i need to find out how buffer box mechanical cable type VSS works,

why is the buffer box way up on back of speedo, and what does it do ? sends something to the engine ECM or ?

why ? and when ? and what kind of signal

instead of making an electric unit at the tranny (like the later style VSS) they put the buffer box in the dash so that its closer to the ECM or ?

looks like 83 was first year for this, judging by what ive seen when disassembling gm trucks, i cant recall seeing one in the dash of a vehicle older than 83, some are black and some are yellow i know, plastic, red and green wires

thanks for any info, links, diagrams etc, i need to learn about this, im a GM guy and more i can learn the better


then i will try and learn about the 93 or so up true VSS system, i think they send like 3000 pulses a mile or something like that ?

thanks

 

Well, do you remember the cruise control servos up on the fenderwell of 70's and early 80's cars? They used to use the signal off those for the cruise control. I'm not sure if they used it for VSS signal to the ECM in GM cars, although I'm sure it was experimented with.

Why? Because mass produced VSS speedos, an optical sensor / buffer box are cheaper than multiple speedo cables per chassis and servos on the fenderwell. Also less work to install when the cars are produced.

I don't know if 82-83 f-bodies had the servo on the fenderwell.

The back of the speedo VSS sends a signal to a buffer box, and that signal is split out to the cruise and to the ECM on some cable speedo equipped GM cars. They also made cars with a cruise servo for cruise and the VSS speedo for the ECM signal.

The signal from the speedo VSS's is 2002 pulses per mile DC signal.

I think my above text explains partially the "why" of the situation hopefully.


Trucks tended to get emissions and other things a little later than cars sometimes. GM was using the VSS equipped speedometers at least in 1981 models if not before that. I had a speedo from a 1981 Canadian GM car and it had the VSS built in. GM was using ECM's in 1980 CA models and 1981+ 49 state models so they probably had the VSS speedos in 79 or 80. I've seen green and yellow VSS boxes.

I'm sure I could scan in 86 or 87 diagrams, just email or PM me and I'll make it happen.

The reluctor rings on the output shafts have various numbers of teeth. On the 4L60-E models, it is I believe a 40 tooth reluctor ring. That means 40 pulses per driveshaft revoluation which is a whole heck of a lot more than 3k pulses per mile.

The # of pulses per mile increases with the higher numerically differential gears. This differs from the earlier VSS's becuase those were still geared to make the pulses per mile signal correct by use of mechanical speedometer gears.

From one GM guy to another...
-Matthew

 

The little yellow box is an optical deal. It literally "looks" at the piece of the speedo that's connected to the cable.

The piece of the speedo that spins is a U-shaped magnet about an inch long, with the legs of the "U" about ¼" tall. The piece that the pointer attaches to is a little metal cylinder about the same height and slightly smaller in diameter, that sits in between the "U". As the magnet spins, it tries to carry the cylinder along with it; the faster it goes, the more force it applies to the cylinder. That's what makes the pointer move.

The magnet is shiny, and the cyl is painted black. Since in all cable speedos the cable rotates exactly 1000 truns per mile (remember, the odometer is nothing more or less than a turns counter) the magnet is rotated at that speed. The little yellow box with its LED and phototransistor sees the magnet's 2 arms go by, once per revolution; since there are 2 arms, that's 2 pulses per rev, or 2000 per mile.

It provides the signal to the ECM so that it knows when to lock up the torque converter, and for various emissions-related decisions even in a stick-shift car.

The factory cruise control in those cars is entirely mechanical and is separate from the VSS and the ECM. It works on a similar principle, with a spinning magnet and an electrical "latch" that captures a spring tightness, and a vacuum valve that opens or closes based on whether the spinning magnet moves the stationary disc with enough force to overcome the spring.