Hi guys,
I'm looking to buy a 99 Blazer from a friend for a farm vehicle.
We were going over it this afternoon and it seems to run well except that the 4WD controls have failed. 260,000km on it (Canada)
My friend says "huh those lights were on last week, I used 4wd in the snow" so we are going to try to get the thing working so I can use it.

There are 3 buttons (NV233 case, I suppose) and they don't light up at all. Not familiar with blazers so I pulled out the dashboard switch module and reseated connectors (no corrosion) and stuck it back in, still no action. Some thumping on the dash while holding 4-Hi got us a click from some relay and a momentary flash of the light.

Reading here it appears that the TCCM is under the passenger kick panel and corrodes, so tomorrow we are going to drive it into a heated garage and try to clean it up. Hopefully this is the only problem.

A couple questions for the forums though:
- Does the truck remember what the last selected mode was or will it always start up in 2-Hi if no buttons are pressed
- 2/4Hi appear to be vacuum actuated, if the vacuum is lost, will I end up in 2-Hi or 4-Hi? Could this be operated just with a simple vacuum switch without the TCCM? I prefer less electronics in charge of critical vehicle systems, personally.
- If the truck is turned off and on with no signal from the TCCM, will the selector motor put the transfer case in low or high gear, or will it remain in the last position it was in?

 

1) the Blazer will re-start in the last selected drive mode it was in.
2) The TCCM sends the signal to the vacuum switch to close which then engages the front differential. If vacuum is lost to the actuator for this, then the front differential will disengage; but I think the transfer case will remain in 4wd.
3) This one I am not sure about, but I think if 4wd is selected and the encoder motor moves putting the t-case into 4wd; then engine shut-off and the TCCM disconnected....the t-case should stay where it was. I would suggest not disconnecting the TCCM with engine running and battery connected.

Also suggest removing the 4wd switch module and disassemble it so all the solder joints can be re-flowed. Several postings on the forum about this. I know it sure fixed my Blazer; had indicator lights not always on and would not go into 4wd every time and when it did it was slow to do so. Cleaned the TCCM contacts...nothing changed....re-flowed all the solder joints on the switch module.....lights on steady, switches to 4wd very quickly now.
Requires some time and need to be careful in taking the switch module halves apart to access the PC board....plastic tabs and all.... but 10 minutes with a soldering iron and then snapping it back together had the results above.

Also the vacuum lines under the hood should be checked....or just replace all of them. Start at the engine and replace one at a time. Also remove the battery, then the battery tray and check the vacuum actuator; diaphragm split or cable to front differential rusted and not moveable. Be sure to check/replace this vacuum line as needed.

Good luck.

 

Thanks for the detailed reply! So the encoder motor is driven in both directions and is not one of those actuator motors that turns to a set point when energized.

Tomorrow became Monday night so I will bring my gas-powered soldering iron and let you know what we find out! Reportedly the 4wd worked fine, just the controls have died, so hopefully the actuator and encoder motor are OK.

I am planning to use this Blazer to pull a trailer 700kms on the highway a week from now, so hopefully we can get it into 2-Hi or I'm sure it will not be happy. Can the encoder motor position be checked with an ohmmeter on the connector on the TCCM, so we know what mode it's actually in if the switch module fails to come back to life?

 

From what I have learned here on the Forum: the encoder motor turns in both directions, normally starts off in 2HI, then goes to 4HI, then to 4LO and then backwards when it returns the t-case to 2HI. There is no ohm meter reading that I am aware of that will tell you what mode it is in. I know that with the encoder motor removed you should be able to tell by the position of the slot that engages to the t-case. I think I saw a picture on here somewhere of what the t-case should look like when it is in each mode....but a search will be necessary.

And you are quite correct, towing on the highway with the t-case engaged will not make it happy or very likely not get you to point A or B....more like somewhere in between.

 

Just one small correction on #2. On a 3-button NV233 the vacuum switch is located on the transfer case and is mechanically operated. When 4WD is selected the the internal fork moves to put the TC in 4WD (via the TCCM and encoder motor). When that happens the movement of the fork pushes up on the spring loaded ball end of the vacuum switch which redirects the vacuum from vent, to actuating the front axle.

 

Yeah...what rockp2 said. I seem to always forget about the three button model t-case and its operation is slightly different from that of the 4 button model.

Just another example of that dreaded CRS problem I have.

 

Before you start soldering on the 4WD switch, Check for your 12V at the TCCM, which is located to the left of the ashtray behind the lower dash. It is a pain to get to, but I had the exact same symptoms (no lights on the 4WD switch, but the blazer drove fine) and I was missing the switched 12V to my TCCM on pin C8). I have a broken wire somewhere under the dash that I am still trying to locate, but I jumped a 12V switched source for temp use and now the 4WD works normally.

 

Thanks for the help guys, we got it fixed!
Pulled out the TCCM and cleaned it and the plug (it was corroded) - this didn't solve the issue but we did get intermittent flickering of the lights now when wiggling the dash module plug. So we cleaned the plugs on the dash module and I popped it open and reflowed all the solder with proper leaded solder.

That was it, it runs through all 3 settings smoothly and quickly.

While we were at it we hooked up my code reader and found codes for MAF and oxygen sensor heater. Cleaned the filthy MAF up, didn't care about the sensor heater (no emissions testing here, thank god) as all it does is slightly speed up getting to closed loop, right?

Everything else looks to be functioning as well as you could hope for a vehicle and engine with so many miles on it. My friend has had no issues with it other than the standard GM issues (water pump etc) and I hope it will keep on rolling for quite awhile longer!

 

Good to hear!