1978 F250 Ranger, 4WD/C6

This started late last winter, although not as severe nor as frequent. At first I could go a mile or more before it would hit. It can be felt as much as heard; it reverbrates throughout the frame.

I investigated one day by manually rocking the forward driveshaft while parked. I observed that the passenger side outer u-joint was failing, as I could rock the shaft an inch or so either way while watching the joint wobble.

I felt I'd made my diagnosis: the u-joint was turning but binding up every so often.

I needed new rotors and pads anyway--an expensive proposition for me at the time--so I decided to tough it out for the rest of the season by leaving it in 2WD. A risky move here in Alaska, of course, but I made it.

So this past week I managed to scrape up the dough to have the job done: u-joints, rotors, pads, bearings, inner/outer seals, all on both sides. Problem solved--or so I thought.

Now it's even worse.

The first thing I noticed after driving away from the shop is that it's now far more difficult to shift from 4H to 2H (while driving). I have to push hard on the stick and after much pressure it shifts with a loud clunk, not the smooth synchronization that it had before the job. Going from 2H to 4H remains as smooth as it's always been.

The funny thing is that I made it several miles at ~50mph/4WD before the BAM! came back. Now that it's here again, it's more frequent and much more severe. It feels as though something with the motor/t-case/trans is going to just up and fall out. It's LOUD. And it shakes the truck.

I don't know whether this is related, but any time the t-case shifter goes into neutral--such as after using 4L or if I accidentally push it past 2H--I have to stop the truck and turn the engine off before I can do anything with it (the shifter). If I don't, all I get is grinding. It's been this way since I bought the truck three years ago. Is this normal?

I'm going to try reaching the mechanic later this afternoon, but I thought I'd post here as well in the meantime. I wonder if anyone has any ideas I could discuss with him.

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

Sounds like some kind of binding in the drive line. Is there anyway there is two different axle gear ratios front to back? Or possibly different size tires?

 

Silly question: What kind of road surface are we talking? Wet asphalt, solid dirt or gravel, ice, snow, lava, what?

 

My bet it's a broken motor, trans, or t-case mount... It may look okay static but there's only one way to find out and that is to use a jack and different lengths of lumber to apply some vertical force.

 

Lava

Dry asphalt, also asphalt w/ice patches.

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

I had the entire rear axle/diff replaced 2 years ago (pardon me, I bought the truck 4 years ago not 3--time flies so fast) and it went for over a year with no problem. So I figured the ratio was OK, even though the (used) replacement didn't come with a tag. I've learned since that it's possible to discover the ratio without a tag, but since it wasn't giving me trouble I didn't research further into how to actually do it.

If it is a mismatch, wouldn't it have shown up right away?

My mechanic took a test drive with me just now; he also seems to think it might be a ratio mismatch. He's going to look at it sometime this week for me.

But I'm hopeful that HIO Silver's suspicion is correct--that sounds like a whole lot easier fix than swapping in a new diff kit.

Whatever the case, I'll post back here with the result once I know for sure.

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

Oh, I sure do hope it's that instead of the alternative (mismatched ratios)!

I hope to know for sure later this week sometime. I'll post back here. Thanks for the tip. I'll tell him to try that first.

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

Another thing to check is the 4wd shift linkage. If it is binding or out of adjustment it could sort of jump between gears.

As far as ratio front and rear, if they were different, it would fight itself constantly on dry pavement in a straight line. It would be very noticeable all the time I think, not intermittent. And would fail quickly.

 

Not supposed to do much 4wd on any dry, hard surfaces.

 

I would agree--unfortunately we don't have the luxury of that option up here in the winter. The roads stay about 40/60 clear/ice all season long.

ALWAYS solid ice at the stop light, so to get moving at all requires 4WD. Don't know how those little sedans do it, but they do. Zip zip!

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

Bingo, will do.

You might be my new best friend :-)

Thanks,
Jeff Bowman
Fairbanks, Alaska

 

I can't say that it's the only thing that will do that .... but I know one thing that will cause intermittent and increasing "BAMS".

A broken retainer pin or bolt for the cross shaft that holds the spider gears in a open differential will allow the shaft to randomly slip out of the case enough to whack into the pinion gear end on revolutions ..... which then knocks the shaft back into the case and chips the end of the pinion gear teeth where it struck.

Usually when it happens, it's a result of an open differential that st some time spun a long time with one wheel only, the other wheel not spinning ..... like one wheel burnouts. There are no bearings in the spider gears, they are not meant to spin at high speed on that shaft. In that long burnout I mentioned they sometimes get hot and one will sieze to the shaft spinning it while breaking that retainer pin / bolt like a pretzel. Getting stuck on a 2wd car with open differential can easily do same thing. They are just meant to spin slowly on that shaft as you go around corners.

Your truck likely has a locking or limited slip rear and open front differential. Getting it in a serious "stuck attitude" where the rear wheels were both spinning and just one front was spinning for long could also so damage the front differential.

It's a Dana 44, easy to pull the plate and have a look.

Did I read you right , are you using 4WD on hard pavement? Part time 4WD should NOT be engaged on any hard dry high traction surface like concrete or blacktop. Even with same ratios, minor variations in tire pressure, tread wear, loading, and any deviations from straight ahead will cause driveline windup which is a binding. Using 4WD with hubs locked on any high traction surface forces the front and rear axles to spin at EXACTLY the same speed with no variation, yet the front travels further around any curve. The windup just increases, the parts stay under a huge stress. Heat builds, gears get hot, metal wears. Sometimes parts break.

 

That is all or most of your problem. I understand you have patchy ice and snow. I drove in Washington State on wet, icy, snowy, you name it--on standard tires with an open diff, 2wd cars. Well, we had snow tires on the back most of the time....but I digress.

When you run a locked 4x4 vehicle on hard surfaces, the driveline will wind up and that tension will be released from time to time. It will take the form of tire slip with a dramatic bang as you are experiencing. It can also take the form of broken axles or transfer cases.

AWD vehicles use viscous couplings and other magic to allow all 4 wheels to turn at different speeds without damage. Not so your hard-connected 4x4. In addition, you almost always have a minor ratio difference. A 3.50 rear axle will be paired with a 3.54; a 4.10 with a 4.09 etc.

I see while I was diddling around here the prior post covers this as well. Sorry to be redundant.

Get your drivetrain checked, and if your tires aren't the best for ice and snow, consider a better set.

If I can get up a patchy ice hill on a motorcycle, or drive on fresh snow with an open diff and Polyglas GT tires, you can drive in 2wd on decent snow tires--I know you can.

 

If you are running four tires of exactly the same size and have the same gear ratio front and back running with it in 4x4 on patchy ice covered pavement isn't going to hurt anything.... I've done the same thing on every truck I've ever owned and never had a problem with any of them related to doing this and my former boss always ran in 4x4 on his 04 F250 and it was never in 2wd unless I shifted it there.... And then he would be annoyed at me for doing that!
FYI, I am not recommending trying the full-time 4x4, there is a shifter lever for a reason.

 

You got it

Thanks,
Jeff Bowman
Fairbanks, Alaska