Hello,


Have an 1985 F-250 4x4 with a Dana 44HD front end. Looking at the 1985 Owner guide under the Transfer Case section the following statement is found: "Do not drive in 4H or 4L on dry hard surfaced roads". Later in the 4WD operation section it says "Do not use four-wheel drive on dry, hard surfaced roads"


I would have though that 4H would have been the preferred mode if pulling a load and the additional traction is needed such as ascending a hill or road driving in heavy rain or icy conditions.


Can anyone explain why you should not drive in 4H on hard dry surfaces?


Thanks


Rgds
nonrev

 

TC's like the 1345 do not have a center differential. The front and rear drive shafts turn at the same speed all the time when you are in 4H or 4L. As soon as you turn a corner, or if your tires are slightly different sizes, then you will begin to experience driveline "windup". This can be relieved by tire slip on slippery surfaces. Or by broken parts on hard surfaces.

 

OP ...


The very last thing you want to do is have it in 4WD with a heavy load and turn a corner, you'll break stuff, most likely the D44HD. It's about traction vs. speed differential front to back. When you turn the front has to go further then the rear, the transfer case can't make up for this like the axle differentials can. Thus if you have enough traction something will break. The good news is that a vast majority of the time you won't have enough traction.


I've driven down snowy highways at 60 MPH in 4WD for hundreds of miles without issue but this was also with a light load and a D60. But it's also a bad habit. I've had a few vehicles with spooled axles(no differential) which has the same effect. This has proven to be that axles can't handle the repeated stress. For example I hauled a heavy load with a truck that had a spooled rear axle. I was REALLY worried I'd break a rear axle hauling that load, I had another with me. But I didn't, instead it broke a few months later when I had no load. I figure I over stressed it when I hauled that load and it just took a while for that stress to snap.

 

Great... good info. I had no idea of the mechanical forces/stresses involved but now that you describe them its clear. I'm thinking since my wife and I will be pulling a vacation trailer that it MIGHT be OK to go into 4H when needed such as climbing a hill IF the road surface accommodated the occasional tire slip to relieve stress (such as slippery from snow/ice or very wet) and even than not for very long and only on a straight run.. Still chancy though I guess.


One question. I've installed a Detroit True-Trac in the front Dana 44HD. Wouldn't this provide some relief from those stresses if the shaft torque got too high on one of the front 4WD shafts?


Well, a second question also. The truck currently has a Sterling 10.25in rear. I plan on rebuilding that sometime this summer/fall. Can you recommend a differential that would help relieve some of these stresses or would it be best to stay with the Sterling.


Thanks again... good info

 

4wheel drive on a straight dry surface will not hurt anything for short distances. it is when you try to turn that you will run into problems.

 

Add in the fact that your front vs rear axle ratio's are close, but not exactly the same. A common example is 3.50 and 3.54 ratio's. Doesn't sound like a big difference but the farther you move, the worse it is out of sync. Four wheel drive is not all wheel drive and should not be used on dry pavement.

 

No issues with that, just avoid turning sharp when all 4 have good traction.

No it won't help relive the stresses, in fact technically it will make them worse. Traction differentials prevent the tire with the least traction from spinning and using up all the torque. But to relieve that tension that is exactly what you need to happen. Thus some of that stress is transferred to the axle with more traction increasing the odds of a break. That said a true-trac isn't going to transfer much with the short duration stresses of relieving the stress from a turn. So I wouldn't sweat it to much.

Absolutely keep the 10.25 it's a fantastic axle, no reason to swap to anything else, none at all. That said there is a better version of the 10.25, ones built after 93 IIRC have a stronger/longer pinion. Your truck did not come stock with a 10.25 so someone put that one in. Before investing in it it would be a good idea to find out if you have a pre or post 93 and considering finding a post 93 to invest in if you don't already have one.


Differential wise, open is going to be best on the stresses but I'd suggest another true-trac.


EDIT
One final note, you said you have a 44HD up front, a stock TTB I assume? Is this truck a regular cab, are you sure it's a 44 not a 50? If it is a 44 in some ways that is a good thing in regards to what you are asking here. With a 44 and the stock hubs you can be rather sure that the weakest point in the 4WD system is the hubs. This is a good thing as it's rather easy to carry spares and replace one quickly anywhere. This is one of the things I do, I have a D60 in my F350 but the same thing applies, I do not upgrade the hubs, I use stock hubs and carry spare stock hubs. Think of them like a fuse in your 4WD system.

And when I say stock hubs, not the automatic ones, the Ford manual ones with the red plastic center ****.