There are 4 different types of transmission fitted to our vehicles, two automatic and two manual. AR25 is the early 4 speed automatic transmission, AR35 is the latter 4 speed box with lock-up 4th.
AW 03-71L / 71LE - AR35 Automatic Transmission - Auto Shift points - Irmscher 4.0L AR35 box - R25-R28 5 Speed Manual Transmission
The first generation of automatic transmissions fitted to Carlton and Senator 12v vehicles. This transmission is manufactured by Aisin-Warner in Japan and is a 4 speed with a lock up in 4th. This box can easily be identified as the shift stick has 1 2 D N R P positions. The box control is mainly mechanical. There is a switch on the shift stick which limits the shift pattern to 1-2-3, a primitive "sports" mode, and also a winter switch which will cause the car to start off in 3rd gear. I like this box, it changes up early and is less likely to change down on a whim like the more "intelligent" latter boxes. It's also apparently indestructible if looked after!
The latter gearbox is electronically controlled which allows adaptive shift patterns under different conditions, temperature mainly. There is also an electronic sports mode and winter mode. The auto box ECU's appear to have their firmware identifications marked on the case itself. Bosch has done a much better job with the enclosure of the ECU, it's a solid di-cast aluminium affair with a decent water seal at one end secured with 6 bolts. The firmware EPROM (removed in the photograph below) is located on the top left of the PCB and held in place with a plastic clip.
Dave Kember and Ian Fotheringham have collected some auto-gearbox ECU firmware images. Dave has supplied me with binary files which can be used to program a 27C256 EPROM to upgrade your automatic gearbox. Dave Kember can supply programmed EPROMs for a modest charge if requited.
Dave outlines the differences and history as:
MKI - gear changes/kick-down/sports mode too abrupt.
MKII - gear changes/kick-down/sports mode too soft.
MKIII - gear changes/kick-down/sports just right but the gearbox sometimes
howls in neutral.
MKIV - gear changes/kick-down/sports just right and no howling in neutral.
A two letter indent code on the auto gearbox controller tells you what
version of firmware is in fitted. The controller (ECU) is fitted under
the rain deflector (i.e. the plastic shield that the windscreen wiper
arms protrude from) on the passenger side.
YU = MKI
HG = MKII
JK = MKIII
KZ = MKIV
ZG = Irmscher 4.0L
Remember, this is only for the AR35 electronic gearbox, not the older AW gearbox.
Firmware: 3336730741
ECU Code: YU
GM Part # 9 60 15 795
Bosch Part # 0260002111
Firmware 3337633072
ECU Code: KZ
GM Part # 9 60 17 208
Bosch Part # 0260002238
We think this last one is the latest EPROM image Vauxhall supplied for the gearbox, it has much better shift patterns, and has been christened the MKIV. One happy up-grader reports:
"Fitted mk4 Auto gearbox chip last weekend, my Senator had the mk2 slushy chip. It's now like driving a completely different car. I am very pleased with the results. The time lag between kick-down and change-down has gone at all speeds (normal mode), gear changes are much much smoother, and sports mode is considerably improved and seems far more responsive."
The Tech1 diagnostics tool can connect to the gearbox ECU, details of which can be found on the Tech1 pages.
Irmscher in Germany fitted the AR35 automatic gearbox with their 4.0L 24v conversion. The Irmscher modifications involve new firmware, a Irmscher sticker on the front of the ECU and red "tamper evident" paint on one of the screws. A friend in Sweden sent me over a ECU to look at. Analysing the firmware file this revision is numbered 33763075 (I am using the numbers located at 0xjjjj as the firmware revision).
Firmware: B337630751
ECU Code: ZG
GM Part # 9 47 12 173 (Irmscher number from the case) 9 60 16 015 (from the firmware and Tech1)
Bosch Part # 0260002169
I don't have a car with the AR35 at the moment so I have not driven and compared the different versions, buy a couple of Autobahnstormers are at the moment!
Change points for the gearboxes:
| Gearbox | Driving Mode | D1-2 | D2-3 | D3-4 | D4-3 | D3-2 | D2-1 | D4lockup | |
| AW 03-71L / 71LE | E (Economy) | ||||||||
| P (Power) | N/A | N/A | NA | ||||||
| AR35 | E (Economy) | ||||||||
| P (Power) |
R25-R28 5 Speed Manual Transmission
I understand that there are three different 5 speed transmissions that have been fitted to the GSi. The early box was fitted before May 1989 and is designated R25. This was replaced with the R25-R28 box which was standardised on MY '90 models onwards. This box can contain two different gear ratios designated (confusingly) R25 and R28 and can also be mated to a hydraulically operated clutch. The 24v got the R28 ratios and the 12v got the R25 ratios.
| Box type | R25 up to MY '89 | R25 as of MY '90 | R28 |
| 1st | 4.044 | 3.954 | 3.808 |
| 2nd | 2.264 | 2.188 | 2.106 |
| 3rd | 1.434 | 1.387 | 1.336 |
| 4th | 1.000 | 1.000 | 1.000 |
| 5th | 0.842 | 0.845 | 0.814 |
| Reverse | 3.748 | 3.534 | 3.403 |
Final drive ratio is set by the differential and is 3.7 on the GSi with or without a LSD (limited slip differential).
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gearbox identification marks, on the older R25 and the newer R25-R28 boxes
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rpm 'v' mph graph for the two different gear sets available for the R25-R28 box