Radeon 4870 in MP2,1 going haywire

[fhturner]

I have a few Mac Pros for various video/server/test uses, the "tail end Charlie" being an 8-core 2007 machine. I've played around w/ getting El Capitan on it (successfully). While I've used a Radeon 5770 w/ it in the past, I'm currently using my "least desirable" ElCap-friendly GPU laying around: a Radeon 4870. The card has worked properly in the past, and it works when booting into 10.7 Lion. But when booting into 10.11 ElCap, the Apple shows up, then the first hint of a progress bar, then screen goes black and the (horrendous) fan on the 4870 starts oscillating between maximum shriek and quieter operation every second or so.

Any idea what's up w/ that? Think this is a GPU or MP or OS fault?

EDIT: A little more background...this is the same 2007 Mac Pro that I've mentioned before having an issue w/ its fans being "on siesta" while the CPU overheats. So could definitely be hardware/SMC/firmware, but why only now and only w/ this card?

Thx,
Fred

 

[haralds]

After many years of service in a 2,1 and 3,1 my Apple 4870 died. The system thought there was a display, but the monitor didn't.
Since I am running High Sierra on that system, I bought a used 4870 since the 5xxx series does not work under that OS on that configuration.