I have an 85 T/A with a cable driven speedo. I read the tech article at http://www.hioutput.com/tech/speedo.html Am I correct in the fact that there is a vss on the back of my speedo? If so, can I use the tranny mounted vss that goes to my electric style speedo to run to the computer so it has its needed input. By the way I have an lg4. Well sort of an lg4. It is now a computer controlled 350 running off of the lg4 computer. I notice the brown wire outputs on the buffer box. I was wondering if they generate the same signal as the lg4 computer needs from the cable driven vss?

Thanks

Ben

This kinda goes with this post but not really but is there any way to usea ADLA scanner to read a cpu controlled carb computer? I know they don't use reprogramable proms so I guess you can't swap in programmable one and mess with these cpu's

your 85 and all the computer 4bbl cars have a 2 pulse VSS which is built into the back of the speedo. The signal goes to a buffer box which splits off to the ecm and to the cruise. If you are using the elec. speedo, the factory ones in 90-92 should be running off a 4 pulse signal. But... I think on the earlier bird elec speedos, they feed the ecm a 2 pulse signal somehow because all the MAF ecm's requre 2 pulse. So... start figuring out how they did that on the 89 'birds and you should be good.

HTH
Matthew

If the speedo located vss sends a 2 pulse that is just what I wanted to here. The way that they do that is there is a little buffer box that converts it from 4 to 2. I think its that way around. Either way, I can send either 2 or four pulse because I have the buffer box. I just need to find out for sure whether it comes from the tranny as 2 or 4. Come to think of it, I think it comes from the tranny as 2 and then is converted to 4 by the box so that the speedo will run right. Anyway, I know I can find it on the tpi board. I have seen it there before, I just didnt know what the comp for the lg4 required.

Ben

the speedo mounted vss is optical 2 pulse, the tranny mounted is a magnetic 4 pulse, need to change the bin in order to switch. cheers, Bob

On the older tranny mounted ones there was a buffer that converted it ether from 4 to 2 pulse or 2 to 4, I dont remember which. It is done in the computer instead of having a seperate buffer or something like that in the new ones. So, does the magnetic vs optical make a difference or just the pulse per revolution? I dont think that there is software out there to reprogram the comp controlled carb computers is there?

Ben

I am sort of confused by the opening question/topic. What follows is based on the assumption that the car originally had only an optical (IR w/ green box behind speedo). If you stick your head under the dash with a flashlight you will see the green box near the speedo or at least a red wire and a green wire coming off the speedo. If this it true, then your ECM is looking for a 2000 pulse per mile (ppm) signal. That is all the optical sensor outputs.
If you want to install an electric speedo in the car, AND the transmission does not have a magnetic pulse generator (4000 ppm) installed you will need a special unit. If you do have this already installed then you need to attach the yellow and purple wires to the "buffer" box. Attach the green/black wire to the speedo (4000 ppm I think) and the brown (2000 ppm) wire to the computer.
Restate exactly what you have and what you are trying to convert to if all of this seems unclear. As I said, I am not sure what it is exactly you are trying to do and what is already in the car.
Hope this helps.
Cheers,
J

Thank you, you have answered my questions, and i have the little green box. I am converting to an electric speedo.