Tach Wire - '91 Volvo 740

Having trouble with the tach signal.  All the references I've found for a 740 ( including this board ) call out the RED / white wire at the coil  BUT when I check it with a DMM it reads 28+ V AC and doesn't fluctuate with engine speed.  I also tried a grey wire common wire coming back from the fuel injectors - but same deal 28+V AC.  I checked my DMM - it's fine.  Any ideas?  I really want to stick to tach mode and I have a second 740 that I've got to install after I get the bugs worked out with this one. 

The info I have states that the tach wire is the RED / white wire at the coil.  Have you checked the battery in your meter? Sometimes when it is weak it will give you a false reading.  If your meter is o.k it might be the system that Volvo use reads diffrently  the tach wire.  (dont know this for sure just quessing)  Good luck with it.  Brian