There was a discussion not to long ago about the PN for the electric solenoid, however, I could not find it in a quick search.

If you have vacuum pressure on the input side of the solenoid, and the solenoid is being energized when in 4HI or 4LO (test with a DMM), then the solenoid is bad and needs to be replaced.

 

When you say solenoid, are you referring to the unit on the firewall? If so, that's what I replaced and it didn't help.

Am I correct in now understanding that I don't have the vacuum switch on the transfer case that I purchased? (Sorry for not clarifying that I had the auto 4wd option... I've posted about the 4wd problem before on here and I know I've mentioned it then, just apparently kept leaving stuff out this time.) No big deal if that's the case, the switch was a whopping $7 or so... just want to know if I don't have to get back under the car.

 

By the way, the solenoid does have constant vacuum to the bottom plug (the other vacuum lines runs to the actuator under the battery).

So I think I'm starting to understand the system... the switch, which has vacuum from the engine, opens when the 4wd button is pressed. This "sends" vacuum ("sends" because I know it's technically drawing air) to the actuator under the battery which in turn pulls that little wire which leads into the transfer case (which I assume should control engagement of the 4wd system).

If this is correct, couldn't I test the switch by disconnecting the vacuum line to the actuator, pressing the 4Hi button, and seeing if there is vacuum at that connection? (Considering I replaced the switch, I'd be surprised if it's bad.)

Now I'm wondering if the actuator under the battery is the problem. If it's not, it'll likely by the TCCM or something else, right? (Would the Tech2 Scan Tool be able to pull any codes if the TCCM were bad?)

 

Yes, you can test it that way. And yes, that is what happens. I would do what you stated as well as test the electrical to the solenoid to make sure that it is getting powered properly to perform its function. If it is not, then... Well, it could either be a wiring problem or fuse, but if I remember the wiring diagrams (do not have the time right now to look them up), the TCCM feeds voltage to this so if you are shifting the transfer case, then the fuse is fine. Tomorrow, I will see if I can dig up a wiring diagram for you.

 

Well, to be thorough as to what the system is doing (since I didn't specify everything, rather I picked up where my older posts left off), when I press 4Hi or Auto 4WD (never really tried 2Lo), the light blinks, the "Service 4WD" light on the dash illuminates, and the system never engages. After a while, the switch will basically give up and illuminate the 2Hi light and that'll be the end of it. I can hear a clicking sound from the passenger's side floor board when the switches are being made, so I assume this is the TCCM working (I have never actually looked at the TCCM - I was never good at removing interior panels without breaking the tabs and stuff). I'm thinking now perhaps I should be looking into the common corrosion problem at the TCCM connector. How does one remove the panel that covers the TCCM? If I remember correctly, I found one bolt holding it on and removed that, but it didn't seem to want to budge so I figure there's either another bolt that I didn't see or just a lot of tabs.

Thus far, I have replaced the switch on the firewall and the dash button assembly.

And I know you probably hear this a lot, but thank you very much for you help with this. I can't imaging how much money you're saving me in diagnostics compared to just handing it over to Chevy and saying "fix it."

 

with the automatic t/case there is no t/case switch-the firewall mounter unit basically takes the place of it. This will cost you nothing-if you want try it. I've had it work betcha 100 times. For how long-I can't tell you, I'm sure it varies but....Disconnect the neg. battery cable then the positive. Pull out one or the other battery bolt. Pull the pos. cable. Then touch both together using the one bolt as a 'bridge' to make sure they both make solid contact for 30-60 seconds or so. Then re-hook back up the positive first then the negative. Now try the 4wd-good luck as for the ABS-if you ohm out the wire on the suspected side and it's out of limits then that is your issue but if the wire is not bad I have had 3 Blazers/Jimmies in our shop in the last month where a scan tool showed(tech 2 tool) left or right side MPH readings 'dropping' out(skipping MPH reports) as I sped up and slowed down causing activation of the ABS. Tried cleaning all 3 of these vehicles to no avail. The fix ended up being a new Timken wheel bearing. If you have those same issues I bet you have a wheel bearing in your future-hey maybe Santa can bring you on but ask him for a good quality one or he'll be coming back to exchange it! Keep us posted .JP

 

Peru, have you tried reducing the air gap (filing down the mounting pad) between the sensor and the reluctor ring inside? The TSB I referenced above states that you should test for ACmV across the sensor as you turn the wheel.

 

Hey Kyle-have you personally done it? What do you 'file' down-the magnetic end of the wire?(probably). Per your previous posts(months ago) I had three Blazers/Jimmies in- in the last 2 months and removed the wire(which I saved because it was ok), tried to blow out/clean out down in the now empty hole-the reluctor ring. Reassembled and with the Tech 2 could turn the hub and view the MPH of this side and see MPH drop out-ie readings 3, 4, 5 MPH then skipping to 7 MPH-missing 6-accelerating or on decel. Or on the other vehicle got 3,4 MPH then skipped to 6 MPH. Never two(so far) exactly the same but similar. I have found that this 'confuses' the a'BS'/activates it slightly. When I take a new hub/bearing out of a box, plug it in(the wiring harness), spin it by hand watching the brake MPH screen of the tech 2 that now all the MPH come in-skipping none. I guess in conclusion of this book I wrote....I've had no luck(personally) cleaning-never 'filed' anything though-you've done it? If so-how?? TIA JP PS thanks and this is a cool site![X(]

 

Yes, I have done this (actually just a few weeks ago). You have to reduce the air gap between the reluctor ring and the end of the hall effect sensor. You do this by filing down the surface which the sensor bolts onto in the wheel bearing assembly. In most cases just removing the rust scale that has built up on this surface. You need to go slow enough to get he ACmV into spec. The PCM is actually using the alternating frequency to determine the speed, not the ACmV output. When the ACmV is low, the PCM interprets this as a drop in signal. I do not know what the minimum ACmV value that the PCM requires, but suffice to say that there is a minimum. Reducing the airgap (not to the point where things touch) will increase the ACmV reading.

 

Hey Kyle-just to put this to bed....on the next one I get with this issue-can I either file the bearing surface where the wire assembly mounts or the end of the wire mounting surface? Would either(or both if needed) achieve the same result? Or is filing the bearing surface the better route? You're at Goulds, right? Do you know Rick Spingler-who atleast used to work there and was a good guy-knew him quite well, in the past. JP[&:]