Driveline "clunk" or "bang"

[poezur]

I have a 1st gen Expy, since new. Been a great truck, always taken care of. Anyway here goes, I know its hard to do surgery over a forum...
I have a "clunk" sound when 4wd high is engaged at speed. It is not constant, but when it does it it sounds awful. I only use 4wd high when it snows and the roads are covered, vs the auto 4wd. I have always been a believer that when the auto 4wd kicks in via a slip, it can't be good on the parts. Anyway, this is a straight line sound, around 40 to 45 mph. You engage the 4wd high and drive up to speed then a loud hard clunk. All tires are in 4wd, but its a scary enough sound that I immediately disengage the 4wd high. Also, I never just turn it from auto 4wd to 4wd high on the fly, I always engage it from a stopped position. In other words, I have never abused this truck or driveline. Any ideas? I have never changed the diff fluid and there is close to 190,000 on the truck. Ideas appreciated...

 

[FordandPolaris]

It sounds almost like the transfer case is not engaging initially. 4H is a mechanical process. Maybe change the fluid in the transfer case and start there. Then you can see what the inside is looking like as well.

On a side note, the A4WD is a magnetic clutch driven system, and is perfectly safe to use in snow. I have 220k with A4WD used in snow and heavy rain and have never had a problem. In fact, 4H is hard on the front components when making turns if both wheels are getting traction. Binds up the front diff since the wheel hubs are locked in and trying to spin the same speed...

 

[toms89]

I have a 1st gen Expy, since new. Been a great truck, always taken care of. Anyway here goes, I know its hard to do surgery over a forum...
I have a "clunk" sound when 4wd high is engaged at speed. It is not constant, but when it does it it sounds awful. I only use 4wd high when it snows and the roads are covered, vs the auto 4wd. I have always been a believer that when the auto 4wd kicks in via a slip, it can't be good on the parts. Anyway, this is a straight line sound, around 40 to 45 mph. You engage the 4wd high and drive up to speed then a loud hard clunk. All tires are in 4wd, but its a scary enough sound that I immediately disengage the 4wd high. Also, I never just turn it from auto 4wd to 4wd high on the fly, I always engage it from a stopped position. In other words, I have never abused this truck or driveline. Any ideas? I have never changed the diff fluid and there is close to 190,000 on the truck. Ideas appreciated...

My guess is you are hearing the clutch release torque built up in the drivetrain.

I would not recommend leaving in 4hi on the road unless it is completely snow covered. (No pavement showing at all) And slippery enough to be necessary. There is nothing wrong with switching between A4wd and 4wd while driving. Owners manual recommends keeping it at speeds less than 45mph. With snow covered roads I will switch to 4wd when I need to accelerate from a slow speed or think I need it to negotiate hill. If I get up to speed or am on partially covered roads I turn it back to A4wd as to avoid any build up of torque in the drive train.

There is no differential between the front and rear axles so any difference in tire size or going around slight turn can bind up the drivetrain as the two axles (front and rear) are turning at slightly different speeds. This will be more pronounced at speed.

Leaving it in 4hi on the street is likely doing more damage than good. Unless the wheels are able to slip it will build up torque in the drivetrain that must be released somehow. Hopefully transfer case clutch releases vs breaking the weakest link in the drivetrain.