4wd system operation 98up Blazers

Pages : 1 [2]

07-10-2011, 08:48 AM
so I had the battery out and checke out my actuator. I can't pull the cable at all. The actuator will hods vacuum if remove the vacuum line & put my thumb on it so the actuator is good(I assume). Now for the million dollar question....How can I fige out if the cable is siezed of if the encoder motor is seized?



My typing is terrible this morning...let me try again...

so I had the battery out and checked out my actuator. I can't pull the cable at all. The actuator will hold vacuum if I remove the vacuum line & put my thumb on it so the actuator is good(I assume). Now for the million dollar question....How can I figure out if the cable is siezed or if the encoder motor is seized?


09-05-2011, 09:57 AM
Turns out this was my problem all along fluid in the switch & lines. I wasteds so much time & money, 3 button switch, TCCM, etc. if I had only checked this first....

Another bit of information some might find necessary:

Since that document was released, the part has been redesigned. The old part was 15664811 and had a green stripe on it. The new part is 89059420 and has a blue stripe on it. You can find more information <a href="http://www.s-10crewcab.com/phpBB2/viewtopic.php?t=3197">here</a>;


09-05-2011, 09:59 AM
What I meant to say was, this was my problem all along....

Document ID# 1350749
2002 Chevrolet S10 Pickup - 4WD

HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components) #03-01-38-009 - (Jun 25, 2003)
HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components)
2001-2003 Chevrolet Blazer, S10
2001-2003 GMC Sonoma
2001 GMC Envoy, Jimmy
2001-2003 GMC Jimmy (Export Canada Only)
with 4WD and Two-Speed Electric Shift Transfer Case (RPO NP1)

Some customers may comment that the HVAC mode switch may be inoperative or stuck in the vent mode. After some diagnosis, oil in the HVAC control head and/or vacuum lines may be found.

When the transmission or transfer case heat up, the oil may expand and find its way into the transfer case vacuum switch. This oil expulsion may cause the vacuum switch seal to fail and allow transfer case oil to be drawn into the HVAC vacuum lines.

Replace the transfer case vacuum switch. Refer to the Transfer Case Vacuum Switch Replacement procedure in the Transfer Case sub-section of the Service Manual. Also, replace all contaminated parts in the HVAC system.

Parts Information
Part Number Description
15664811 Switch -Transfer Case Vacuum
Parts are currently available from GMSPO.

Warranty Information
For vehicles repaired under warranty, use:
Labor Operation Description Labor Time
K4120 Switch, Vacuum Control Transfer Case - Replace 0.6 hr
Add Diagnosis Time 0-0.3 hr
Add With Skid Plate 0.2 hr
Any additional time for replacement of HVAC components must be submitted as Other Labor Hours and requires appropriate authorization.

09-13-2011, 01:50 PM

Here are the assembly drawings for the vacuum system control for the front diff.


This website has tons of GM assembly drawings!!!


11-23-2011, 03:06 PM
OK, but what about us older Blazers?? I have a '96 that I am trying to troubleshoot for what appears to be an uncommanded engagement at highway speeds.

What happens is that while running at 55 mph, occasionally a high pitched whine starts and then gradually gets lower and loud as heck. I pull over down to 10 MPH, hear a loud clunk and all is well.

I pulled the battery, and checked the function of the vacuum actuator and when I start the motor and have someone switch the electrical switches in the cabin from 2wd to 4wd nothing changes. There seems to be vacuum at the actuator at all times. There is an electrical switch that is supposed to control the vacuum right behind the battery.

The question is, when is the switch supposed to supply vacuum to the actuator? in 2wd or 4wd? When choosing 4wd, is that switch supposed to have 12v applied to it? Maybe I'll apply 12v directly from the battery and see if it works.

I should also mention, that when driving in snow last weekend, the 4wd function DOES/DID work.


11-23-2011, 03:12 PM
I would unplug the vacum line from the transfer case and see if fluid comes out of the line. If it does let it drain and replace the vacum switch on the transfer case. It fixed my problem.

11-23-2011, 03:14 PM
In my truck, the vacum pulls the actuator to shift it into 4wd I'm pretty sure

11-23-2011, 11:39 PM
OK, I'll give that a try. I was under it this afternoon and it sure looks like it would take a contortionist to get the vacuum line off the transfer case, but I'll take another look tomorrow from the drivers side. Does the skid plate have to come off?

I see older posts do mention the perils of having fluid in the vacuum lines. That might explain why my heater won't blow on the florr anymore either.

Thanks for the tip. I'll be checking back in a day or two to follow up.


11-24-2011, 06:37 AM
I was having probelms with the with it not changing between floor/dash/defrost for along time, but I never put two & two together. If I had payed attention to what I read on here I would have saved myself some money, grief, & time.

11-24-2011, 10:30 AM
I'm reading through all of you older posts and am getting some good info. However, this morning I lifted the right front tire and started the engine. I was able to rotate the tire freely, which means it is not in 4wd. That seems to contradict the fact that my vacuum actuator is pulled all the time the engine is running. (???) Maybe the actuator is NOT pulled in 2wd. More investigation needed, here.

Nevertheless, I'm going to heed your advice and remove the TC vacuum switch. If there's oil in the system, that's what I need to know.

I might even replace the thing, regardless. If it leaks vacuum, it would explain my heater issue.

On a side note, I saw that there is an electrical vacuum switch that controls the front axle actuator located between the battery and reservoir. I don't see where anyone in these forums say anything about that one. It's a puzzler to me to think there are TWO actuators controlling the front axle.

Your thoughts?

11-24-2011, 10:08 PM
The vacuum switch on the transfer case is located on the driver side, inboard of the driveshaft yoke. In the image below, you can just barely see the three black ports of the switch behind the manual shift lever. If you have electric shift, the encoder motor takes the place of the shift lever and adds another obstacle when it comes time to replace the switch:


One vacuum line comes from the vacuum reservoir under the hood and should have manifold vacuum whenever the engine is running. The second vacuum line runs directly to the vacuum actuator under the battery tray. The third vacuum line tees into the transfer case vent directly above the vacuum switch and then runs up along side the transmission dipstick tube. There is a filter at the end of the line and it's zip tied to the dipstick tube. That line vents the vacuum actuator to atmosphere when 2WD is selected, (no vacuum to the actuator). When 4WD is selected, linkage within the transfer case actuates the vacuum switch. The vacuum switch connects manifold vacuum to the actuator under the battery. The actuator pulls a cable that operates a clutch on the right front axle shaft, just inside the axle tube. There is an electrical switch at the clutch that tells the transfer case control module when the axle is locked in and the 4WD light illuminates. If the vacuum switch binds and does not completely vent the axle actuator, a slight vacuum will still be applied to the actuator and cause it to partially engage the clutch on the axle. If/when this happens, you hear the noise you're hearing. If you select 4WD, and the switch is not totally seized, manifold vacuum will be applied to the actuator and the noise will stop. The switch is under $20 and the easiest way to get to it is reach over the frame crossmember from rear to front. IIRC a 7/8" deep socket and a 3/8" ratchet is the ticket. Gotta hold your tongue just right and cross your right eye, being a contortionist helps too ;)

11-24-2011, 11:42 PM
Thanks for that complete description and photo. I have the electrically controlled device, so I suppose I'm in for a treat tomorrow when I replace it. Also, your "partial engagement" theory makes sense.

Next question - Then what does that electrical vacuum switch do that is in-line with the vacuum hose going from the actuator to the TC? It's located between the battery and reservoir on my '96.


11-25-2011, 08:33 AM
Can you post a picture of it? Starting in, (I think) '99 there is an electrical vacuum switch on the 4 button systems mounted on the firewall above the distributor but I don't believe it was used on '96, unless someone added it for some reason. There might have been a factory TSB for it, not 100% certain of it though. Trace the vacuum line from the transfer case switch... IIRC the vacuum line to the actuator runs along the passenger side to the actuator under the battery.

11-25-2011, 09:52 AM
Eureka!! I just pulled the vacuum hose assembly from the TC switch and sure enough, my fingers were bathed in flluid!! Sure enough, the switch was stuck. Does the transfer case use ATF? Sure looks like it.

Next question - I plan to disconnect the vacuum ball and actuator from the system and blow out the lines. Will that damage any HVAC diaphrams? Do I need to blow the lines out at all?

I'd like to replace the whole hose assembly, but it seems to run along the top of the trans and I'm laying on my back in my garage floor, so am not keen on messing with it.

I'll send a picture of that switch we're talking about later. Right now, I'm off to the parts store to get a new switch.

11-25-2011, 11:15 AM
The transfer case takes DexronIII. When you have fluid in the vacuum lines, it means vacuum is leaking past the seal in the vacuum switch. When this happens, it creates a negative pressure, (vacuum) inside the transfer case and ends up sucking transmission fluid past the seals lowering the transmission fluid level and drawing it into the transfer case, raising it's fluid level. When the transfer case fluid reaches the vacuum switch, it enters the vacuum lines. Every component in the vacuum system, including HVAC lines, actuators, reservoir, mode switch etc must at least be disconnected and checked for oil and cleaned out if necessary. Do not apply pressure to the HVAC actuators, it will rupture the diaphragms inside them. More than likely the output seal on the transmission will need to be replaced along with the input seal on the transfer case if they continue to leak externally after the vacuum switch repair. On a side note, fluid in the lines can cause the HVAC actuators to operate slowly and sometimes not at all. It can also cause slow operation of the 4WD actuator under the battery.

11-25-2011, 11:43 AM
Wow, trans seals, TC seals, that's more possible bad news than I thought.

I'll check the trans fluid when I warm it up this afternoon. I also saw the hose routing on the driver's side of the trans, and it isn't as daunting as I thought. I may replace the hoses. When I pulled the hose off the actuator yesterday, it was dry. As soon as I figure out how to attach a photo, I'll send one of that other switch I was talking about.

In the mean time, R & R of the switch was a piece of cake.

11-25-2011, 11:47 AM
It is possible that after the switch is replaced and there isn't vacuum inside the transfer case, the seals might not leak, cross your fingers.

11-25-2011, 12:33 PM
Thanks for your help. I tried to attach the photo of that other switch I was talking about, but I think it's a mute point now that the trouble has been found.

You saved me tons of time, sanity and money.

snorander - if you're reading this, your help was much appreciated, too, as it put me on the path to the switch in the first place.


A. J.

11-25-2011, 04:49 PM
Glad there is at least partial success. Warm it up to operating temp and check the transmission fluid level. If the fluid level is not down noticeably, you probably caught it before it sucked fluid through the entire vacuum system. At any rate though, check those lines and clean/replace them as necessary. Then keep a close watch on the transmission and transfer case fluid levels and make sure they aren't leaking.

11-28-2011, 12:21 AM
I'm back. I still am having a heater/defrost issue.

1. No floor heat mode
2. When under a load, the heater switches from vent to defrost, becaue of low vacuum.

What I've done so far:

1. Tested the vacuum canister. No leacks
2. Tested the check valve. Works as it's supposed to
3. Tried to find the heat mode actuator under the dash, but gave up, due to impossible access

I hear no hissing in the dash.


11-28-2011, 03:34 PM
If the air flow changes to defrost on acceleration, (low vacuum) it's an indication of a vacuum leak either in the HVAC system, the 4WD system, or a faulty vacuum check valve. You'll need a hand held vacuum pump and some time and patience to find the leak. Start by applying vacuum to only one of the systems to determine which system is leaking. Then trace the line to locate and repair the leak.

11-28-2011, 09:57 PM
OK, one last question, then.....can you tell me where I can find the thread with instructions on how to remove the HVAC control panel from the dash, so I can check for oil contamination? In the mean time I WILL replace the hoses.

Thanks for your help.

02-28-2012, 02:04 PM
OK, one last question, then.....can you tell me where I can find the thread with instructions on how to remove the HVAC control panel from the dash, so I can check for oil contamination? In the mean time I WILL replace the hoses.

Thanks for your help.

The dash comes apart pretty easy on these, a few screws & bolts, and it snaps right out, well that's how it is on my 2001. Should just be a couple of bolts to pull the hvac out once you remove the dash cover. Mine occasionally doesn't like to switch out of vent mode, it will after a while. At this point I just live with it. It doesn't happen that often so it doesn't bother me anymore LOL. Good luck Shark I know you can do it! Listen to Old_Master he is very wise when it comes to thses S-10's :)

09-21-2012, 01:06 PM
I have a 96 GMC Jimmy 4X4. I can shift into 4LO but not 4HI, the accuator engages the front axle and everything works fine in 2WD or 4LO. Can anyone explain to me why i can't go into 4HI?

11-09-2012, 07:19 PM
i have a 1999 s10 blazer 4x4 with the N1 (3 button)
the diaphram under the battery pulls the cable just fine
the 3 buttons (2hi, 4hi and 4lo) seem to work ok
they make noise when i do each one and the lights flash for a second then stay solid

4lo makes a louder noise then going to 4 hi or 2hi.

if i goto a dirt road and put the 4x4 into 4hi OR 4lo and put my brakes on and step on gas only my rear tires turn

my front dont

can someone help


11-10-2012, 07:45 AM
Welcome to the forum.
I think I can fix your problem in two simple steps.
1. Stop break torquing a 4wd vehicle. You're going to break stuff.
2. Go start your own thread in the proper location.

11-10-2012, 11:34 AM
Welcome to the forum.
I think I can fix your problem in two simple steps.
1. Stop break torquing a 4wd vehicle. You're going to break stuff.
2. Go start your own thread in the proper location.

first of all i do it on gravel
second of all this is the appropriate section
i read the whole 6 pages and there is all kinds of different questions to this thread
i believe mine is the most on topic
so i still waiting for help

01-02-2013, 03:52 PM
I have just bought a 98 blazer 4x4. the 4x4 was not working after reading the above articles & repalcing all the vacuum lines & cleaning the transfer vacauum switch. I was able to get it to lock in. when I put it in some mud I got stuck. The right front was spinning the left front was not. Is this the way it should work or do I have a problem with the left side locking?

01-02-2013, 06:20 PM
when I put it in some mud I got stuck. The right front was spinning the left front was not. Is this the way it should work or do I have a problem with the left side locking?

Normal. The tire with the least amount of traction will spin. Only the right side locks to the differential when you select 4WD. The left side is always connected.

03-03-2013, 05:46 PM
Thanks muddog321. Great description.

My problem was that I couldn't shift into anything other than 2-high. Lights would flash, stop, and I'd be back in 2-high. Problem was the vacuum hose from the firewall vacuum switch to the actuator under the battery. Replaced the hose and I'm back in business. I couldn't have done it without you. Many thanks!

Jimmy Ford II
08-25-2013, 08:01 PM
maddog, One question pls regarding your AutoTrak 4x4 operation write up. I too am having this very trouble. The only thing I haven't confirmed in your instructional is the front axle indicator switch mainly because I don't quite follow that instruction. But, my question is, I've got S4X4 light in 2WD without even attempting 4WD. What of this would throw the light in 2WD? Tx again.

Jimmy Ford II
08-27-2013, 10:06 PM
[2. See if the actuator pulls the front axle cable to engage it. If not and all hoses are good the diaphragm may be leaking on the actuator. If it engages but the light is still flashing remove the indicator switch on the front axle (only electrical plug in there) and manually push it in with the connector plugged in and see if the light stops, if so replace.]

As I understand this muddog's step 2, to test the front axle indicator switch, if I remove it and push it in manually and that works, does that not confirm that the switch is good? and that the axle is not fully engaging so that the differential internal workings are not pushing in the indicator switch correctly?

12-23-2013, 10:46 AM
P { margin-bottom: 0.08in; } Hello I was searching on google and ran across this.
I have a comment for all.

Your TCM can get so jammed with it's own codes, even if it still works, it will no longer let the 4WD system work at all and will just give you a service 4WD light even if you have fixed other problems with the 4WD system.

After any repair attempt therefore clear it out by disconnecting the battery - for 30 min put it back on and then turn the key from off to on (not start) five times before starting. Sometimes this will even fix all your problems if you clean all the connections to the TCM. The dealer can do a reset with their reader. They won't even tell you about it. A lot off check engine shit in cars works this way.

The transfer case shift control module is equipped with the ability to store diagnostic information, which is useful to a technician in the event of module, component or wiring failures. This information can be retrieved from the transfer case shift control module. On the older cars you could ground pin 13 and the codes flashed in 2wd 4wd select leds with a 3 sec interval between them. Later cars like my 1999 Jimmy the basterds took this away from us.

Code "1" is "RAM standby power" and is considered normal, may be caused by clearing the codes.
Code "2" is "Encoder fault"
Code "3" is "TCCM motor circuit"
Code "4" is "RAM/ROM failure"

You mom and pop shops will know less about this system than you do but they will have the $1500 scanner to get the codes. A few will tell free code reading in hopes off getting jobs. Check a bunch off shops to find out but DON'T phone them, this will just piss them off that they had to take their time answer.

Find a single guy independent that specializes in GM and wants your business back if you want an honest fix. The kid that works at the dealer doing side jobs won't have the scanner.

All this being said I did this in my 1999 Jimmy and now my 4WD HI works fine but after driving about 1 mile my Service 4WD light comes on and I will have to stop and turn the car off and on to clear the light and shift back to 2WD. It is very cold in WI so this is good enough for me for now. I suspect it is a vacuum problem since that would defiantly change while the car is being driven. I know it stays in 4WD when the light comes on.

I suspect I might have to reset the TCM again before to long because it will stop working altogether again. I did however clean the connectors to the TCM so maybe not.

02-26-2014, 07:24 PM
I havent asked any questions or anything on here but i would just like to say thanks to everyone who posted answers about this. im waiting for the actuator that goes on the firewall to come in the mil and hopefully that fixes it. if not youll be hearing from me again.

02-26-2014, 08:13 PM
Welcome to the forum!

If you need help, it would be best if you start a new thread identifying your specific problems with the 4WD.

08-08-2014, 06:53 PM
I have a 99 blazer and 4 high work perfectly. The problem is when I engage 4 low it will make a clanging noise then go back to 2wd. Some times it will work perfectly. Any help would be greatly appreciated.

01-05-2016, 08:10 AM
A seal between transmission and transfer case went bad , filled transfer case up into vacuum line and disabled vacuum switch.to check to check fluid level in t case. Hope this helps someone

01-05-2016, 01:45 PM
The vacuum switch on the transfer case controls vacuum for the actuator under the battery tray when performing a mode shift. Selecting a range shift has no effect. Most likely a problem with the TCCM, dash switch, encoder motor, or wiring.

Please check the date before posting in dead threads ;)

07-16-2017, 10:10 AM
My 2000 Blazer had 4WD service light come on this week. It was stuck in 4 wheel auto and would blink the Lo light when you pushed 2WD but it stay in auto mode. I checked vacuum lines were good since many suggested they were a possible issue. I got the Tech2 out and it came back with code B2725 - Automatic transfer case mode switch malfunction. I wasn't sure if this was the switch on the dash or on the transfer case? Then I read a article in the Avalanche forum Google threw at me when I searched code B2725 http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,82307.msg1757910.html#msg1757910
I did same as RSmike, took the switch out of the dash. Popped apart and re-soldered all the connections. Everything is good again and shifting with no error message on dash. If you were not able to solder orthe switch had issues I saw Dorman sold a new switch for $17, Part #901-060. Kevin

07-17-2017, 08:30 PM
To support my repair yesterday I found GM had a TSB 02-04-21-004 on this problem.

Service Bulletin
File In Section: 04 - Driveline Axle
Bulletin No.:
Subject: "Service 4WO" Indicator Illuminated,
(Replace Transfer Case Shift Control Switch)
Models: 1999-2002Chevrolet and GMC elK Pickup and Utility Models
(Silverado, Sierra, Avalanche, Suburban, Tahoe, Yukon, Yukon XL)
with Four Wheel Drive and NVG 236/246 Transfer Case (RPO NP8)

NP8 means 4 button autotrac

Some customers may comment on the Service 4WD
~ indicator illuminating. Upon investigation, the technician
may find DTC B2725 set.
The condition may be due to a intermittent fault in the
transfer case shift control switch.
Important: The transfer case shift control module
should not need replacement. DO NOT replace
P/N 15749703 unless diagnosis points to the module
as the malfunction.
Replace the transfer case shift control switch using
published Service Information procedures.
Parts Information
Part Number DescrIption
15709327 Transfer Case Shift Control Switch
Parts are currently available from GMSPO.
Warranty Information

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