1990 F150 4WD *FAILS* compared to 79 F150

I have had a 1979 Ford F150 pickup truck for about 7 years. It was a piece of junk when I bought it, but I did some work to it and got it in halfway decent shape, except for the badly rusty bed/box, which is
beyond repair.
This old 79 truck with it's 400 engine and auto transmission sucks gas like crazy, and the engine is properly tuned. I probably get 8 MPG at the most. Considering the price of gas, and the fact the bed/box is starting to literally fall off the truck, not to mention the transmission seems to not want to engage when I first start the truck, (yet works fine after I get it going), among other problems, I decided it was time to retire this truck as a road vehicle, and just keep it as a farm vehicle until it totally dies.
I am a farmer and use my truck to haul loads of hay and pull heavy trailers, etc. I also live in the north and need something to get me thru snow and ice, or mud on my private farm roadways all year around.
In October I decided it made more sense to buy a newer truck in better shape, rather than stick any more money into that 79, especially since I could not afford to drive the thing with gas being $3 a gallon. I found a fair deal on a 1990 F150 truck with a 302 engine, 4speed manual transmission and fairly decent body. This truck ran well, but needed brake work and the front universal joints replaced. I bought it, replaced the joints and brakes and ended up with a decent truck with a bed/box that would not fall off when going down the road. It preformed well when loaded with hay, and got much better gas milage. I was happy until we got a snowstorm last week. After I cleaned up the deep drifts on my roadway with my tractor, I put the 1990 truck into 4WD and could not get out of my farm. The truck just can not bite into the snow, even with fairly good tires. I finally parked it, and got in my old 1979 truck in 4WD and was out of the farm in minutes.
WTF !!!!!!
Why does the old 79 truck (with worse tires) bite into the snow, while the 90 truck just spins (with better tires). Does anyone have any ideas? Both are in 4WD. All I can figure is the older truck is heavier. I know the engine is smaller on the new truck, but the problem is not engine power, it's the lack of ability to bite into the snow.
However, someone told me that the newer 4WD Ford trucks dont have the same differentials and they lack something, and are not really true 4WD ?????? I dont know if I understand that ?
What's the deal with this? If I cant use this newer truck to get thru snow, I may as well sell it now, because it's not doing me any good sitting in front of my house, unable to use in winter. In fact, my wife's 1989 full size Chevy station wagon gets thru the snow better than this 90 F150 with 4WD.
Just so you know, the 4WD is working correctly on all wheels. While I did my own brake work, I had a shop replace those front univ. joints and they checked the whole 4wd to insure it worked.
I'm puzzled......
George
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Does the 1990 have a limited slip rear axle?
How do you know the 4WD is working?
If you don't have a limited slip rear end, one rear wheel and one rear wheel will spin freely, while the other 2 wheels don't turn (even though equal torque is applied to all the wheels). If you are on an even surface with uniform frictional characteristics, this is not a problem. But if you are on an uneven surface with highly variable frictional characteristics (like soft snow), then this can be a big problem. In this case you are reduced to a situation where the maximum torque you can apply to a wheel on an axle (either front or rear) is the amount applied to the wheel with the least traction on that axle. Because of the way soft snow packs in front of the wheels, it is pretty easy to build a barrier that makes the truck difficult to move. Since only one wheel on each axle is turning, it will melt the snow and make a super slick surface which means the total torque applied to the wheels is nearly zero, which won't move the truck against the walls built up in front of the none moving wheels. With a limited slip rear axle, both rear wheels are turning and can burn through the snow and get a gripe. Back in 1979, Ford also offered limited slip on front axles, so in theory you could have all four wheel turning. Later Fords don't offer limit slip front axles, only limited slip rear axles (and that was an option). It is possible that you have a limited slip rear axle, but the clutch backs are worn out. Pick-up trucks are often worse in snow that cars because they are so light in the rear end. And without a limited slip front axle I can see a case where both rear wheels and one front wheel are spinning without being able to move the truck because the rear wheels don't have enough weight on them to develop enough traction to overcome the snow bank in front of the front wheels (one of which is not spinning). Also, even though your 1990 might have good tires, are they wider than the tires on the 1979? My Father never had any trouble getting stuck with his old trucks, where as my newer trucks can get stuck on wet grass - mostly because the wider tires tend to float on the surface and not cut down into the mud or snow.
My advice is that if you really want to go on snow - get chains.
Ed
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On Wed, 26 Dec 2007 20:42:17 GMT, "C. E. White"

Thanks for the advice. I dont know if I have a limited slip rear end. How do I find out? I know the 4WD is working because the mechanic tested everything and showed me all wheels were turning when the truck was on the rack. What he fixed were both front universal joints and ended up changing on of the lockouts in the process. I know chains help, but having to install and remove them several times every day when I leave my private road and enter the highway, is not anything I am willing to do in the cold. If I do not have this limited slip rear end, I wonder if I can change the rear end from an older truck that has one. Otherwise I guess my only other option is to sell the truck and find another older one in good shape.
I'm glad I kept the old truck, or I'd be stuck here. I must go uphill to exit my farm, and my roadway is about one mile long before I get to the county road. We keep the wife's car up near the road and drive the old truck to get to the car, then take the car. It's a pain in the ass, but it worked with the old truck. It's not working with the new one.
Is there a way to ID what rear end I have? Will the rear end from another 79 F150 fit in a 90? I know where there's an identical 79 F150 to mine, with a blown engine. I was going to buy it from the guy for parts before I decided to get a newer truck. I think he still has it.
As for the tires, the older truck has 235 75R 15, the newer one has 225 75R 15. Basically the same thing.
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There should be a vehicle certification plate on the edge of the drivers door or in the door jam. There will be an axle code listed. If the axle is a limited slip type, the axle code will be one letter and one number. If the code is two numbers, the differential is an open style (not limited slip).
Ed

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well just my 2 cents,,i also have a 79 ford f150 and a 90 ford f150 the 79 has a straight front end(solid) and the 90 has independent front end it makes a difference in the mechanics of the trucks in 4x4 mode,,can go anywhere with the 79,,leave the 90 home if theres lots of snow,,,for gods sake dont buy a 99 f150 like i did,,will leave that at home till the snow is gone,,lol
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Too light of front driveline components. A cast iron block does help weigh the wheels down but nothing beats a solid front axle (except those high gas prices).
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On Sun, 30 Dec 2007 11:50:26 -0600, snipped-for-privacy@webtv.net (Jeff Guenther) wrote:

Thanks Jeff
I looked at the front end and I can see the difference.
It looks like I'll be selling this 90. It's not doing me any good. I live on a farm and my almost one mile long private road is bad in winter. THe 79 is getting me up it, and the 90 does nothing but spin on the snow. I have a feeling the 79 is also heavier. Seems the farme is beefier, and it has an automatic trans, which is know is heavier than the manual in the 90. The 79 is really falling apart, so I hope to find another one in better shape, and with a smaller engine. The 400 eng in that one gets around 7mph. Cant afford that these days. The 90 seems to get at least 15, I didnt drive it enough to know exactly.
By the way, it was running great, even if it didnt get up the roadway. I have not used it for at least a month, and now it will not start. The starter spins and all that, I hear the fuel pump, but it wont fire up. It was just sitting there, so there was nothing done to it. I suspect fuel line freezeup. I wanted to spray some starting fluid in it, but where the heck do I put it. There are two large hoses for the air intake, and a black plastic box which I assume is the air filter. Then there is a scoop that goes to the grill. That scoop seems to be the only place to put it without ripping stuff apart.
I have never owned a fuel injected vehicle before now, so this puzzles me. I think carb engines are much easier to work on. I have to pretty much re-learn engines now.
By the way, if I get an 1980 (something) truck, when did they switch the body, what year. Otherwise, I may have to get a GM truck if the newer Fords cant get thru the snow. Of course I dont know how those are built either. I have always owned Ford trucks and Chevy cars. Both are preferences of mine, but I may have to change ???
Thanks
George
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