Alpha 4100 CPU fan

Rob Doyle radioengr at
Wed Jun 10 04:00:12 CDT 2015

On 6/9/2015 8:08 AM, Adrian Graham wrote:
> Folks,
> Trying to source a non-DEC fan for a 4100 CPU since getting them in the UK
> is more and more tricky as time goes on. The originals are 12V 0.17A 60mm
> 3-wire and I've found some almost-identical-but-not-quite 0.18A variants.
> Needed to rewire the plugs to match (12V, sense, gnd) but the machine
> reports 'CPU0 Fan Fail' despite the fact the fan is running at speed.
> Is +0.01A enough to make the monitor board think the fan's failed? Board
> itself is fine as it works with its existing fan.
> Cheers,

On a Pee Cee, one of the 3 wires is a tachometer output. The tach signal
is a logic signal that pulses when the fan rotor is in a particular
position. Some fans pulse once per rotation. Others are multiple times
per rotation.

Why do you think it measures the fan current?

What the the tach signal look like on the good fan?

Is the tach signal pulsing?


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