On a 2002 4dr 4wd with the four buttons (2HI, 4Hi, 4Lo, Auto) system; front diff is not locking.
The acutator is good under the battery.

My question is regarding the vacuum switch mounted on firewall center. I put a vacuum gauge on the actuator side and selected 4Hi, no vacuum. I have vacuum going into the switch. So easy....bad switch....removed vacuum switch and checked to see if voltage is at electrical connector when selected....yep....12 volts....

Applied 9 volts from a battery across vacuum switch to see if it would switch....nothing. So I measured the resistance across the terminals and it was about 88 ohms...which seems high. I then applied 12 volts using Blazer battery and it switched...but I think I got a bit of smoke....just did a quick tap when applying voltage.

Reconnected everything and still no 4wd.

Anyone know what the coil resistance should be for the vacuum switch? Or should I just replace it?

Thanks

 

There is vacuum to the switch.
You have voltage when you select 4WD.
The switch doesn't work.
Might want to check ground for the switch, and make sure the connection at the switch is good.

 

Since I measured a good 12 volts at the vacuum switch connector, I assume that it is good.
I have ordered a replacement GM switch; will see if problem fixed next week.

 

The TCCM switches the ground side of the circuit, not the positive. I would recommend taking Captain's advice on testing for a good switched ground before throwing parts at the problem.

Wiring & vacuum diagrams are in the tech article sections.

 

Looks like I killed the vacuum switch when I applied 12 volts from the battery. Measured the resistance across the terminals.....zero...so I have shorted it out.

I had ordered the replace switch anyways....good thing.

Installed switch today, drove to a patch of loose gravel, pressed 4 HI button....light on 4HI came on steady in about 3 seconds. Yes!!!!

Tried 4LO...quickly switches, light steady.....in 4LO...!!!!!!

Went back to 2HI....felt front diff unlock....yep back in 2HI.

I am good to GO!

Got the switch off of ebay, supposed to be a GM part....appears to be.