'85 300TD tach issues (Don't worry I've searched the forum)

[benzwagon440]

Hi all,
trying to get my tach working again. I know it's not vital but I'd really like to have it working - as I get my trans and engine dialed in I'd like to have that reference for myself and I have never really tried to tune any engine.

I've searched the forum A LOT, here is what I've done:
- pulled speed sensor and wiped clean
- checked voltage from speed sensor, checks out fine
- changed OVP relay with another from a PnP car. Mine was really rattling. Not sure the 'new' one is working
- OVP relay fuse (it was blown)
- Pulled connector off the EGR computer, cleaned the ***** out of the pins and pin connectors, and replaced the female connector on the bottom which looked the worst

Known issues:
- OVP relay. From a PnP car so can't really tell if it's working as it should
- There is one part of the instrument cluster circuit board that has burned through. Very minor, no damage to the board, however that burned through spot is behind temp, oil and fuel gauges which are all working fine.
- Lots of corrosion on the EGR computer pins and connectors. Nothing seemed to have penetrated the module itself but there was definitely moisture in the area

After all of that, when I am accelerating, the tach needle is all the way to the LEFT, when I let off the gas the needle swings all the way to the RIGHT. So clearly the gauge is getting some information, the speed sensor is putting off what it should, the strange gauge behavior is not what I would suspect from a bad OVP relay. All this seems to me to point to a bad EGR computer.

For what it's worth the previous owner disconnected the vacuum lines to the EGR, which I have not plugged back in. Not sure what it's worth.

Am I missing something? Anyone have a similar experience?