Vehicle speed sensors. 94 k3500

[StevesHDgarage]

Hey guys, I've got a 94 k3500 with a 454. The speedometer doesnt work but the tach does. I bought a vehicle speed sensor for the trans but it looks like there is 2 of them on the driver side of the trans and 1 at the end of the transfer case. The one at the back end of the trans doesnt have a pigtail connected to it and I cannot find one in the wire harness any where. Take a look at the pics below. The forward sensor on the trans was very hard to get a pic of, so it's not very good but that one has a connector plugged in. Any help would be appreciated.

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[stutaeng]

So, some 4L80e units have the input shaft and output shaft speed sensor on the transmission units themselves. Other 4L80e units only have the input shaft speed sensors and rely on the transfer case for the output speed sensor. These are obviously the 4x4 4L80s only.

I've never paid attention to see if my '00 K3500 has the 2 sensors on the transmission or something like what you have here... Basically, what I'm saying is the speed sensor that has nothing connected to it is redundant, so that's why it's not connected.

Sounds like you are trying to fix a speedometer?

 

[StevesHDgarage]

Ok. Yes I'm trying to fix the speedometer when I noticed that. It threw me off thinking maybe that was my issue but searching all over the harness I could not find a pigtail for the sensor. Thanks for shedding a little light on that for me.

 

[stutaeng]

Ok, gotcha.

I honestly don't know enough about the GMT400 to know where the speedometer gets it's signal. From the rear VSS? I think the tachometer comes from the camshaft position sensor and I had that once cut out randomly on my 4.3 (along with a SES code.) But never have lost speedometer.

Hopefully someone can chime in. Lots of folks here with invaluable knowledge willing to help...

 

[StevesHDgarage]

Ok, gotcha.

I honestly don't know enough about the GMT400 to know where the speedometer gets it's signal. From the rear VSS? I think the tachometer comes from the camshaft position sensor and I had that once cut out randomly on my 4.3 (along with a SES code.) But never have lost speedometer.

Hopefully someone can chime in. Lots of folks here with invaluable knowledge willing to help...

It would make sense that a 2wd would utilize that rear sensor while the 4wd utilizes the sensor on the transfer case.

 

[stutaeng]

Did the speedo quit on you? Or bought it like that? Does your tranny shift okay?

I would think a faulty VSS would cause the tranny not to shift (ECU doesn't know it moving or how fast is moving?)

 

[StevesHDgarage]

Did the speedo quit on you? Or bought it like that? Does your tranny shift okay?

I would think a faulty VSS would cause the tranny not to shift (ECU doesn't know it moving or how fast is moving?)

I bought it like this. Tranny doesnt come out of second gear. I thought I would start at the speed sensors. The rpm guage does still work.

 

[stutaeng]

I bought it like this. Tranny doesnt come out of second gear. I thought I would start at the speed sensors. The rpm guage does still work.

I think you are on to something there. 2nd gear only is limp mode.

I don't know on the TBI setup, but on the Vortec you hook up a handheld scanner and see what the speed is. Probably the same on the TBI. If you don't see any speed, the ECU would put it into limp mode....

 

[StevesHDgarage]

I think you are on to something there. 2nd gear only is limp mode.

I don't know on the TBI setup, but on the Vortec you hook up a handheld scanner and see what the speed is. Probably the same on the TBI. If you don't see any speed, the ECU would put it into limp mode....

I'm gonna try replacing the VSS on the transfer case and see what happens. It shifts from 1st to 2nd and that's it. Too bad its OBD1 and not 2

 

[PlayingWithTBI]

Too bad its OBD1 and not 2

You can still get a scanner for OBDI and see your speed as well as other stuff.