Hi, BrianB.
For the history of ABS on our NSX, please refer to the following link on NSXCB site.
http://www.nsxcb.co.uk/showthread.php?6286-NSX-Health-Check-Service&p=75279#post75279
The photos are based on RHD model so the orientation of the modulator and the position of the hard line fittings would be different from those of LHD models but you will get the idea.
From hardware point of view, apart from the obvious difference in the shape of ABS reservoir, you may not be able to tell the difference in the solenoid package between the 1st and 2nd gen ABS.
2nd gen has better protection against the contamination around the solenoid body and this is the one you bought.
As others mentioned, you can install the 2nd gen system in place of your original 1st gen one.
However, you need to be aware that there are two different ABS setting within the 2nd gen system due to the introduction of the larger caliper/disc design from 97 onwards.
I don’t know which version of 2nd gen ABS you bought but the ABS pistons and the balancing springs for the rear brake are different compared to the early 2nd gen model and you won’t be able to tell the difference from the outside.
From software point of view, again, the ABS controller was changed (as per warrenw’s post above) due to the above reason but the main reasons were the changes in ABS warning light circuit, extra error codes, chassis loom, etc.
Therefore, unless you modify your chassis loom, you will be forced to use your existing ABS controller on your 91 model even when using the 2nd gen 97+ spec ABS.
It will be plug and play and it will work but strictly speaking, the ABS setting won’t be the same if you bought 2nd gen 97+ ABS and if you are still using your original 91 brake caliper/disc design.
Having said this, the brain power and control of 2nd gen (including the 1st gen) ABS are so primitive and considering 99% of the ABS usage would be under panic braking situation, you won’t notice the difference between the ‘pre’ and ‘post’ 97 classic 2nd gen ABS setting.
You will only notice the huge improvement once you upgrade it to the latest one-body Bosch style ABS that can be seen on 99+ (I don’t like using the year model so some will call it as 00+ depending on the country spec) NSX.
The reaction time and control detail have improved so much that you will never want to go back to the classic ABS.
If you have access to the skid pad, you can test one of the major loop hole in classic ABS but it is out of the scope for this thread so I’ll leave it for another time.
Kaz