Yo Bob! That's right up my alley. 91 is rather a difficult year as it has attributes that make it exclusive. Both 90 and 91 have the extra little circuit board mounted at the bottom, back center with a little white cover. My experimentation leads me to believe this is related to the SRS system.

Next, 91 has a female spade lug to the driver's side of the white enclosure on the back that houses the speedo head. This is the ECU signal to the check engine light. It's there because, since some 91s have ABS, the check engine light is moved left one space next to the service indicator to make room for the ABS light. I think that's contact 235, by the way. It lights the second idiot lamp from the left. It takes the only male spade lug wire behind the dash.

A 90 cluster has the SRS board, but does not have the female spade lug. However it can be added. I just pull the insulator sleeve off a connector and solder it in. You have to swap out the light legends on the left side of the cluster so your check engine light is in spot number 2.

89 cluster has the extra female lug on the back. That will be the glow plug lamp for diesel engines, available in Europe in '89. However there is no board for SRS. Having done this swap is actually what leads me to believe that little board is SRS related (hanged if I can determine from the schematic). After testing a particularly irksome '89 cluster in my 91, my SRS system threw a code and I had fits getting it to clear even after putting my cluster back.

All 89-91 clusters are LH2.4/3.x compatible and have only four pins on the crescent connector. The only caveats are the SRS issue, the extra lug 235 and the warning light positions. These will work in LH2.2 cars too.

From memory, the ECU for LH2.4 gets it's speed signal from the round cluster plug #6. (Note to Lucid, the Bentley lists the ECU as LH2.2 for 89-93, and it's listed in the wrong grid.) 90-91 cars don't require a speed signal wire to the cruise ECU, done through the same contact? I'll have to check that.

On 86-88 clusters the wire color on pin 6 of the plug is different (red) and I've never been able to find where that goes on the 2.2 schematics. Cruise ECU takes it's signal from the double male spade lug on the passenger side of the speedo enclosure from 86-88, so that's no clue. I have put LH2.2 clusters in 2.4 cars though, and they run fine. I just don't know if the ECU is getting a signal from pin 6, I've never checked.

I'm a little hazy on the next section, but if memory serves:

The speed signal to the LH2.4 ECU is for injector pulse management, as I understand it. That's from a Bosch LH book I once read--again, if I remember correctly. It gives an automatic trans car some degree of engine braking on de-cel. Something about idle characteristics too. In other words, if it was missing, as a result of putting a 2.2 cluster in a 2.4 car and not having that signal present at pin 6 for the ECU, I'm not sure how noticeable it would be. Certainly if the car is a manual, the engine braking issue is moot.

Hope that helps!
Dave