normal for rpm sensor to not work at 7 volts?

[htne]

I've got a HSF here, solid copper, with a 60x60x25 mm fan on it. I picked this thing up used, and do not have a brand name. The fan has two separate wire leads, one with two wires going to a standard 4 pin molex connector, and one connector with one wire going to the motherboard cpu fan connector to provide the rpm signal to the BIOS. At the default 12 volt, the fan turns at +/- 7,800 rpm (according to Motherboard Monitor and the information in the CMOS setup screens). I rewired the Molex connector to provide 7 volts to the fan, and it is somewhat quieter. Louder than I would have expected, but quieter than at 12 volts. The problem is that the rpm sensor no longer works. When I powered it up, the BIOS halted the POST and told me the cpu fan was not working, did I want to continue. When running, MBM says 0 rpm. I was just wondering if this is normal behavior when a fan is undervolted?

Thanks in advance.

 

[Thraxen]

Can you see the fan turning? I'm not sure about 60mm fans, but I know most 80 mm case fans stop turning at or a little below 7 volts.

 

[woodie1]

Many fan monitors require a minimum RPM or they don't read. My Soltek MB wouldn't read the fan RPM when I fed it 7V.

 

[Kraith]

You say that the 3pin connector for the RPM signal only has 1 wire? I'm thinking that the fan then uses the ground from the 4pin connector for comparison with the RPM signal wire, and if you run the 7V trick, the ground is actually 5V, so effectively, you are comparing the RPM signal with 5V. To fix this, either run 5V on the 4pin (maybe this is as quiet as you want), or get a fan controller, the 7V trick won't get you the RPM signal in this case.

Another possibility could be, as already stated, that the motherboard doesn't support a low RPM reading. I find this unlikely though, since the fan runs at 7800 (!) RPM at 12V. Anything above 2000 RPM should show up on most motherboards.