Need some help

[mwolf380]

I have a 2003 Chevrolet Silverado 2500Hd 4x4 with 6.0 LQ4, 4L80e and manually shifted transfer case. Burned up the transmission so I picked up a used 4L80E out of a 2004 Suburban 2500 with the 6.0. 

 

Well finally get it installed and during the installation, had 1 plug I could not figure out where it went to. Well after scratching my head for a few days discovered the replacement transmission is missing a speed sensor. Swap it over from the old transmission.

 

Take it for a drive and no speedo. Anyone run into this before? 

[davester]

Speedo is run by the sensor at the back of the transfer case.  IIRC from when I converted my truck from 2 to 4wd, the 2wd 4l80e has a sensor at the front and a sensor at the rear, and when converting it over, the sensor on the front is still used/plugged in, and the sensor in the rear isn't used anymore, the plug that would go into it, instead plugs into the 4wd harness, which then plugs into the sensor at the rear of the transfer case (it's basically a short extension cord to that sensor).

[mwolf380]

The harness in my truck has plugs for both front and rear on the transmission. Also none of the harness is modular, it's all one piece. So there's no 4wd harness per say, it's just part of the engine harness.

 

The truck the used trans came out of was the exact same drive train (engine and transmission) other than being a year newer and a suburban not a silverado.

 

 

[davester]

Having a '06 2500HD, and from doing a lot of research on how the 4wd setup works on that generation truck, yes, it is modular.  The 4wd harness goes from the front diff,  up to the engine fuse block, where it plugs into the main truck harness and has ground point, then back to the the transfer case, and makes a U going forward to plug into the end of the harness going to the transmission (namely, the plug what would go on the VSS at the end of the transmission if it was a 2wd truck).

 

It plugs into the rear of the transfercase so that it can read the actual vehicle speed.  If it used the transmission output shaft speed, it would need another sensor to know whether the transfer case was in hi or lo (which it doesn't have).

[mwolf380]

Ok, this is my first Silverado. I dont have any plugs on the rear of the transfer case. I will have to look again. 

[davester]

Here's a pic of the transfer case:

The black plug at the rear is the VSS plug, the front, white plug is the 4wd sensor.

[mwolf380]

I think you may solved this for me. I will respond tomorrow if that is the case. 

[mwolf380]

So still baffled. However I did discover this plug. Any idea what it's for?

[mwolf380]

Davester, thank you so much, that last post helped so much. I figured out that the plug I have a picture of above went to the plug for the second speed sensor! Plugged in and issue is fixed! Thank you so much again.

[davester]

Second speed sensor?  The sensor on the front of the TC, above the manual shift lever, is the 2wd/4wd mode sensor.

 

There's no VSS sensor on the rear of the transmission (or at least, it's not used, my '06 has a plug that goes into it instead of a sensor).  The harness along the transmission plugs into the transfer case harness instead of where that plug goes.

[mwolf380]

So....the transfer case harness had the second plug off of the VSS in the transfer case that just looped back to the plug that would have plugged into the 2nd VSS on the transmission, had it been there. 

 

But as soon as I plugged that plug on the transmission harness to the transfer case harness, the speedometer came to life the abs light went out, and the service brake system message went away. The transmission now shifts properly too.

 

Thank you again for your help. Doing all this work (new engine install, transmission replacement, new fuel lines, etc) all solo I was bound to miss a few things.