Dude, Ur in luck.
I just went through the mothjer of all troubleshooting the Fr33kin Speedometer.
All I know is that the transmission sends data to the 4 out Buffer (yellow) box. The yellow box takes the signal from the trannys VSS in the DRVR side tail of tranny(Plug) and turns data to signal which is used for speedo, cruise control and ECM.
That Sounds Easy right?
Well in case you check all electronics in car and replace or just verify that they are 90%.
You may need to jack-up rear of car and pull out the VSS sending unit. Only 10 mm bolt and lots of tranny fluid, after the drain look at VSS unit and check gear, its plastic it wears out over time. if it a little worn replace it, go here(
http://www.redrider.us/Speedometer_Gears.htm), count the teeth and order same color, no one else carries it any more, AMACO $2000, diy $11 plus shipping for a 2 once plastic wheel. HA! I had an expert at a auto parts store that sweared that I had a speedo cable, cause he had 3 of these cars and he knows about transmission. Ha! Even when I tried to return the cable, he insisted that I'm (lectric speedo on 86 TA) wrong. well I just kept on diggin aroud, got the service manual for the 1986 thirgen(get the book, it's like the car bible. Amen) checked the speedo electric schematic, and there is is. VSS on tranny to VSS buffer box to speedometer. pulled out the VSS and whala, very worn driven gear (red 37 teeth) Dam!!! I feel like I just hit a bases loaded home run. I won't fix anyones car, I only fix my cars, 86 TA, 00 Expedition. after paying $600 for a tuneup, I vowed not to get suckered again. Thanks to service manuals and the internet. I keep all my money and learn a couple of knew things and sometimes, not to do that again
