4WD slips into 2WD

[Caan]

Newbie here,

Just bought a 87 GMC Suburban 2500 4X4. Don't know much about mechanics. Here is a problem I noticed. I decided to engage 4WD and drive it a little. I locked the hubs and then put it 4WD H. I drove about 35 mph. The dash light was on indicating 4WD. Suddenly for no reason the light went out. I looked down at the shifter and it had slipped back into 2WD. I stopped put it back in 4WD and drove another 2 miles. I then made a u-turn, noticed some mild vibrations, and it slipped back into 2WD. I put it back into 4WD H and droove home. I did notice a slight and mild humming noise coming from the undercarriage. Could this be a serious or expensive problem?

[bake74]

     A coupld of things.  The first thing I would check is the linkage for the transfer case.  If it is too tight it would slip out of 4wd.  ( I might be wrong here, if it is electronic controled then it might have other issues ).
     Second, I believe 35 is the max speed you are soppose to drive in 4wd.  The humming noise is probaly normal.  Most all 4wd vehicles make a noise when in 4wd and you drive fast. (even the newer ones make a slight humm )
     If I am wrong about the linkage part on the 87 someone will chime in and say so, otherwise check to see for yourself how you engage 4wd, mechanically or electronic.

[pholliday1]

Right on about linkage. No electronic 4wd on 87 just floor lever. Your burb "should" have new process 208 transfer case. These were good units and were used in most military gm's made  to include the military blazer however military trucks that needed pto for equipment used the np 205. If the 208 has problems its generally linked to low fluid and yours should be checked it just takes simple atf. To determine if you have a linkage problem of or transfer case problem, you should disconect linkage from transfercase  underneath burb and manually put in 4 wheel drive.Then drive it and  see if it "pops out" if it does then the tranfercase is the issue. generally the linkage becomes worn and has "slop" and can be adjusted from underneath. slight hum is normal with more gear noise in 4wd low as for 4wd hi you should be able to drive at normal speeds with no issue.

[thirsty]

I then made a u-turn, noticed some mild vibrations, and it slipped back into 2WD.

Check your tranny mount too.

[DustyRusty]

Caan, you didn't mention what type of surface you were driving on.  If it was a hard surface like concrete or asphalt that could be all or part of the problem.  A vehicle w/ 4-wheel drive should not be driven on hard surfaces w/ the transfer case in 4-high or 4-low.  It will cause binding in the driveline and this could be the reason for it dropping back into 2wd.  Otherwise, I agree wi th the others.

Rusty

[Caan]

Thanks for the quick responses. I'm gonna look into the linkage issue.

If I was driving to fast in 4WD H, is it normal for it to shift back into 2WD? Is that what it is supposed to do? Maybe there is no problem at all. The transfer case looks brand new and shiny. It looks as if it was replaced or rebuilt recently. The previous owner didn't elaborate on any work he had done on the Transfercase. Not sure if it is the original. But if it looks brand new, maybe there isn't any problem at all.

[thirsty]

If I was driving to fast in 4WD H, is it normal for it to shift back into 2WD? Is that what it is supposed to do?

It should not just come out of 4wd unless you take it out.

[Blazin]

X2 on the linkage. I put new motor and trans mounts in an 84 I had years back. I had to readjust the t case linkage after because it would slip out of 4x4 with the new mounts.

[Engineer]

If I was driving to fast in 4WD H, is it normal for it to shift back into 2WD? Is that what it is supposed to do?

It should not just come out of 4wd unless you take it out.

Ditto this.