i tried to do a search first but no luck, im trying to find a location where i can get a signal for the rpm and mph guages from the instrument cluster. i have an 85' iroc with analog (needle) gauge and want to convert to my own digital style guages. is the signal going to instrument panel analog between 0 and 5v or is it pulses? can i tap the reading from the wires going to panel or directly from the ecm?? thanks all

 

For RPM you will use the tach feed which comes from the ign coil and ends up at the tach.

The speedo is harder. You have two choices. Install a 2Kppm speedo sensor on the trans and feed it to the ECM and your new speedo or you can use the speedo sensor in the car now. The existing sensor is built into the speedometer. The yellow box on the back of the cluster outputs this signal to the ECM and the cruise.

Now if you happen to be running a 7730 or other later ECM they have a 2Kppm output to drive a speedo or cruise. However you still have to have a 4Kppm sensor to feed the ECM.

 

hhmmm, ok thanks, where can i read more about this? any references, diagrams, schematics, etc? thanks!

 

can someone tell me what the pulses are? or voltage? is it like a pulse per mile, 2 pulses per mile, etc ... also, what about the tach, what signal is fed to the guages ? thanks again all...

 

ok, whats the 2k ppm and 4k ppm? i did research on this site and other sites but no one really says what the pulses are? does the ecm need to see a constant 2k or 4k pulse ( kinda like a clock/osc ) to give an output? or do these pulses carry info? what i need is the actual data the ecm spits out to the guages so i can connect to my own guages.... thanks

 

I'm pretty sure ppm means Pulses Per Mile. The stock sensor outputs 4000pulses/mile, then either the "yellow box" (vss buffer) or a 7730 ecm will break it into two signals: 2000ppm ans 4000ppm.

on the vss buffer the pins are:

yellow...vss high input signal (4000ppm)
purple...vss low input signal (ground)
pink/black...+12v (battery)
brown...ground
red...2000ppm cruise control output
brown...4000ppm speedo output
black/white...ground

 

sorry to be a pita, but again, is the 2000 ppm a direct relation to speed? ex, if im doing 25 mph, is the output going to be 25X2000 = 50,000 pulses?
or is the 2000 ppm just constant or fixed number the computer expects to see?

 

If you travel 1 mile... the sensor will send 4000 pulses to the ecm/buffer. It doesn;t matter if you're pushing the car at 2mph with the ignition turned on and the engine not running or if you're doing 100mph.

I think the gauge/ecm counts the time between each pulse, then assuming 4000ppm, calculates the vehicle speed. It also assumes stock speedometer gears, stock rear gear ratio, and stock tire diameter.

 

That is how it is done in the ECM. The nice part about the magnetic VSS is that there is a divider value in the code so you can adjust the MPH the ALDL spits out and what the ECM uses for the MPH calcs internally. This is for gear changes.

The optical VSS part of the ECM code doesn't have this because gear change values are done at the transmission with a plastic gear change. You can't fudge things like you can with the magnetic VSS.

The ECM speedo output to the speedo gauge has very course divide settings in the ECM when using a magnetic VSS.

 

does my 85' have a cable driven vss? and would it better to get the pulses from the ecm output that connects to the cluster? i m assuming thats not a 2k ppm but a converted pulsed signal from ecm? is it linear so i can write a program for it... ex, if im doing 35 mph, would i get 35 pulses, 70, etc or does the ecm have its table programmed internally? i may just have to get out the 0'scope.. thanks again...

 

Some cars have the mechanical VSS and some have a magnetic.
Look at the firewall of your car and see if there is a speedo cable.
If there is a speedo cable then you have an optical sensor that spits out a 0 - 5v squarewave . That square wave has the shape of MPH * 2,000 IF YOU HAVE THE PROPER SPEEDO gear in the trans. for your rear end gears. Example, 30 MPH * 2,000 PPM = 60,000 pulses/hour. 60,000/3600 = 16.67 pulses/sec or 16.67 Hz.

In the ECM it assumes your trans. gear is correct so it uses the 2,000 PPM all of the time with no divide or multiplies to fudge things.

Figure out which VSS you have first. If all you want to do is connect to the VSS output from the ECM or maybe from a VSS buffer box if you have one (it is a box bolted to the ECM) then it will be the 2,000 PPM signal used for the cruise control.

 

the car is already apart and in boxes, getting ready for paint, but when i was taking the car apart i didnt notice a cable coming from the trans tunnel. what else can i check to see what i have? its an original 85' z28, 700r4.. it does have cruise control cuz i saw the assembly on the pedal..

 

If the car is apart you can check a few things. Look for a green or yellow box bolted to the ECM on the pass. side. Look under the dash at the speedo and see if there is a yellow or light green box about 1" x 2" in size. connected up to the speedo. That is the optical to elec. conversion box. If it helps, every 1985 Camaro I have seen had a mechanical cable driven speedo with the optical (2,000 PPM) VSS on the back of the speedo. The output went to a buffer box and the fed the cruise, ECM, etc.

I would design for the 2,000 PPM output. The output wire colors of the VSS buffer box is floating around here at TGO. Do a search for "vss buffer"

 

or read post #6 in this thread. haha.

 

the only box i seen was a little black box bolted on top of the ecm. and i was told it was a module for the cruise control i think, ill have to check my older post and see.... but i dont see any colored boxes. is it a big box? or something small, like a deck a cards or something??? thanks again...

 

Yes, that box bolted to the ECM is the vss buffer box and creates the signal to go to the cruise control. It has the outputs you would sample with your new devices that you are making.

 

can u tell me what colors are assigned to each wire? my gm srvc manual, chiltons and haynes dont describe this vss signal/ecm outputs in detail.. thanks alot....