I've been working, when I get time, on my 86 Spt Coupe. It has a 2.8 and 5 spd. I don't think I should list everything that has been going on for fear of puttin you all to sleep.

Basically, the car runs pretty good but needs to get a few things settled so I can get it through emissions inspection.

The current problem concerns the Vehicle Speed Sensor. It is mounted on the instrument panel on this Camaro. Some of them have them in the trans but I was honored with this stinker.

I am getting a code 24 both by reading the light and from my AutoXray Scanner. The scanner can watch about 25 items working in real time while I drive the car. The speed shown on the readout is totally accurate. Yet I get a code 24. I checked the resistance of the lead to the ECM. It was at about 2 ohms. By cleaning the contacts I reduced it down to about 0. I also cleaned the connector on the VSS and the little propeller like thing that the sensor reads inside the speedometer. Still I get an accurate speed reading and an error code.

I believe as a result of all this my idle speed is going a little nuts also. It idles a little high and not smooth enough until the code 24 starts. (I'll work on that later. ) Then it revs up to somewhere between 1500 and 2000 RPM. If I am driving at the time stopping and waiting for a bit seems to allow it to subside. The change in speed is very fast. It's almost like a switch gets thrown. It seems to happen about, not exactly, when the ecm switches from open to closed loop and back.

The question is do I buy the new VSS, about $69-79, and see if that fixes it? Am I missing something? Should I sell the car to the first person with a few dollars to spend on a car close to 20 years old?

FYI-I have thus far: repaired all vacuum leaks I can find, replaced the engine temp sensor for the ecm, replaced the IAC, cleaned the worst of it off of the throttle body plate, set the timing and intial engine speed. I replaced the ignition module, pickup coil, plugs, wires, rotor, dist. cap and probably a bunch of stuff I don't remember right now. The car wasn't used for a couple of years before I started all this.

 

The sender on the speedo head sends a low level signal to the VSS buffer module if my diagrams are correct. Later TG's use a different ECM with the VSS sender in the trans. You will need to check for pulses into the buffer module before condemming the head. VSS controls EGR operation and can screw up driveability.

 

Thanks Danno

Your understanding of all this is a little better than mine.

The VSS is on the dash on my beasty. It's a little yellow box with three leads going to what I believe is a light emitter and receptor fastened to the back of a hole in the speedometer case. Inside the case there is what looks like a shiny metal propellor(flat) that I think serves to make and break the light beam to time the pulses. There are three leads from the yellow box to one of the connectors for the instrument panel. I've had that in and out so often I can probably pull and reinstall it in about five minutes. Crazy stuff!

I've been checking the VSS through an AutoXRay Scanner that is grabbing live data. If I drive down the road with the scanner doing live monitoring the speed readout seems to match the speedometer readout. That's where it gets wierd. If it's accurate to the scanner you'd think it would be accurate to the ecm.

I checked the wiring---I think I got it right. The connectors weren't color coded exactly right but maybe the diagram was off. In any case the connection I was looking for was from the VSS to the ECM plugs. I did only find one going to the ecm. I think that's what there is unless I miss my guess. If there are others then that might be the trouble. If I remember right, of the three on the VSS only one is routed to the computer.

You mention a buffer. Do you know if thats integral to the ecm or is there another component that I am not familiar with?

Thanks for the advice on this.

 

The yellow box is the Vss buffer. It reads optical pulses the sends it to one line to the ECM, A10 I think. There a only 3 wires on the early one, the later electric speedo uses a 4 wire unit with a magnetic generator.

 

Thanks again

I have the yellow box with the three wires. I found continuity between the box and the ECM. Not sure at this point which connection. At this point I think I am going to have to order the sensor. They provide it all as one unit. I don't think there is much else can be done at this point. If it eliminates the problems I think it will the price is worth it. If not, it'll be worth it from the point of view of knowing that's not it. It's mostly likely the problem anyway.

I'll post the results of all this. It will be a few days as they have to order it in.