Question o' the day.........

The speedometer is a little "quirky" in my V8 Fiero. For those that are familiar with
the V8 swap and VDO aftermarket gauges, do I need to wire the gauge directly to the
wires coming from the transaxle, or can I use the 4K/mile output from the ECM
as the input to the speedo? I'm asking because I am using the 4k/mile output
from the ECM and the speedo isn't registering right at all -- needle bounces all
around the dial, I've only put on a few miles driving around the development, and
the odometer is reading 45 miles -- I've maybe driven 3............

Bonus question -- if I wire the gauge directly to the two wires coming from the sensor
in the transaxle, how many pulses per mile are being output by that sensor?

thanks in advance!

-jeff d
Fort Worth, TX
198? Fiero GT/V8
1973 TVR 2500M

TTT..............anyone? anyone?? pleeeeze??

-jeff d

Jeff!

You can hook the speedo directly to the output of the VSS. Is your VDO Speedo a LCD version? If so, it should be just like mine and mine came set @ ~ 125000 pulses per mile (ppm) and you would have to reset it to the Fiero's 4000 ppm. This URL has the instructions that I used to reset mine.

http://www.springfieldstreetrods.com/Install-InstructionsSpeedo.htm

The output from the TPI ECM depends on the input of the VSS. If the ECM isn't getting what it expects to get, you are going to get cr@p out (i.e. GIGO - Garbage In, Garbage Out). I am running my VDO Speedo in parallel with my ECM (i.e. one side of the VSS is the signal and the other side is grounded (at least inside the ECM it is grounded) so I have the Purple wire (B9) grounded and the Yellow wire (B10) as the signal. From the Yellow wire, I "Y" it to the ECM and the Speedo and ground the other wire (i.e. I ground the Purple wire from the VSS, the B9 connection to the ECM and the ground wire from the Speedo.) My Speedo works and the ECM sees a speed signal.

Paul

BTW, the B9/B10 pins are the pins used by the Camaro/Firebird 90-92 TPI ECM. Your Corvette ECM may have other pinouts.

[This message has been edited by Paul Herder (edited 10-10-2004).]

Paul!!!

Thanks for the help! Just what I was looking for........................

But to clarify........I only need run one wire to the gauge? One of
those is signal and the other is ground?? Just wanted to
make sure, otherwise, I was going to use the VDO instructions
for an inductive sensor and run the yellow and purple wires
to speedo pins 7 and 8.

I don't think that the speedo is necessarily seeing a cr@p
signal ---- but the 4k/mile output of the ECM is represented
as a square wave, and I think the speedo is wanting to see
a sine wave.

Interesting story.........warmed the car up and drove a bit
around the development last night. I didn't think I was driving
over 120 MPH and I was only gone a few minutes.......put over
10 miles on the speedo on a road that is only 1.25 miles or so.

Back to the Future anyone

-jeff d
198? Fiero GT/V8 (basically reassembled)
1973 TVR 2500M (basically disassembled)

IAW the TPI computer wiring diagram I have been looking at, the purple wire at Pin B9 is grounded internally in the ECM.

With that knowledge, I just gounded the purple wire out of the VSS, grounded the wire that came out of pin B9 and grounded the wire off of the Speedo. I believe that you only get one wire out of the ECM to drive the Speedo anyway so the speedo has to have the other wire connected to something so I put it to ground. That is, of course, if you connected your Speedo to that output. My ECM converts the signal in to something else out for the Speedo and since the VDO Speedo was happy with the direct output of the VSS, I just connected the Speedo in parallel with the signal going to the ECM.

Once I got the ppm setting correct, my speedo functions properly.

I have a circuit diagram that I used on a Quad 4 ECM to the Fiero's Speedo so that the output of the Quad 4's ECM would drive the Fiero Speedo but I don't think that circuit would do you any good since you are not running the Fiero Speedo.

Clear as mud?

BTW, the VDO Speedo also has another method to adjust the readings. You set the Speedo into Adjust mode and hit the button to start and then drive exactly one mile. You hit the button again and the Speedo is set! I plan on using my GPS system to drive as close to one exact mile as I can and then check the ppm reading compared to what I have it adjust to now. You can use mile markers on the road but they are NOT exact, just close enough for gov't work!

Paul

[This message has been edited by Paul Herder (edited 10-11-2004).]