Blazer - 2 WD light not on and transfer issue

Blazer 1999, 4wd 4.3l, 4 dr
I have noticed for some time that when I am in 2 wd the 2 wd light wont come on. The light is good as if I change from 4wd hi or 4wd lo I see the light come on. It just turn off after a bit of driving. Im not sure if this is an indicative or a problem or just something a bit loose.
Also, when I use either 4wd when I shift back to 2wd the truck still feels like it is in 4wd. It is not until i drive a bit that there is a big loud thud that you can feel and hear as well than I can tell the truck is in 2wd. I will think this is an issue with the transfer but before I start taking things apart I want to hear about what you experienced guys have to say about it. That way I will know what to look for or even have parts ready for prospect resolution.
Thanks!!!

Re: Blazer - 2 WD light not on and transfer issue

Dont start gutting your X case yet. Your 4wd system is 'part time' not 'full time'. That means that you should not drive it on hard surfaces in 4wd. It is shifting in 4wd but thats causing "drivetrain wind up". This is harsh on U joints, the Xcase,,,,,,,,,actually everything from the Xcase to thw wheels.

When you shift it back into 2wd the system isnt "free" its bound up and cannot release itself. The drive train has to equalize before its able to let the front axle go free.

Try this, shift it in 4wd and wait for engagement. Then while driving, knock the transmission in N and then click the 2wd button and watch how much more quickly it shifts into 2wd with no thud. Once it shifts back to 2wd, then drop the transmisssion back to D.

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Now for the light.

You are using two modules for this. The PCM (powertrain control module) and the BCM (body control module)

The PCM is doing its job by shifting the X case once told to by the BCM. The BCM gets its command my you pushing the button. The light is controlled by the BCM once the PCM confirms its done its job by shifting the X case. The lights blink while its working then goes solid once its confirmed. More than likely its a BCM issue but not worth the $$ to diagnose. You could pay about $50 bucks for the dealer to relearn or reflash the BCM but thats no guarantee. You could attempt to let it relearn the existing program by disconnecting the batter for 5 minutes allowing the Modules to loose their soft memory or learned memory.

Give it a whirl and let us know.

Re: Blazer - 2 WD light not on and transfer issue

sportsmanphil said:

Try this, shift it in 4wd and wait for engagement. Then while driving, knock the transmission in N and then click the 2wd button and watch how much more quickly it shifts into 2wd with no thud. Once it shifts back to 2wd, then drop the transmisssion back to D.

.

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I've been doing this for 5 years on the Blazer, 3 years on the Explorer, and 1 year on the Trailblazer, its very easy on the drivetrain and shifts almost instantly. Smooth and easy means it might last until I can afford another one. LOL.

Re: Blazer - 2 WD light not on and transfer issue

Nandy, we had a similiar issue with a 1988 S-10 pickup. Ours was not shifting out of 4wd either. On those trucks, that is a vacuum operated cable, not electric, like the newer trucks and the full sized.
So, try it phils way, and if it doesn't seem to help, open the hood and look for a round ball that is a vacuum canister, in the left front corner area.. About the size of a softball, as I remember. Unplug the hose from it, and plug the end of the hose. (We would put ours on the battery bolt, which is very close). If that causes the truck to come out of 4wd, then that's your issue.

Re: Blazer - 2 WD light not on and transfer issue

TilliamWe said:

Nandy, we had a similiar issue with a 1988 S-10 pickup. Ours was not shifting out of 4wd either. On those trucks, that is a vacuum operated cable, not electric, like the newer trucks and the full sized.
So, try it phils way, and if it doesn't seem to help, open the hood and look for a round ball that is a vacuum canister, in the left front corner area.. About the size of a softball, as I remember. Unplug the hose from it, and plug the end of the hose. (We would put ours on the battery bolt, which is very close). If that causes the truck to come out of 4wd, then that's your issue.

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Two parts to a Blazer 4wd system. The system you are talking about is what locks the front axle but does not shift the x-case.

A 1988 "should" have used the thermo unit (a heat element) and not vacuum, but I wouldnt bet the farm on that statement. The 80's Blazers (my fav of them all) mostly had a manual X case shifter in the floor. Once the X case was shifted, the shift rod in the X case pressed a vacuum switch which took engine vacuum and sent it to an accuator which either turned on the thermo lock unit (or pulled on a vacuum accuator) which caused the front axle to lock to the now powered front drive shaft. So why have a vac or thermo lock on the front axle? This system prevented the need of manual locking hubs.

The thermo units could be ran to a switch on the dash which would allow the user to put their Blazer in 2WD Low range.

The problem with the thermo unit was in very cold weather the thermo unit would/could cool enough to disengage the front axle forcing it back into 2wd; which is why GM switched to the Vacuum accuator in the late 80's. I thought that happened after 88 in the Blazer but I could be wrong on that.

Re: Blazer - 2 WD light not on and transfer issue

sportsmanphil said:

Dont start gutting your X case yet. Your 4wd system is 'part time' not 'full time'. That means that you should not drive it on hard surfaces in 4wd. It is shifting in 4wd but thats causing "drivetrain wind up". This is harsh on U joints, the Xcase,,,,,,,,,actually everything from the Xcase to thw wheels.

When you shift it back into 2wd the system isnt "free" its bound up and cannot release itself. The drive train has to equalize before its able to let the front axle go free.

Try this, shift it in 4wd and wait for engagement. Then while driving, knock the transmission in N and then click the 2wd button and watch how much more quickly it shifts into 2wd with no thud. Once it shifts back to 2wd, then drop the transmisssion back to D.

***********

Now for the light.

You are using two modules for this. The PCM (powertrain control module) and the BCM (body control module)

The PCM is doing its job by shifting the X case once told to by the BCM. The BCM gets its command my you pushing the button. The light is controlled by the BCM once the PCM confirms its done its job by shifting the X case. The lights blink while its working then goes solid once its confirmed. More than likely its a BCM issue but not worth the $$ to diagnose. You could pay about $50 bucks for the dealer to relearn or reflash the BCM but thats no guarantee. You could attempt to let it relearn the existing program by disconnecting the batter for 5 minutes allowing the Modules to loose their soft memory or learned memory.

Give it a whirl and let us know.

Click to expand...

I have never fully believed the "shift on the fly 4WD" thing. I have always either come to a complete stop or let my foot off the gas and let the truck coast for a couple of seconds when shifting in or out in any 4WD vehicle. My thought was the same as yours.

Re: Blazer - 2 WD light not on and transfer issue

I agree with you to a point. Which bings me to the next step of this saga.

Some X-cases have whats called a viscous coupling. All 'full time' cases have this and some part time cases; which allow them to be driven in 4wd on hard surfaces.


The viscous coupling acts as a center diff, allowing more than 50% of power to be sent to the front or rear axle. This is great for road manners in 4wd but sucks if you need more power making it to where it counts off road.

Also the viscous coupling will ware out and start acting up in full time suv/trucks like the Cherokee's.


Personally I would rather have the clunk, bad road manners and a more strong and simple X-case.

Re: Blazer - 2 WD light not on and transfer issue

Thanks for the input guys. I do not use my 4wd if in not in the mud and in need to. Gent, this not a clack or click, when this THUD happens passengers is the blazer jump of their seats and start freaking out. I tried the "shift in the fly" process today with my unaware friend in the passenger seat (he is an avid 4wd driver) and he freaked out when he heard and felt it. Picture some one slamming his fist in your dash, that is how it feel. Maybe I need to make a video. This started after I left that blazer unused for over a year since I had the 4runner. Before that you could only hear a small click when moving between 4wd and 2wd. I have always stopped to make the shift as if I recall right that is what the manual says.

Re: Blazer - 2 WD light not on and transfer issue

my F150 4X4 says to put tranny in "N" shift 4X4 then put tranny back in drive
let her roll about 10 feet to engauge front end. to take it out of 4X4 just the reverse of above.to release the auto lock hubs back up about 10 feet.
but my ford has a stick for 4X4 not a button

woosterken

Re: Blazer - 2 WD light not on and transfer issue

Shizzy said:

I have never fully believed the "shift on the fly 4WD" thing. I have always either come to a complete stop or let my foot off the gas and let the truck coast for a couple of seconds when shifting in or out in any 4WD vehicle. My thought was the same as yours.

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I can assure you that until we developed that vacuum issue (and I assure you it was a vacuum system Phil) on our 1988 S-10, it would indeed shift on the fly, exactly as described in the manual. No clunks, no clicks, it just worked.
The loud clunking you are experiencing is definately not a good thing. Can you tell if it's in the transfer case or from the front differential. We all know it's not at the hubs, cause they are engaged All the time. The in out for the front axle takes place in the front axle differential.

Re: Blazer - 2 WD light not on and transfer issue

How could I tell? They are in the same general area. You think having someone walking by it will help? I don't have a way right now to lift all 4 wheels. Not sure if the sound will still be that bad or even happen that way... Suggestions?

Re: Blazer - 2 WD light not on and transfer issue

Nandy said:

How could I tell? They are in the same general area.

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Not to me they aren't, but i know what you mean.

Humor me, okay? Get it out of 4 wheel drive. Then find that vacuum hose I talked about. Unplug it. Then pull the transfer case lever back into 4 wheel high. This should isolate the issue to the transfer case, cause the front differential will not "lock" into gear. Drive the truck like you normally would, then take it out of 4wd and see if you get the huge clunk. if you do, it's probably the transfer case. If you don't it's the front differential.
I'll bet a nickel that it's the front diff, not the transfer case.

Re: Blazer - 2 WD light not on and transfer issue

Sounds like a plan.

Re: Blazer - 2 WD light not on and transfer issue

That plan may not work.

With out the front diff engaging via vacuum, the you will not get the drive train to bind up which is why its making such a clunk noise.

See, as your OP says, it doesnt clunk going into 4wd, only coming out (post wind up).

JUST BY CHANCE! have you checked your tire sizes? I have seen an incorrect tire size thrown on a car or people that replace in pairs accidently get a set that is larger or smaller than the other (all of which could make the wind up worse)

Have you knocked the trans in N and coasted while before you shifted the X-case back to 2wd yet?

Re: Blazer - 2 WD light not on and transfer issue

But Phil, if the clunk is the front differential coming out of a bind when shifted back into 2wd, it will isolate the problem. I agree, it's not a perfect plan, but it's the first thing to try, if you have no idea what is going on.
Good call on tires. I once saw a Subaru that had two "transfer cases" destroyed, cause the tire store put a 205/70 instead of a 215/70 on it, and the dealership didn't see it until the second gearbox was destroyed.

Re: Blazer - 2 WD light not on and transfer issue

The front disconnect is not really under much stress. More than likely its noise free. Mostly because the front diff is open (letting one tire turn faster than the other) so its cant get bound up. But I do understand the idea behind the test.

As for his test theory, you really dont need to disconnect it, but pinch it off. Use two small blocks of something and sandwich the vac line. Then clamp some vice grips over the two blocks. This will prevent damage to the vac line and close it off 100%. Just make sure you do this before you shift it into 4wd.

Now for the noise. IMO I have heard hundreds of blazers shift in and out of 4wd over the years. Some were quiet as a mouse and some were noisy and a handfull would make you think "HOLY CRAP".

A good audible metallic CLUNK is very normal. Something easily heard standing around 70 feet from the vehicle.

I will go dig through some TSB's and see if I can come up with something. However more than likely if the shift mech is binding or striking something its going to require opening up the X-case. (which really isnt all that bad)

Re: Blazer - 2 WD light not on and transfer issue

Nandy,
Have you been on the Jimmy / Blazer forum ???
I will post the link if I can find it again.
My 98 Jimmy with 300,000 km on it works fine in 2WD. I 4WD when we need it during the winter sometimes the indicator light will go out but it remains in 4WD. (you can tell by the way it set's off on an icy road) push the 4WD button again and it comes back on.

Re: Blazer - 2 WD light not on and transfer issue

"As for his test theory, you really dont need to disconnect it, but pinch it off. "

I just say disconnect it, cause when you turn it the other way it fits right over the end of the battery cable bolt, like it was made for it! And ours didn't fall off the bolt, even after much driving, cause someone (okay, me!) may have forgotten to hook it back up once or twice!