I am trying to make room for an aftermarket tachometer where the stock one used to be. Its just a little too deep so I need to cut out a hole in the back of the cluster. In trying to figure out what everything is on the back so i can trim the "circuit board" behind it I found a yellow box that connects with a cable on one side and looks like it has a little light on the other that goes into the speedometer. What is this and what does it do? This is on a berlinetta with the dual needle.

Thanks!

 

Hmmm: The yellow thing on back of the cluster shown here?

 

That is part of the VSS (Vehicle Speed Sensor).

 

Yes, that thing! What does it do? I put a non fuel injected 350 swap in the car so if its just for cruise control/top speed limit am I okay not putting it back on?

 

Originally it would tell the ECM what speed the car was moving at. It would also affect the cruise as well.

If you have neither of these items you can likely remove it.

 

The black piece near the speedo cable connection is the 'optic head.' The yellow box is the 'buffer' (the whole business is generally simply called the VSS). Here's the way it works from my 85 GM Shop Manual:

"This is the device that will supply the vehicle speed input to the controller. The optic head portion of the VSS is located in the speedometer frame. A reflective blade is attached to the speedometer cable/head assembly. The reflective blade spins like a propeller, with its blades passing through a light beam from a L.E.D. in the optic head. As each bade enters the L.E.D. light beam, light is reflected back to a photocell in the optic head causing a low power speed signal to be sent to a buffer for amplification and signal conditioning. This amplified signal is then sent to the controller."

In short, the VSS sends a signal to the ECM and to the cruise control module on your car (since your speedo is cable driven). It does not affect how the speedo or trip odometer reads.

JamesC

 

Just a warning:
Be careful not to rip/cut/tear the "circuit board" aka the plastic film with the
copper lines running thru it on the back of the cluster.
It carries the power to your dash light/guages etc.................

 

Yeah the whole fuel injection wiring harness and computer are gone. Out it comes, this simplifies tach mounting substantially. Thanks!

As far as the plastic film you can easily see where every wire goes, careful scissor skillz will leave you with a trimmed but still functioning harness for the dash lights/gauges that you need. All the lights/tach function/oil pressure/warning lights on the right side in my application can be removed since they no longer function correctly.

One other thing you guys may be able to help with is which of the 3 pins going to the old tach are the signal wire? or even a wire color going into the back of the plug? I could run a new one but would rather just use factory wiring.