Maker Pro
Maker Pro

Tachometer Problems - 4th Channel Not Working

Anyone here have experience with multi-channel tachometers? I'm using
a Deuta Werke type DF9 tachs with 4 channel output. These are
typically used in heavy duty vehicles and things like commuter trains.

The problem is that I don't get any output from the 4th channel, and
this is being seen as a pattern throughout our entire fleet. The
outputs are open collector type. The tach datasheet says to use the

Vout = ( (15 * Rload) / (1500 + Rload)), and cautions that Iout needs
to be 10 mA max.

All I get when I setup channel 4 is a 24VDC signal with some noise on
it. Rotating the tach has no effect on its output.

Are there any special tests I could perform to check the integrity of
the channel 4? I'm worried that maybe channel 4 was never internally
hooked up in the large batch of these tachs that we own. Is it
possible that unused outputs of tachs can be damaged if left floating
for a long time? Is there any local distributor or rep for Deuta

Thanks for the help.