I bought a VSS plug pigtail from GM, and I need to wire it into my harness. Problem is, the GM pigtail is NOT color coded! It has two white wires. I need to know where the purple and yellow wires correspond to on the VSS plug itself, either A or B.

This is the last piece of the puzzle I need to be able to get my 86 Firebird running this weekend. Any help is appreciated.

 

Hm weird. My diagrams don't say, but my Hayne's manual does . It has A - yellow, and B - purple. It's for a TBI, but I can't imagine them changing it for a TPI.

 

I'll be under my car tomorrow swapping speedo gears and the givernor, if you don't have an answer by then, drop me an email so i rememeber to check for you.

 

It don't matter, just hook them up. That's why GM don't color
code them, it don't matter.

 

I wanted to say as much, but i really wasn't 100% sure. I will back you up that it shouldn't matter though. AFAIK one is ground, the other is a reciever with a pull up, such that a VSS pulse just pulls down the signal to the ECM

 

The VSS signal generator is essentially a differential signal and the polarity does not matter. Most connectors purchased from GM have only white wires. The wires are white just because they are white(cheaper to manufacture), not because it does not matter. They also cost an a lot.........MAF connector ~ $50
J

 

$50 eh? What's the PN?

Kinda glad I bought a $15 connector from SLP

Matthew

 

I would think it would make a difference as to which goes where. One wire is the 2000PPM signal and the other is the 4000PPM signal.


VSS 4000 PPM - yellow wire ECM pin B10
VSS 2000 PPM - purple wire ECM pin B9

This is for a 90 - 92 TPI system.

I'll wire it as JZA said. My guess would be either the speedometer will read correctly, or be either half speed, or double, depending on which wire goes where.

 

The purple wire goes to ground inside the computer. The yellow
wire splits inside the ECM, one leg goes to VSS input and the
other leg goes to 12 volts through a pull-up resistor.

Think of it as the yellow wire being hot and the purple as ground
providing a reference voltage and the VSS sensor as a generator
adding modulated current to the reference voltage.

It does not matter which wire goes where. The ECM is just
counting the pulses in the reference voltage be it a + pulse
or a - pulse, the ECM don't care.

 

yahtzee

 

Sorry Ed, had to edit it just for the fun of it. Imposing half of an
A/C cycle on a D/C signal is crude.

 

Ok, I'll take your word for it. Just wondering.. where did you get this information? From ECM circuit diagrams?

Only thing I have available to me is some reference sheets from a factory manual that has the ECM pinouts.. but does not diagram the circuit inside the ECM itself.

So is it strictly a 4000PPM sensor, and the ECM just divides the output to the 2000PPM and 4000PPM for the speedometer and cruise control?

 

I don't know where you are getting the 2000PPM and 4000PPM
from but Pulses Per Minute is what the ECM counts.
If you are not moving the ECM sees 0 PPM. I would have to
look it up but the ECM calculates MPH from PPM. It takes a certain
number of PPM to move the speedo one MPH. Lets say it takes
10 PPM to make the speedo read 1 MPH. Then it would take 100
PPM to read 10 MPH and 1000 PPM to read 100 MPH.

That's just an example, it's more like 30,000 or 40,000 PPM
to read 55 MPH.

 

ok maybe I am just messing my info up here and getting the input and output confused.



according to this the output is 4000 pulses mile - goes to the speedometer
2000 pulses mile goes to the cruise control


you referred to pulses per minute.. so am I just getting the two confused? As in, pulses per minute is what the signal is from the VSS, and pulses per mile is the the signal from the ECM?

 

OK, what you are looking at there is the VSS signal comming into
the ECM. Current is flowing from B9 (ground) through the VSS to
B10 (positive). Once inside B10, The resistor shunts the current
to keep from overloading the VSS input circuit. The VSS input is
taped in front of the resistor between the VSS sensor and the
resistor. The VSS sensor adds its modulated current to the current
flowing from B9 and the ECM reads the pulses as the current
flows into B10.

The ECM then does its voodoo on the signal and sends it to the
speedo as 4000 pulses per mile and to the cruise as 2000 pulses
per mile.

I was messed up there, it is pulses per mile.

 

OK, thanks. I think I have it all figured out now.. except for the "voodoo" part lol

Sorry to keep asking questions, but I'd rather understand the way it works rather than just plug it in and not have a clue what's going on.

 

No problem.

 

The MAF connector from GM for $50.........you don't even want the PN cause like you said, you can buy it cheaper anywhere else. I bought one back in the day when I had no clue and did my first TPI swap. Actually, not much has changed, I still get ripped off and still have no clue. I learned not to buy pigtails from GM though.
J

 

The reason the polarity (wire connection) doesn't matter is because it is an A/C signal. A/C is a changing voltage / current that is referenced to one side or the other....still A/C. It is circuit theory, and not a GM thing.
J