1981 F100 Transmission problems
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1981 F100 Transmission problems
My sons first car is an 1981 F100with an 4.9 and a 3speed manual with overdrive. It runs great but has a problem: I can jack all four tires off the ground and with the truck in neutral they all spin freely. When I put in any gear including reverse the truck feels like it wants to move but will not move a single inch. with the rear tires off the ground it will spin them in first gear but makes a strange noise then when they stop spinning forward they spin backwards like a wound rubber band. I'm clueless.
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From: Virginia
This tranny does not have a good reputation. If it looks like it's the culprit, look for something else, lots of different trannies will fit.
Did this truck run ok when he first got it? Or was this a project you bought? Or did he powershift it without saying anything and ruin the tranny?
Did this truck run ok when he first got it? Or was this a project you bought? Or did he powershift it without saying anything and ruin the tranny?
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This tranny does not have a good reputation. If it looks like it's the culprit, look for something else, lots of different trannies will fit.
Did this truck run ok when he first got it? Or was this a project you bought? Or did he powershift it without saying anything and ruin the tranny?
Did this truck run ok when he first got it? Or was this a project you bought? Or did he powershift it without saying anything and ruin the tranny?
Cargo Master
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From: New Zealand
Borg Warner T18, & New Process 435 are both common in these trucks.........both are strong & will bolt in, but are non-overdrive.
You also need to determine that the problem is/isn't in the rear axle.
You also need to determine that the problem is/isn't in the rear axle.
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The truck drove perfect before this problem in the past clutches have gave me warning they were going to die.
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as far as what trans I have I dont know, where would I find that info? I checked under the hood and found nothing about the trans identity. Its a long narrow trans with no visible shift linkage so it must be on top or inside the trans. I would go out and look but it freezing here in Michigan.
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What is the TRANS code on the Certification Label on the driver's side door jamb? If you post a picture of that sticker, we can tell you all sorts of stuff about how it came out of the factory. 
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Probably an aluminum SROD or top shifted unit if an OD. OK trans for light duty.
With the wheels off the ground, does it turn the tires in all gears?
If you slowly apply the brake during this test, do the wheels stop without killing the engine?
The spinning backwards is a bit odd. The trying to move is what leads me to the clutch.
Differential failures result in either a free spinning pinion and no axle motion, or rough operation with some lockup or near lockup, or complete lockup.
A trans failure is going to result in some serious grinding and again, rough engagement, lockup etc.
With the wheels off the ground, does it turn the tires in all gears?
If you slowly apply the brake during this test, do the wheels stop without killing the engine?
The spinning backwards is a bit odd. The trying to move is what leads me to the clutch.
Differential failures result in either a free spinning pinion and no axle motion, or rough operation with some lockup or near lockup, or complete lockup.
A trans failure is going to result in some serious grinding and again, rough engagement, lockup etc.
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I will unhook the driveshaft and check the rearend again when the weather allows me to. I am stumped also I have a pretty good working knowledge of older cars but the spinning backwards is something I have never seen. Unless something in the reverse gear is trying to engage.
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From: Virginia
Like I said this tranny does not have a good reputation, so that's why I would agree with you and suspect it is the problem.
Like someone else said, some of the 4 speeds with the granny 1st will fit this truck, like the NP435 and the t18. But you have lots of choices if you want to change a few other things. You could install a 5 speed with overdrive out of a little bit later truck if you wanted to convert to the later hydraulic clutch system.
What are your priorities? Cheap and quick would be one of the granny 4 speeds. If overdrive is a priority, then rebuild what you have or get a later 5 speed and convert to a hydraulic clutch. You could put a automatic in it also, but you would need a few extra parts to make it all work.
All these trucks are the same up to about 1996, so if you get a donor or get all the stuff you need, you can bolt it all in and it will work. Would he like 4x4? That's a bolt in proposition also. Just find a old f150 with 4x4 and a good tranny, and bolt the frontend under it, the tranny and transfer case, and the driveshafts, and you have yourself a 4x4 truck.
Like someone else said, some of the 4 speeds with the granny 1st will fit this truck, like the NP435 and the t18. But you have lots of choices if you want to change a few other things. You could install a 5 speed with overdrive out of a little bit later truck if you wanted to convert to the later hydraulic clutch system.
What are your priorities? Cheap and quick would be one of the granny 4 speeds. If overdrive is a priority, then rebuild what you have or get a later 5 speed and convert to a hydraulic clutch. You could put a automatic in it also, but you would need a few extra parts to make it all work.
All these trucks are the same up to about 1996, so if you get a donor or get all the stuff you need, you can bolt it all in and it will work. Would he like 4x4? That's a bolt in proposition also. Just find a old f150 with 4x4 and a good tranny, and bolt the frontend under it, the tranny and transfer case, and the driveshafts, and you have yourself a 4x4 truck.
Cargo Master
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From: New Zealand
I don't know about the wheels spinning backwards after going forwards - is this separately (different directions), or together?
You're almost saying two different things - locked up, but also driving in low gear?