91 corvette
VSS output is good (spun with a drill), measured signal at the ECM connector; Continuity from ECM output to CCM is good; Dash lights up, but speedo reads 0; no error codes
Drove car monitoring with tunerpro, speed=0
Cruse control also stopped working.
Car seems to run and shift fine

Is there a way to check the signal output of the ECM or CCM with a multimeter? Anything at the dash to check?

Bottom line, how do I figure out what to fix?

 

Your failure certainly seems to be the (output) of the ECM towards the CCM although it seems you mention "IS GOOD" - two of the controlled units 1. speedometer 2. cruise don't work - the 3D would be the "speed controlled volume. You need to check the outputs of the ECM. Do you have an FSM? The diagnostic tree is "best I recall" quite straight forward. I believe all that's involved checking the ECM out put is a test light, no DMM required.

What were your results when you spun the VSS with a drill when measured with the DMM? I believe it's where you start again. Maybe check the VSS spinning with a drill but measure at the VSS and not the ECM. When did you loose speedometer/odometer? Just quit or you were doing something maybe related at or to the ECM etc.

 

I'm having this same problem right now. I took my cluster out to fix some broken dash plastic and the speedometer didn't work after I put it back in. That's not 100% true -- it works when it's cold but stops when it warms up. Next step is to take the cluster apart to see if I can figure out what's wrong. Being an electronic engineer can be handy…

For reference purposes: I measured the DC resistance of the VSS at the dash connector (earlier cars have the VSS signal go to the dash first and then to the ECM). The resistance I got was 435 ohms. That verifies that the VSS and wiring are OK. I measured 405 ohms directly on the VSS, so there is some resistance in the wiring and connections.

On my car the output of the VSS is a sine wave (it's basically an AC motor operated as a alternator). The signal from the dash to the ECM is a square wave, so the dash does some manipulating to it. [ EDIT ] I just read that the signal from the VSS is 4000 pulses per mile but the dash divides it in half and the signal to the ECM is 2000 pulses per mile. [ /EDIT ]

I don't know how the later VSS signals work. It's possible that the ECM sends a square wave to the dash, but it could be a digital signal.

I have heard that the newest VSSs put out a square wave and use a reluctor pickup instead of gears. I don't know what year that started.

 

I had a problem with a short in the harness were it went through the dash, ran a new wire.
http://forums.corvetteforum.com/c4-t...nd-cruise.html

 

Mine does it once in awhile and does not affect anything else so it doesn't bother me. Cant' look at the speedo when autoxing anyway.

 

Check your speed buffer, I believe the 91 also have a SB which controls your speedometer and cruise. On the earlier C4 the speedometer will work without a ECM, so that is not the problem (I don't think) but if the buffer don't convert or relay the pulse to the speedometer or cruise, neither will work, just my thoughts.
Randy

 

 

I am back at my spedo problem. I pulled the center console found a bit of a mess. Several wires from the ECM have been replaced. The g/wt VSS wire and the tan serial wire. It is a D- job but the wires have continuity. They also did a real hack job replacing the stereo. It is ugly but not sure why it would not work. All the different modules are just shoved behind the dash.

Anyone have a photo of where these modules go? Trying to figure it out from the FSM but a photo would be nice.

The good part of taking out the dash is I now know where my rattles are coming from. About half the screws were not replaced from the previous job.

Back to the spedo. Wiring is ugly but appears correct. I will do some rewire and get rid of the twisted taped connections, but don't think that is the problem.

Speed buffer is new to me. Any suggestion of how find it and test it?
thanks

 

The output of the VSS is a sine wave. You can measure that with a DVM set to the AC volts mode. According to Gordon Killebrew's cluster book the voltage should vary between about 2 volts and 30 volts, depending on the speed.

The VSS output from the ECM is a square wave. You can measure that on the DC voltage mode. It should be about 1/2 the voltage when there is no VSS input.

 

Anyone know if the speedometer will display with the key on but the engine not running?

I set up a spare VSS and ECM (using a drill to spin the VSS). Without more effort, I can't crank the car with this setup.

I have good varying VAC from the VSS and VDC from the ECM.

Speedometer should read out with the key on, engine not running? Right?

 

I checked all the electronics, everything works.
My dad once told me, if everything works and it still does not work, your checking the wrong thing.

Answer is: transmission is not spinning the speed sensor.
Gear on the sensor looks clean, no missing teeth.

Can any help with a first thing to check in the tranny?

 

There is a retainer clip inside the output housing that holds the drive gear in place on the output shaft. The output shaft has two holes in it and the drive gear can be mounted in two locations. If the drive gear is in the wrong location it will sort of mesh with the driven gear but not correctly and the gears will get chewed up.

Another situation I have heard of is that the clip wasn't installed correctly and the drive gear slipped on the output shaft. To find out if either of these scenarios applies to you, take out the VSS and look into the tailshaft housing. The drive gear should be centered front-to-rear in the VSS mounting hole.

Here's a nice pic that has been circulating for a while that shows the drive gear and the clip quite nicely. I grabbed this image from Google Images, but I don't know where it came from originally -- probably some company that rebuilds transmissions. It shows a speedometer cable drive, but the rest still applies.

 

Anyone have trained eyes that would recognize this gear as, maybe needing replacement?

No obvious problems, but I have never seen one before.

the "clip" is not obviously there, but I cannot rotate or move the gear by hand.

 

trying to find a new sensor gear. count 36 teeth on gear. found many different sizes, but not a 36 teeth. It is a neutral color.

all help appreciated
dell

 

Here's a gear chart I found somewhere. You might have better luck by searching for the GM part number: