own risk--i have not personally verified all the entries, and it's possible a previous owner, or a repair shop, may have substituted or altered parts (on my cars or yours). Also, let me know if i missed something!

*** A3 w/ AHU engine <--> A4 w/ ALH VE engine
fuel injectors (4 total: 3 without electrical wire, 1 with)
glow plugs (4 total) for engine cylinders
AC compressor (where applicable based on electrical connector)

*** A3 <--> A4
relays (where applicable):
... 18 (if specs call for "18/100", i assume either 18 or 100 is usable)
... 53
... 100 (if specs call for "18/100", i assume either 18 or 100 is usable)
... 109
... 173
... 180 (only until 2001)
miscellaneous fuses, nuts, bolts, screws, etc.
marker light bulbs
cabin air filter (where applicable--absent in some A3's)
tools accompanying the spare tire

***A3 <--> A4 Jetta Wagon
tail light bulbs (where applicable--definite for my 2003 Jetta Wagon)

*** A3 1998-1999 <--> A4 1999-2001
radio

*** A3 1997 <-- A3 1998-1999 (backward compatible only)
*** A3 1997 <-- A4 1999-2001 (backward compatible only)
radio

*** other:
other parts may be modifiable, a poor fit, etc. (see discussion below for some examples)
diesel injector fuel pipe(s) can be repurposed to make injector removal tool(s)

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SOURCES:
* My own 2003 Jetta Wagon and (sold) 1998 Jetta
* "Volkswagon Jetta, Golf, GTI, Cabrio--Service Manual--Including Jetta III, Golf III, and TDI--1993, 1994, 1995, 1996, 1997, 1998, and early 1999", copyright 1999, Bentley Publishers
* "Volkswagon Jetta, Golf, GTI--1999, 2000, 2001, 2002, 2003, 2004, 2005--Service Manual--1.8L turbo, 1.9L TDI and PD diesel, 2.0L gasoline, 2.8L VR6", copyright 2005, Bentley Publishers
* Information from this thread's discussion (thanks, you all!) up until
... 14 March 2018.

The ECU in the A3 and A4 is completely incompatible (the Mk3 has no CAN bus). I wouldn't worry about saving used glow plugs either, though they are the same. IIRC the coolant glow plugs are different.

Relay 109, and the "standard" 53 and 100 relays are the same.

The only exterior bulbs that are the same would be the 194s in the marker lights. Every other lamp is different, even the headlight bulbs.

If you're good at welding you could probably attach the "wrong" cat to the car's downpipe... but the downpipes themselves are different. 1998 - 2001 radio wiring is the same. A newer radio could be installed in a 1997 car, but a 1997 radio shorts out the K Line in a 1998+ car.

Fuel injector assemblies are the same through 2002. 2003 has a different wire connector (this can be swapped). AC Compressors are the same, with 2000+ cars having a different electrical connector (this can be swapped). I believe the alternators are the same as well, though the pulley is different.

-J

The fuel pump (assuming injection pump on the engine) is just as swapable from MK3 to MK4 as it is MK4 to MK3. Neither is bolt on and requires significant modifications to make work but it can be done. MK3 uses a 50mm mounting diameter while the MK4 is 68mm. Plus the electrical connectors are different.

I assume, compu_85, that you mean the fuel injector pump assembly? Not the #3 fuel injector (the only one with an electrical connector)?

Your memory of the coolant glow plugs matches up with what i observed.

I broadened the scope of the tread a bit to include PD engines. Does the AC compressor work in PD's as well--connector permitting?


My 2003 TDI Jetta Wagon takes identical taillight bulbs out of the 1998 TDI Jetta that i'm cannibalizing--departing from what my Bentley's manual indicates.

I added some relays to the list after checking the relay panel on each car a couple hours ago... But where is the 180 glow plug relay in my Wagon? It's not on the panel (despite what the Bentley's manual indicates).

***
Thank you both for the information!

The glow plug relay is completely different on your 2003. It's mounted up near the ECU, in the wiper cowling.

GP relays are the same from 1997-2001.

Wagon lamps are different than the sedan / Golf lamps.