[Vwdiesel] Electroyisis and heater core/radiator

[cvaf4u at rhinocat.com]

I'm about to embark on changing out my heater core.  The instructions 
state to check with a digital voltmeter with the ground lead to the 
neg of the battery and the positive lead to the radiator core.  If 
it's greater than .300 volts, the aluminum heater core will fail.  
Well I measure mine and it's -.360 (it's negative).  I know there's a 
couple of experts on the list.  To me, if the voltage is negative it 
should be ok by the directions, but to me, there's got to be an 
electrolysis issue going on if there's any reading, positive or 
negative.  Is my logic faulty?  

Bud
85 VW Golf 1.6 NA
http://www.rhinocat.com/cvaf4u