GMT800 VSS Questions

[stan1688]

I’ve been searching the net for this info but haven’t found all that I’m needing and I’d like some advice.

I’m in the planning stages of doing a solid axle swap on an ‘02 Silverado 1500 2wd. From the information I’ve found, I want to run the second VSS in the 4wd adapter to the tcase (swapping in a rebuilt 4l60e) so the computer doesn’t act up in 4low. It looks like I’ll have to add a clamp-on tone ring to accomplish that. I’ll be running a drivers-drop np241 if that matters.

Is that what I should be doing? Do the trucks that come with manual shift tcases still have a TCCM?

I’m turning here since I know there are some individuals who know this stuff much better than me (@ZooMad75 for one).

 

[ZooMad75]

I’ve been searching the net for this info but haven’t found all that I’m needing and I’d like some advice.

I’m in the planning stages of doing a solid axle swap on an ‘02 Silverado 1500 2wd. From the information I’ve found, I want to run the second VSS in the 4wd adapter to the tcase (swapping in a rebuilt 4l60e) so the computer doesn’t act up in 4low. It looks like I’ll have to add a clamp-on tone ring to accomplish that. I’ll be running a drivers-drop np241 if that matters.

Is that what I should be doing? Do the trucks that come with manual shift tcases still have a TCCM?

I’m turning here since I know there are some individuals who know this stuff much better than me (@ZooMad75 for one).

A manual shift t-case lacks the TCCM. All it really has is a shift position switch to trigger the front diff actuator, 4wd indicator and 4Lo input to the PCM.

A driver-side drop 241 should already have a VSS output as those came on trucks that had an electric speedo. Look at the rear output on the 241. You should see it's lacking a speedo cable connection for a simple 2 wire connector in the same place.

To keep the transmission from freaking out you need a 4wd low input. Here's the schematic for a 261 manual shift t-case that would have been an option on a '02 4wd.



You can see in the center the Transfer case shift control switch. It actually has two switches within it, but in your case, with a SAS the second switch to signal the front axle actuator really won't be used. But the other side is just signaling ground when the shifter is in the 4lo position. So you need to make sure your harness has that circuit (it probably doesn't) and you'll probably need somebody with HP Tuners or EFI live to turn on that PID so that the PCM will look for the ground signal to indicate the t-case is in low range.

You really don't have to use a factory switch if you can't find it. But a micro-switch set up to close and complete a circuit to ground when the shifter is in the low range position is all you need.

 

[stan1688]

Yeah, my 241 does have VSS.

How difficult would it be to add a transfer case shift control switch, or a micro switch?

If I did put the other VSS in the adapter before the tcase, that would essentially make it equivalent to a 2wd, right? How would it act in 4low?

 

[ZooMad75]

I don't think the 241 could use the same switch as the 261. You could go an easy way by just putting a switch on the dash to manually ground the circuit when you put the t-case in 4lo. Otherwise, you'll have to build a bracket and put a micro switch on the t-case that the linkage will touch when it is in 4lo.

As far as the VSS goes I don't know why you'd want to run one on the trans if you have the one on the t-case. If you run one in front of the t-case it won't read correctly in 4lo.

 

[stan1688]

I was just getting ready to reply.

To add a switch, it looks like I would need to add a pin in the C2 plug at pin 16 if it’s not already there. Is that right?

I was going to ask if a simple toggle switch could be added, but you just answered that.

 

[ZooMad75]

I was just getting ready to reply.

To add a switch, it looks like I would need to add a pin in the C2 plug at pin 16 if it’s not already there. Is that right?

I was going to ask if a simple toggle switch could be added, but you just answered that.

You got it. C2 terminal 16.

 

[stan1688]

One more thought:

The 241 has a shift indicator. Could that be wired to terminal 16? So the whole process is a little more "stock", and I don't forget to turn a toggle switch off.

 

[ZooMad75]

One more thought:

The 241 has a shift indicator. Could that be wired to terminal 16? So the whole process is a little more "stock", and I don't forget to turn a toggle switch off.

The 241 shift indicator is a two wire unit that only shows the unit is in 4wd, not 4lo.

 

[6.2Blazer]

As mentioned above you want the VSS in the tailhousing (output) of the transfer case, which is the way the trucks came stock. You also need an input to tell the truck it is in 4 lo.

FYI: a guy in our club did a LS/4L60E/NP241 swap in his truck and for some reason decided to install an VSS in the transmission output for some reason. Obviously worked great in 2hi or 4hi but acted very weird in 4lo. Shifts points were way off along with the speedo output, basically the ECM was freaking out because it thought the truck was going almost 3 times faster than actual and it would shift real early. He finally went back to the VSS in the transfer case output. At some point he got a new ECM for tuning but it must have been out of a 2wd truck and didn't have the option for the 4 low input in the programming (he had assumed they were all the same and you could turn that input on and off by software, but was wrong).

 

[stan1688]

Good to know about the programming. Thanks.

I had read the bit about putting a VSS in the adapter from an article in Petersen's. But running it in the tcase is definitely the way to go.

From what I've ready, if the 241 is new enough and has the 3 wire switch, one of them can be ran to the ECM to tell it when in 4lo.