85 V6 SE 4spd manual.

Justinstalled a 120 cluster, tach works perfectly, speedo reads exactly half. While driving with the ALDL hooked up, winALDL reports correct speed of 40 mph, while speedo reads 20. And this goes for all speeds through 80. Ideas?

[This message has been edited by spc15tdime (edited 05-19-2012).]

The ECM (and thus the ALDL) gets it's input from the speedo's output. Bad speedo is what it looks like to me.

The speedometer circuit takes the raw VSS signal and provides signal conditioning and processing for both the speedo calibration (4000 pulses per mile) and the ECM input (2000 pulses per mile). If your speedo is reading exactly half, then it seems the signal processor has failed for the speedometer drive. A new speedometer is the remedy.

Do the trip/Odometer's also read half?

You can try metering out all the resistors to make sure their values match their indicated values, maybe replace the capacitors, but it could just be the aircore meter driver chip like blooz said.