4WD will not engage I think

[Crazychopstick]

I've been having trouble getting the 4hi to engage. Haven't taken it off road in 6-7 months. The light won't illuminate and I have tried engaging it at various speeds. But all on paved terrain.

Its a 97, auto, 3.4, with J-shift tc.

Fluid levels are good and clear of debris.
Vacuum is present at the check valve, all lines to and from the diff are in place and intact, the VSVs read vacuum on each separately and switch according to TC position.
The TC also engages properly I believe, since the front DS spins free in 2WD and is locked in 4WD, also low gear functions.
The ADD actuator appears to be in working order, and it will push air on the respective vacuum receptors ( I havent had it on a hand vacuum pump but Im willing to bet the results are the same) it slides easily by hand, as does the ADD sleeve.

At this point I'm out of ideas. Anything else I should check?

 

[TheDurk]

I've been having trouble getting the 4hi to engage. Haven't taken it off road in 6-7 months. The light won't illuminate and I have tried engaging it at various speeds. But all on paved terrain.

Its a 97, auto, 3.4, with J-shift tc.

Fluid levels are good and clear of debris.
Vacuum is present at the check valve, all lines to and from the diff are in place and intact, the VSVs read vacuum on each separately and switch according to TC position.
The TC also engages properly I believe, since the front DS spins free in 2WD and is locked in 4WD, also low gear functions.
The ADD actuator appears to be in working order, and it will push air on the respective vacuum receptors ( I havent had it on a hand vacuum pump but Im willing to bet the results are the same) it slides easily by hand, as does the ADD sleeve.

At this point I'm out of ideas. Anything else I should check?

Are you certain the front diff is not engaging (aside from the light). If you turn while on pavement, you will feel it bind in 4wd. Of course, only do this at very low speed for a test. True 4wd (not AWD) should not be used on pavement.

If the only problem is in fact the light (or lack thereof), you'll want to test the sending switch on the diff for continuity. If it is in fact not engaging, your problem has to be the actuator, given that the vacuum is switching as it should.

 

[Nelsonmd]

And if it isn't just the light, then check to make sure that your vacuum actuator is actually holding vacuum. Mine switched just fine, I had full vacuum on the line, but the actuator had corroded and the seal had failed, even though it looked to be in perfect condition from the outside (i had even removed it twice, cleaned it, and put i back on and still couldn't see that it was corroded). if you have a vacuum pump (I used the HF brake bleeder vacuum pump), then attach it to both the outlets on the actuator and make that it hold vacuum and doesn't leak.

 

[OkiSurf1996]

It could be a jerk like mine was. Spent about 3 hours covering everything in the ADD system...pulled the center console up, darn thing had unplugged itself at the J shifter.

 

[Crazychopstick]

And if it isn't just the light, then check to make sure that your vacuum actuator is actually holding vacuum. Mine switched just fine, I had full vacuum on the line, but the actuator had corroded and the seal had failed, even though it looked to be in perfect condition from the outside (i had even removed it twice, cleaned it, and put i back on and still couldn't see that it was corroded). if you have a vacuum pump (I used the HF brake bleeder vacuum pump), then attach it to both the outlets on the actuator and make that it hold vacuum and doesn't leak.

I went ahead and pulled vacuum on it, it actuates at 15hg/in going both ways and holds that vacuum fine.

Good call to TheDurk on the ADD sending switch. Probably should have been the first place to look. Continuity is open for both positions of the actuator, so that appears to be the problem. I'll update once I get a new switch on and make sure everything works.

 

[Crazychopstick]

It was the switch. Replaced it with Dorman 600-554 and all is well, light comes on when engaged, still need to go offroad to test everything out.


Thanks again guys.