Hey there!
I just recently became owner of a 1989 Chevy Van G20 with a petrol 5.0 liter V8 small block and THM700-R4 transmission.
She sure is rough both on the outside and inside but I have great interests to turn her into a fine jewel of my own.
First of course, any tips and tricks for a new you could say classic Chevy Van owner?
Secondly, and this has been bothering me for quite some time already, the old cable speedo/odometer doesn't work with the mechanical 4-speed transmission. Any suggestions or ideas on what to do with it? I have seen both aftermarket speedos and conversion kits to make analog and digital work with each other.

Eager to hear any help thrown my way!
Kind regards!

 

Been wondering about this same thing for years; and posted on here awhile ago with no responses. I have a 1990 g20 and the speedometer runs at half even with a new speed sensor (no cable on mine) and the odometer stopped working. Going to start cruising some other forums.

 

Welcome from Florida. Have you checked to see if the cable is broken?

 

Pre-1996 OBDII computers are a bit hard but there have been tools fir diagnosis out there for a while. Look up Win-ALDL (Assembly Line Data Link). You need a laptop, serial port or a good USB to serial adapter, and a cable that you have to make yourself. I made one ages ago with parts from RadioShack. Some simple soldering with very few and cheap components. You need this to see if the computer is getting the right number of pulses.

 

just circling back. kinda doubt this would solve your issue if you haven't sorted it out by now but someone on another forum recommended looking at the DRAC module for my weird issue of speedometer running at half speed - in my 1990 g20 it's located behind the dash on the drivers side, in a white case. I simply swapped it out with one from a junk yard and it worked perfectly. either the original one was calibrated wrong or was just faulty.

who knows, maybe yours go burned out and isn't relaying the needed info to the cluster?
hope you've solved this by now but if you haven't good luck!