I know a lot of 850 drivers have had their ABS or ABS/TRAC control module failed at low mileage. My too (ABS/TRAC) went out of service in less than 50k-miles but after replacing the control module it was cured.

But what I learned was that those 850s without TRAC can probably add TRAC by installing a control module that is marked "ABS/TRAC" rather than ABS alone. The reason I think it will work is because my 850R comes with TRAC but the replacement controller module that was shipped to me was the wrong one, it was ABS ONLY. Nevertheless, it's been installed and ABS works, just No-TRAC but the TRAC warning light does not stay on either so you can't tell TRAC has been erased.

I looked at the ABS pump on both cars with and without TRAC, apparently they are the same, so I think if you mount an ABS/TRAC controller to an ABS ONLY car, you'll have TRAC by simply doing that. The circuit is in the module, not in the mechanical parts.