1994 Firebird 4L60-E Transmission

[RedEye]

I have a 1994 Firebird that has the 4L60-E Automatic Transmission.

On the same day my transmission quit shifting, (all gears) my speedometer also quit working. All the transmission people tell me I need to re-build my transmission.

I am not buying that the ELECTRONIC transmission that relies on the car speed to shift and the speedometer both going out at the same time being independent problems.

I suspect it is the PCM or the speed sending unit. I do not know exactly how the PCM gets speed information used to shift. Does this input come from the transmission or how can I determine if the PCM is getting the car speed?

Would this information be in the diagnostic codes? (With the car not moving while testing for ?can you read car speed? would this be a test?)

It is my understanding that the speedometer itself is just a analog meter displaying what the PCM tells it to, (not the sending unit to the PCM) the PCM must get an input from somewhere, (thinking straight from the transmission) and that is the part I need help with.

I need to determine if the PCM is getting the speed input it needs on the current car speed but do not know how to determine that, not knowing exactly where that input comes from.

Anyone have a speed sensing problem cause 4L60-E problems?

Any Help would be GREAT!

Thanks!

[LS13RDGEN]

The transmission has a gear on the output shaft. A sensor on the tailshaft reads the gears teeth as a digital signal (ones and zeros) and sends this signal to the pcm. The pcm then sends a voltage to the speedometer on the dash board to display how fast you are going.
First thing I would do would be to check grounds on the block, frame and chasis. Then check the connections on the speed sensor on the tailshaft (make sure it hasn't come loose) then check continuity from the connector to the pins on the pcm itself.
As far as seeing if the speed sensor is actually sending a signal to the pcm, you will need an instrument that can read a pulsing signal.
A DMM might work if you put the car up on jack stands, key on, engine off, splice into the two wires on the speed sensor with the DMM, put it in drive and have a buddy sping the tires or drive shaft. :D

[RedEye]

THANKS!

That is pretty much what I think the problem is due to both the speedo and shift going at the EXACT same time.

I will do the continuity between the trany sensor and the harness at the pcm to be sure it o.k. (as far as you know the wire should not stop anywhere else betweeen the tranmission and the PCM?)

Also since this is a car moving only signal, do you know if a diagnostic will pick up anything if the VSS if not sending info?

Finally someonme that is not telling me to rebuild my transmission!

Can I measure the voltage on the 1-2 shift solenoid? Should this solenoid have voltage when the PCM is telling it to shit into 2nd? Have any Idea what voltage?

Thanks again!

[LS13RDGEN]

If the shift solenoid is not working, it will light up a code for sure.
A connector popped off on me and the truck immediately set a SES light.
The VSS can trigger one of 3 SES codes if I remember right.
P0500 VSS Circuit
P0502 VSS Low input
P0503 VSS Dropped to fast

As far as I can remember, the VSS wire goes right to the PCM and doesn't stop anywhere.
Might want to hunt down a pin diagram of the PCM so you will know what VSS wires go where. :D

[RedEye]

Well I got my Transmission going again!

The problem was a short in the wire between the VSS and the PCM. There are 2 wires Purple and Yellow coming from the VSS on the transmission. These should go to pins 29 and 30 on the PCM. I ohmed them out and found the purple one was not making it from the transmission to the PCM. So I ran a new wire from the PCM to the transmission and the transmission now shifts like new and speedometer works as well.

Seems the 4L60-E needs the vehicle speed to the PCM to shift correctly, and that 1 wire was the whole cause.

Before my transmission went out I did not even know I had n electronic transmission or an electronic speedometer. But now I do.

I just wanted to thank everyone that gave me suggestions and helped me track this problem down. THANKS!

[LS13RDGEN]

Gladd it was easy to fix. That's always a blessing. :D