1981 F-150, ~170,000 miles, 302/C6 2WD/9" 2.75. Street truck.


Hey guys. My truck has had slippage issues with its C6 for a while. If I put any kind of miles on the truck, whether it's around town or on the highway (enough to get the transmission hot), I start having weird shifting issues, as well as sometimes a delay in going into gear, and when it goes into gear it squeals as it engages. It can be full of trans fluid and still do the same thing.

Shifting issues are:

When I take off from a stop, the truck will start moving, and at 5-10mph I'll feel a big clunk (as though the transmission is "catching" as it slips);
Early shifts from 1-2 (the way I drive, it usually shifts around 1500rpm from 1-2, early shift is around 1100 or so);
Slow shifts from 2-3 (you can hear the engine speed rise during the shift)

I was looking at a nice TCI unit to replace mine, it's under $800 in my JEG's book and features a lot of nice modifications like a shift kit, manual/auto valve body, and more.

A local shop I priced wants over $1000 to rebuild my trans to STOCK specs. That TCI, even with shipping, should cost less and it's a performance unit.

I figure since I don't have $800-$1000 to blow right now, I'd look at the torque converter, since it's been suggested to me that a worn one could cause problems like what I'm experiencing. That's like a $100-$200 investment compared to the price of a new/rebuilt trans.

Does anyone have any suggestions as to which brand, model, specs, etc. I should look at? I'm running the OEM one right now and I bet anything would be an improvement. I am open to all suggestions.

 

Ever considered buying a manual truck, similar motor type and bootlegging everything off of it? It could be a cheaper option.

 

That's my plan eventually, to convert over to a ZF5. But it'd take my truck off the road for alot longer than if I just dropped in the TCI unit which should bolt right up. Not sure about tailshaft length on the TCI vs. my stock C6 (TCI length is 13" or 13.5").

 

Updated first post with my truck's mileage. I guesstimated since I changed clusters a good while back and didn't adjust the new one to match. lol

 

How about testing the TQ theory by installing a used one? Should be pretty cheap, although a fair amount of work.

Having said that, I thought the only way a TQ failed, at least ours that don't have clutches or switch-the-pitch vanes, is for the bearings to fail. If the were the case I'd think you would have vibrations due to the converter getting off center. Any such symptoms?

 

Not saying the transmission couldn't have problems but the torque converter does sound like it's going bad.

I agree that trying a used converter is an option, as you might end up buying a new converter then finding out your transmission is bad. Then you have a converter filled with who knows how much junk that you wouldn't want going into a new transmission.

A C6 is cheap to rebuild, not to mention you could probably find a used one on craigslist for as low as $100.

 

I guess I missed something. All the symptoms I latched onto were about shifting, and I don't see the TQ making a difference there as that's all tranny. What did I miss?

 

Go with a Huges Fuel Mizer converter! I really like them as an upgrade to the C-6 the temps run about 30 degrees cooler and the slippage is way less.

 

There is NO WAY your problem is the torque converter. I'll bet you that if you replace the torque converter and do nothing to the trans it will still have the same problem. Your transmission is worn out, not the torque converter.

 

You could be having a problem with the governor, and there's a trick that can sometimes fix it.
In an empty parking lot, put the truck in reverse and give the truck full throttle and go as fast as you can safely, then stop abruptly. Repeat several times.
Sometimes the governor get junk in it and this can clear it. It's also free.
I learned it from a auto tranny rebuilder many years ago and it's worked to sort out shifting problems on several vehicles.
Mark's right; you don't have a TC problem.
Does the C6 have a vacuum modulator? If so, check the hoses at both ends of the steel tubing and pull a vacuum on the modulator to see if it leaks; if it does, replace it.
Good luck.

 

Yes, the C6 has a vacuum modulator and I should have thought of that. Bad hoses or a ruptured modulator could cause at least some of those problems. Modulator is at the right rear at the bottom of the tranny. Should have a hose from it to a metal tube, and then another hose from the other end of the tube to the vacuum tree on the engine or on the firewall.

 

For clarification, it was mentioned to me that there is a type of clutch inside my TC (I'm looking around and from what I've read, it's what the stator is mounted to), and that if this wears out, it could cause problems. That's why I suspect the TC.


I could do that. Though if I'm going to do that much work anyway, I might just spend a little and get a new unit...because if the TC is my problem and a used one fixes it, I'm not going to want to pull it back apart to put a new one in....lol

I don't doubt the transmission has some issues. It -is- 30 years old and has had a pan leak for years (new gasket didn't fix it either). Been run low on fluid countless times, but I always filled it up ASAP. Though if putting a new TC on it will fix at least some of the issues or at least improve performance, that might help it last til I can afford a new unit. Or a used one.

I was just thinking about that the other day, looking for a parts truck on CL with a good trans.

Doesn't the TC drive the pump in the transmission? If it were slipping a ton, that would mean it's not driving the pump at the right speed, which in turn would result in lowered fluid pressure....


Yep, I replaced it just a few years ago. When I got the truck, the steel tubing was long gone, having been replaced with a length of rubber hose. Not knowing any different, I did the same thing and put new rubber hose on.

Just recently I trimmed a bit of the rubber hose to get rid of a spongy section where it connected to the mod valve. I checked the hose and mod valve and noticed no trans fluid in either.

Speaking of which, I had read on here that if I get a modulator valve with the black stripe on it (mine has the green stripe I believe), it's supposed to make it shift harder and whatnot....but of course, nobody at the parts stores knows anything about where I'd get one of them. lol

 

The C6 was also used in cars and had a total lifespan of something like 30 years - mid 60s to mid 90s I believe.

But there are at least 4 different bell housing patterns/tranny cases available for the thing, so be sure you find one that will work for you.

If it were me who had asked the question, and a former Ford automatic transmission engineer told me the problem is here and not there, I would believe him.

 

There is a one way clutch on the stator and if it fails you lose the torque multiplication. That affects the first 20-50 feet of launch, but it makes zero difference to any shifting.

Yes, the torque converter drives the pump. It is driven by a splined shaft. Either it drives the pump at exactly engine speed or the shaft is broken and it doesn't turn the pump at all. If the pump isn't turning the trans will never engage in any gear. It isn't possible for the torque converter to drive the pump at a slower speed.

 

That's exactly when I get that weird clunk I described in my first post. First 20-50 feet. What sort of symptoms are caused by this a bad stator clutch?

Alright, I didn't know that.


What are some things I could do to my transmission to improve shifting or to make it last longer, until I can get another one? Any adjustments?

Don't know if it changes anything, but I forgot to mention; when I'm having the shifting issues, if I shift the trans manually, there are no delayed or slow shifts. Still feels like it's slow to take off.