Do the 90-92 Broughams have on board diagnostics like the 91-93 deVilles? On the devilles, you access it through the ECC, and it shows up in the FDC. Where does it show up on the Broughams?
I don't know what ECC and FDC is but you can get it to show codes the same as any other Cadillac with the cllimate control buttons and it shows them on the climate control too. They had that since the early 80s it wasn't anything special just for for those DeVilles.