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Originally Posted by MStefak
Doesnt a drac just determine speed ?
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It does, but it's required when you cross the 91-92 line. From 88-91, the VSSB
INSIDE the cluster sent speed information to the ECM, Cruise, and ABS computers. The VSS fed speed only to the cluster.
In '92, they moved the VSSB out of the cluster and renamed it a DRAC. The VSS sends it's signal to the DRAC, and the DRAC sends the speed info to the cluster, ECM, Cruise, and ABS.
Note that the various boxes require differing signals, thus the need for some kind of signal conversion. VSS signal is like 16 pulses per driveshaft rev; ABS requires something like 144 pulses, ECM something else.
If you have a truck made in '88-91, the clusters are a direct swap, although the 91 clusters require moving pins around in the dash connector to match the cluster. But no re-wiring is required.
'92 and later clusters CAN be swapped in, but rewiring is required, as well as the addition of the DRAC. There's an article in the FCSU section on how to do this.
-blaine