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Old 08-25-2013, 09:01 PM   #91
Jimmy Ford II
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Re: 4wd system operation 98up Blazers

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.
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Old 08-27-2013, 11:06 PM   #92
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Re: 4wd system operation 98up Blazers

[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?
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Old 12-23-2013, 11:46 AM   #93
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Re: 4wd system operation 98up Blazers

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.
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Old 02-26-2014, 08:24 PM   #94
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Re: 4wd system operation 98up Blazers

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.
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Old 02-26-2014, 09:13 PM   #95
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Re: 4wd system operation 98up Blazers

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.
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Old 08-08-2014, 07:53 PM   #96
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Re: 4wd system operation 98up Blazers

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.
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Old 01-05-2016, 09:10 AM   #97
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Re: 4wd system operation 98up Blazers

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
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Old 01-05-2016, 02:45 PM   #98
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Re: 4wd system operation 98up Blazers

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
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Old 07-16-2017, 11:10 AM   #99
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Re: 4wd system operation 98up Blazers

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...tml#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
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Old 07-17-2017, 09:30 PM   #100
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Re: 4wd system operation 98up Blazers

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.:
Date:
02-04-21-004
April,2002
CHEVROLET
,.--.
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

Condition
Some customers may comment on the Service 4WD
~ indicator illuminating. Upon investigation, the technician
may find DTC B2725 set.
Cause
The condition may be due to a intermittent fault in the
transfer case shift control switch.
Correction
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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