Hmmm. the '85 still had a vacuum actuated EGR valve. Now, the vacuum is sent to it via the vac amplifier that is electrically controlled by the EGR module...

The EGR module is a consumer of the RPM sensor signal along with water temp and rack position. It combines this data to know when to active the EGR for best NOx reduction.

It doesn't amplify the signal. Passing the signal through without the module gives me accurate tach and the compressor cuts out when expected. My '84 did't have a tach amplifier either. I'm not sure the actual wiring differences that changed from 83-84, but the 85 just routed the same rear rpm sensor to through the EGR module that the 84 used.

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'95 E320 Wagon
'85 300TD "Puff The Magic Wagon" - SOLD
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition - SOLD
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter