hi all can anyone give me some advice please, I am in uk, and I have an 88 R20 suburban 454ci with a thm400 autobox, problem is my speedometer became "jerky" at over 50mph this lasted for some time then it stopped working altogether, I obtained a new cable and fitted it but this didn't cure it, I then obtained a Speedo head from ebay for a 1987 truck{almost identical except trip meter and speed sensor, but guess what , yeah that's right it still doesn't work, my Chilton/Haynes manual doesn't show the Speedo drive gear, does anyone know where I can download a thm400 tech drawing from. thanks xxxxxl
ps lmc don't list it , and classic industries don't either.