Aftermarket Speedo Buffer

Thought I would post on here a bunch of information I was able to cobble together from various sources on Pennock's and other forums.

Lots of folks want to replace their Fiero interiors with a newer, updated interiors or replace/upgrade their gauges. The process, aside from cutting and fitting the parts, gets a bit more complicated on Fieros when the Speedo is to be replaced. The Speedo replacement is not a simple swap.

Fieros have a VSS buffer built into the gauge clusters that receive the VSS signal from the transmission sender and reshape it to a 4000 ppm and a 2000 ppm square wave. These "Buffered" signals go to the Fiero ECM, Speedo and Cruise Control units. Later GM vehicles have this buffer moved into the ECM but the Fieros are all in the gauge cluster.

When replacing the Speedo, the buffer needs to be retained in the system. There has been quite a bit of debate on Pennock's but JScott1 (who did most of the investigation) says that he only found '88 Models working for a 4000 ppm output on Pin M.  He could not get the other modules working.  I will assume his thought process in this list. If you have an '88 year and model, you are in luck as you can reuse the buffer from your gauge cluster. If you do not, like most people, you will need to do one of the following:

1. Obtain an '88 GT gauge cluster and pull the buffer board from it
      These are getting harder to find as there are far fewer '88 GTs than other Fiero models and years

2. Buy a later model speedo buffer from a GM
      The 80's Firebirds and Camaros up to '90?? I believe, are the best source for this buffer. There are also other models but I don't honestly know them all. A search on Pennock's should give the models and years this buffer module is available

If you go with option 1 and can find one, you essentially plug the buffer board into your speedo connector in your dash wiring. Make sure when you put the board in the circuit, it is in a box or something that will not allow it to short out on the metal in behind the new dash.  You will need to pull your Speedo signal from the M pin on the connector section for '88 gauge cluster and Pin N from non '88. That is the 4000 ppm output and best output to use to your speedo.   The full pin out list is below.

Speedometer board Pin Outs (84 non cruise, 85-88):

A - not used
B - not used

C - not used
D - not used
E - not used
F - not used

G - not used
H - not used
J - not used

K - not used
L - not used
M - 4000 pulse per mile buffered VSS (88 only)

N - 4000 pulse per mile buffered VSS (86-87) (In question as per above).....
P - black/white - ground
R - puple/white - from the VSS (low)
S - Yellow - from the VSS (high)

T - yellow/black - Ignition power to cluster
U - Dark green/white - 2000 pulse per mile Vehicle speed sensor signal to ECM and Cruise

If you go with option 2, these are much more easily find on ebay or at your local wrecker and the box looks like below.

The colour may be different (I have a white box) but the shape is the same and the yellow seems to be the most common
 

The wiring diagrams and labeling in the next two pictures show you how to hook this module up to the speedo connector. The actual position of the wire colours in the module may be different on each year but the colours remain the same. When matching up the wires to the speedo connector, make sure you match the wire colours rather than the position. It will probably easier to use a new connector but just make sure the input and output wire colours are matched up
as per the Fiero wiring diagram and the module colours.

Cheers
Don







Edited by Linlor