99 Blazer 4WD not working, replaced everything

Joe Snyder

Asked by Nov 17, 2015 at 09:47 AM about the 1999 Chevrolet Blazer 4 Door LT 4WD

Question type: Maintenance & Repair

I have been fighting with my 4WD since this summer, trying to get ready for
the winter.  I've poured over countless posts and YouTube videos on it, and
I have replaced everything I can think of.  

The problem seems to be standard - after resetting the Transfer Case
Control Module (unplugging for a couple of minutes), you press the 4WD HI
button, and it flashes, the relays click, then it just reverts to 2 HI after about
10 seconds.  Do that about 3 or 4 times, and it just clicks once and goes
back to 2 HI.

I got the blink code from the TCCM initially, which showed a bad transfer
case actuator.  I replaced that, and same problem.  

I replaced the vacuum actuator underneath the battery tray.  It showed
signs of having fluid in it at one time, so I thought that might be it - nothing.  

I replaced the vacuum valve on top of the transmission.  I re-routed all the
vacuum lines coming from the vacuum valve on top of the transmission -
there are no leaks.  

I read that there was a problem with transmission fluid making it into the
vacuum system, but the other vacuum actuated valves for routing cabin air
work just fine, so there is apparently vacuum available to run valves.  

This version does not have the 4WD actuator solenoid valve like this one:

There is a vacuum resevoir in the driver side fender, which I found the
vacuum line disconnected from.  I thought AH HA!  Finally something visibly
wrong.  I reattached that, and still the same problem.  

Even though there was no corrosion, and I could hear the relays clicking,
as a last ditch effort, I replaced the TCCM.  Sure enough, same problem.  

To the best of my knowledge, it is a 100% completely new 4WD system.  
Does anyone have any idea what I could be missing?  Thanks for your

3 Answers


are you aware that there is a vacuum actuator located under the battery that when 4x4 is engaged it activates the front axle i had the same issue i put the back on jack stands and engaged 4x4 and looked under and the front axle was turning but wasn't engaging into the front so id check that and also look because i noticed that when i replaced the vacuum plug on the transfer case there is a vent side and if you get dirt in that side can really mess things up

Joe Snyder

Yes, I replaced the vacuum actuator underneath the battery tray. I'll double check for any other vents, but I've replaced all the vacuum lines, so them being clogged with dirt already is unlikely.


Only other thing I can think of is if by chan5you put the vacuum hoses on wrong or if it is like mine and splits from the break booster tgere may be a problem in the line elsewhere making in not get correct vacuum

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