Electronic speedometer help

[critter2655]

Having issues with a electronic speedometer. First off here's the truck, it's a 87k20 with a lt1 and a 4l60e and the transfercase with the VSS (241 I think). The dash wiring harness and transfer case were out of a 91 suburban. The engine wiring was sent off to be redone for the lt1. Here is my current issue. The VSS is sending a signal to the ecm, so the trans is shifting normal. This tells me that side of things is good. However the speedometer works only when it wants. I have 3 of these clusters, 2 do absolutely nothing and 1 works sometimes. Does anyone know how or who rebuilds these? Also how can I test one on a bench. Is there any replacement parts? Also do these clusters have a drac? Or do I need to somehow add this? Any And all advice is much appreciated.

 

[yevgenievich]

They use a separate drac. Can be tested on the bench using a function generator or 555 timer.

Picture of the drac board(from my old 85 with 91 cluster)

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[critter2655]

So how would I test this, by doing so what could I learn from it and where could I find parts?