Knock sensor ??

[JanDebL at aol.com]

Are there any knock sensor experts out there?  My DMM reads open with any and 
all combination of connections when I test the sensor on one of my 87 5ktq.  
The sensor on my other 87 5ktq gives a reading.  I didn't find any info for 
testing in any of my manuals, bently, bosch etc.  The VW bentlys have a test 
specs. 

Is the later style VW unit (with the through bolt) a direct replacement?