Hey Kyle-you're human! I did the same thing...This thread is getting sooooo long that I forgot some of the details. We all have to solve his issues so he can move on to bigger and better things! Jerry

 

The strange thing is the noise it makes when I push the buttons. It's not an unusual noise, just not what I'd expect.

When I FIRST push 4Hi or Auto 4x4, the TCCM does not click. When it switches back to 2Hi (after timing out or if I do it manually), it will click. From then on, it will click any time I try to switch between modes. When I switch between 4Hi or 2Hi (didn't try with Auto 4x4, come to think of it), it actually sounds like the transfer case is engaging. I popped my head out of the car while I was doing it and I heard a quiet "clunk" when I'd press the buttons. This would lead me to believe the transfer case is shifting, but I know that it is not (4WD is definitely not working). I'll take a multimeter to the connector for the switch either later today or tomorrow and post with the results. Based on your previous post, I should not get voltage to the solenoid when in 4Hi? Or just not a full 12V?

 

It should be a normally closed solenoid. As such, it should not have voltage when the axle is not requested to be engaged (ie 2HI or A4WD when not in a slip condition) and have +12V when the axle is requested to be engaged (ie 4HI or A4WD when in a slip condition).

Have you tested the resistance of the switch? I know you said that you replaced it, but did you replace it with a new unit or a used one? That thread up there also lists some trouble shooting guides that may help in your situation.

 

I tested my old switch and it was hardly out of spec (talking only a couple ohms, but I replaced it anyway). The unit I replaced it with was new from Chevy.

One other thing worth noting - the 4WD system does work once in a blue moon for a short while. Over summer, I was on my way to my friend's shop to pull trouble codes and engaged 4WD on the way so that the light would be on when I got there. Go figure, the 4WD system worked fine that day. After making a stop and getting back in the car, it stopped working. Similarly, a month or two ago there was a very rainy day, so I tried to use it and it worked for a while.

When the problem began (shortly after I bought the Blazer last January), it was intermittent - 4WD normally worked, but worked progressively less until now when it hardly ever works.

 

Well I didn't get a chance to test the connector (been really busy at work and we had a lot of snow over the past few days), but the issue has become a little more interesting.

Today I had to go to the store, so I figured I'd try the 4WD system just to see if it'd help me out today. Well, I pressed 4Hi and it began blinking... but the Service 4WD light didn't come on. As I'm driving, I start to realize that 4WD is working, though the light is blinking and not going solid. A mile and a half or two later, the Service 4WD light comes on and I lose 4WD.

On the way back from the store, I tried it again... pressed 4Hi, it blinked, but I had 4WD. Just to test it, I put it back in 2Hi, go back to 4Hi, and the Service 4WD light comes on and it doesn't re-engage 4WD.

I've gone out a couple other times this evening and it worked briefly one other time.

Any ideas?

The only thing I can think of is wiring + something else. When the 4WD was working, the light was blinking... which makes me think it's the device on the TC that sends to signal to let the computer know whether it's engaged or not. And the fact that it works once in a while (under very wet conditions at least) suggests faulty wiring.

As usual, I appreciate any input. It's going to Chevy on Thursday if I can't fix it on my own.

 

Chevy's going to look at it Monday or Tuesday. The problem is absolutely electrical between the vacuum switch and the TCCM - I am able to force 4WD engagement by connecting the two vacuum hoses at the switch with a vacuum connector. I will do this if I need 4WD this weekend (which is likely).

It'd be annoying if the brand new switch is also faulty, but it'd also be a relief if it wasn't the TCCM or wiring.

I've asked this before and nobody answered (and it seems like everyone's done helping me, but I'm going to ask again): if the TCCM were bad, would it be able to throw the vacuum switch code? (I just want to know if a bad TCCM is a possibility, but I'd suspect a bad TCCM couldn't throw any codes.)

 

As stated before, you can test the wiring yourself. If you want to dismiss suggestions and have a dealer fix it for you, that is fine, but it does not change the fact that diagnostic suggestions were made and not taken. To make it a little simpler on you, the TAN wire at the solenoid should be +12V and connected through the IGN3 fuse in the fuse box. The Gray/Black wire should be connected to the F4 cavity on the 32-way connector at the TCCM. An easy continuity test can be performed to determine if the circuits are intact. Before I had stated that the +12V is what was switched. Prior to locating the connector views I have now, that was an assumption I had made based on typical systems. It now looks like the ground is what is switched. Here are those connector details:

Front Axle Solenoid:

32-way connector at TCCM:


A faulty TCCM is a theory, however, testing that is beyond the capability of what most DIY people can do. You can replace it, but you would be spending money on speculation alone. You can take the chance as it probably would be cheaper than having the dealership diagnose the problem.

There are other threads that have had many links to outside references for the NV236/NV246 transfer cases which run through these same diagnostic suggestions.

Ofcourse, testing the wiring may not tell you if the system is functioning properly or not (other tests are detailed in other posts). I believe that when the TCCM commands a mode switch, the engagement of the front axle is the last thing in the process as things are verified. If the TCCM does not sense that the transfer case has been properly shifted, it likely would not continue on with the rest of the steps in the process.

 

I never said I wouldn't test the connector for power, it's just not so pleasant when it's below 0 or snowing and I don't have a garage (I'll be visiting my sister this weekend and I'll be using her garage for further diagnosis). The only question I asked twice was about whether or not a faulty TCCM would throw any error codes at all. If a bad TCCM can't throw codes, then there's no reason to suspect the TCCM since Chevy did pull the code for the switch. (Unless I'm misunderstanding something, the TCCM would be what records and sends error codes to a Tech 2 scan tool.) I'm not going to replace it just for the hell of it, I'm just trying to eliminate things to look at. If I can't find the problem this weekend with a multimeter (those diagrams help, btw), Chevy's only charging me $65 to find the problem... and I'll go from there.

But since the beginning of this thread, I've eliminated at least one likely culprit (the vacuum actuator), and repaced another (the vacuum switch)... since 4WD is clearly capable of being engaged, the process of elimination leaves me somewhere between the vacuum switch and the TCCM and I just wanted to know if I can eliminate the TCCM. The system also began behaving differently, so I thought I'd post about that to see if perhaps you (since you seem to be the only knowledgeable one on here with these systems) had seen exactly that before.

Sorry if you think I'm bugging you with the same question rather than testing the voltage at the connector, but the things I've done and observed aren't instead of testing the connector - I'm just covering all my bases progressively. If there is no voltage at the connector when I test it this weekend and the TCCM is good (which I suspect it is for the reasons previously stated), that leaves a wiring problem... and wiring is something I can do. I'd rather go out there and say "if this happens, then this is the problem... else this other thing is the problem" rather than testing the connector, coming back here, and saying "does that mean wiring or TCCM?"

 

Although I know nothing about these systems....and probably banging my head against the wall...If you can connect those two hoses, on the firewall, above the distributor-and get 4wd....you either have a solenoid issue(As previously/previously stated)or an electrical/ground issue to it. Maybe my previous responses didn't show up...later JP

 

had an 01 in today with a flashing 4hi 4x4 light when requested-no 4wd.(auto 4wd function) Although I know nothing....It had vacuum hooked to the lower nipple on the vacuum acuator/solenoid-GM # 15165891. When 4wd was requested-vacuum DID NOT pass thru is to the upper hose. Most likely could have connected the two hoses and regained 4wd but there is more to this than that. Unplugged the two wire plug connecting to it. As Kyle stated, this solenoid works on reversing the ground so armed with this knowledge...In 2wd/key on, one wire has batt voltage. The other has 5volts. When 4wd is commanded, the 5volt wire becomes the ground. The dash light flashes thru all of this.(4wd light). I never got vacuum thru it at any time until I stole this unit off a 00 Blazer that was in the shop-with the same set-up. As a result...a good unit will 'click'(relay inside/points), my bad one did not. The good one, of course did. Part of this part is a metal case-I thought that there was a possibility of stuck points inside, so I took my handy 3/8's" S and K ratchet and gave the metal portion of the case a 'whack'...and whola-works like a charm now. For how long, I don't know. But I made a note of this in my notes on the vehicle(stock vehicle-for sale at our shop)....we'll see. There you go...JP[:'(]