1981 F150, 302/C6, running B&M Trick Shift fluid for several months now.


With the truck in Drive, at part throttle I can go from 1-2 okay with no problems, but sometimes when I get to around 35mph, right when the transmission would normally shift to 3rd, it feels like the truck quits pulling as hard. It feels like something is slowing the truck down, like if I was downshifting to slow down. Engine RPM doesn't change or anything, speedometer reads the same as well. It will not shift to 3rd automatically when this happens unless I let off the gas, at which point it will immediately and firmly shift to 3rd.

Other times, it will shift normally. If I manually shift, the shifts are firm and quick. Full throttle shifts are fine.

I am sure the fluid level is a bit low, but I am almost certain that's not the issue here. I have run it very low on fluid in the past and it's never done anything like this. I know the B&M fluid is more like Type F than Dex/Merc, but I started running it since it firmed up my shifts and helped with some slippage issues I was having when the trans would get hot. I switched months ago and never had any issues.

I am going to check the modulator valve and its vacuum hose in the morning. Last time it was replaced was way back in like 2005.

Also worth mentioning, when the truck is still cold (a little above 100F water temp), 1-2 shifts are soft if I'm in Drive, or very harsh from 1-2 if I'm shifting manually. Once the truck is warmed up things even out.

 

Well I think I found my problem. The end of the vacuum line leading to the modulator valve was pretty mushy, and I think when it would be about time to shift from 2-3, the line would collapse on itself. I cut off the mushy section and rerouted the hose accordingly and it now shifts normally. 2-3 shifts are on time and firm.

Though something's got me confused....if I recall, I put a purple or green stripe modulator valve in last time I replaced it, but today when I was up under the truck, I had one with only writing on it, and it seems to be much smaller than the green or purple stripe valves. I'm wondering if someone might've changed it out at a shop without my knowledge....? Either that or I'm just gettin old and forgot.

 

First, I'm glad we could help.

Second, congrat's on such a cheap fix. But, the rest of that hose is gonna go mushy too, so you might want to replace it.

As for the modulator, I didn't think you'd notice.

 

Yes, I would have never figured it out without all your wise council. haha

I was thinking about that. I think the one portion I cut off was mushy mostly due to the fact that my transmission has a leak at the pan, and it coats everything around it with fluid. I get paid tomorrow so I will grab a new length of hose and fix it proper. Even though my current fix is technically proper....lol.

lol. Really though, I thought for sure I put in one of the colored stripe valves. And I thought the one I put in was bigger than the one in there now. No telling, it's been years since I last touched it. lol.

I am noticing that the truck is still holding 2nd gear a little longer than I'd like at times, but I don't have that weird slowing-down feeling like before. I checked yesterday and it is a little low on fluid so I am about to pick some up and top it off and see what happens. If I can track down a small enough screwdriver I am gonna play around with the adjustment screw on the modulator valve and see what happens too. Just gotta keep track of how much I turn it and which way so I don't end up running around in 2nd all day. lol

 

Guess what!


Found my problem! And two more, to boot!

Come to find out that the vacuum connection I had the mod valve connected to was jammed up with crud, so it wasn't pulling barely any vacuum. Hose was a bit on the mushy side so I got 5ft of reinforced 1/4" fuel line (the kind with the little strings inside) and ran a new line from the valve to the biggest vac connection on the vac tree (aside from the brake booster connection -- that one's just a bit TOO big) and capped off the cruddified fitting. Pulls a ton of vacuum now and the truck shifts like brand new. Shifts are on time, very firm (not harsh, but very firm) and best of all, no hanging up between gears.

And then as I was rerouting the preexisting vac lines accordingly, I had the engine running and noticed a hiss coming from the back of the block. I'd have scratched my head but I was trying not to fall onto a running engine. Anyway, I for some reason thought to pinch down on one of the PCV valve vac lines (I had a large one and a small one ran to it), and when I did, the idle speed went up a bit and the idle smoothed out to what sounded like perfection.

Turns out that by running two vac lines to the PCV valve long ago, I had unknowingly created a vacuum leak. Never noticed it til today and all this time I thought I just had a bumpy cam and didn't know it. Used what was left of the new fuel line I bought to make up a new PCV hose and capped off all remaining unused fittings.

So now my truck shifts correctly, idles correctly, and all the hesitation and bogging I was getting if I'd floor the pedal is gone. If I drop the hammer now, the truck picks up and hauls you-know-what. Even sounds different. I'm wondering if I'll notice an improvement in MPG as well, but I may be dreaming on that one.

 

Sounds like you did some nice diagnostics work Kirby. I have those moments too when you post up a problem. My truck is really lacking power, its getting crappy fuel mileage too. Something smells kind of burny.

Never mind I had the parking brake while I was driving.

You will be suprised how vacuum effects these carbed motors. It does funny stuff to the EFI trucks as well. A buddy of mine showed me a trick to find vacuum leaks. You spray the hoses with carb cleaner. Use the straw and just slowly trace the hoses from one end to the other. When you hit a leaky spot it will speed the motor up from drawing the combustible material into the engine. Brake cleaner will work also.

 

Good job Kirby! You'll be pleased with having fixed it yourself as well as a truck that performs like it should. I find fixing things very rewarding.

 

Many thanks. You will be glad to know that I also got the taillights fixed recently as well.

I forgot to mention that everything I mentioned doing in my previous post was done in the O'Reillys parking lot. LOL.

And yes vac leaks do some strange things. Before I fixed all this, there'd be times I'd want to show off for a friend and try and chirp the tires, and it'd either bog out or try to die on me if I floored it. Now it runs like a scalded ape in any gear (something I have to discipline myself on...the motor doesn't use much if any oil currently and runs like a top, but if I keep getting into the pedal and abusing it then that could change...)

I have heard of that trick. If only I had tried it before I ran out all those tanks of gas with a motor that wasn't running right.

Thanks! I am already very happy with how easy it was to fix and how well it's running. You are right, it really is rewarding to fix things myself (or with friends) and then realize how much of an improvement it made. I-beam bushings are a prime example, not only did I learn by doing them with a friend, but I now have a truck that rides (somewhat) like a new truck. Granted, I still need shocks, tie rod ends, probably a few other steering items, new balljoints, and an alignment, but I am a man of independent and limited means so things come along as I can afford them....lol

 

Well the dreaded half-shift is back. I was just cruisin' around today and felt it pull back like before. So tomorrow I am gonna get back under it and see how much vacuum I'm pulling at the mod valve, and pull it to make sure something's not broke. I am gonna put some vacuum to it and see if it holds. I am wondering if maybe it's my problem, perhaps the diaphragm in it is starting to leak and it can't hold enough vacuum to allow proper shifting? If I manually shift I have no issues with weird shifts, aside from those hard shifts when it's still cold.

Could it be that something in the trans is out of adjustment? Bands, maybe?

 

How long has it been since you changed the fluid? The reason I ask is that your symptoms of shifting fine with the lever but not in automagic are the same as what I had on the C6 I put in the '82.

It sat in a field for a few years and apparently things gummed up in the control body. And even after draining and refilling, including the torque converter, it still did it. A dose of Sea Foam's transmission magic helped, but it still took several hundred miles before it always shifted automagically.

 

If the band was loose it would slip. If its not slipping then the band is fine.

 

Oh ok, I didn't know. I remember reading about band adjustments in my old Haynes manual (no idea where it's at now), and couldn't remember what it was supposed to help. I get some slip in the summer on long trips but the Trick Shift fluid helped with that. That and keeping my speed down.

I changed the fluid when I changed the filter and pan gasket, but I didn't drain the torque converter. Right now it's running a mix of B&M Trick Shift and whatever's left of the Dex/Merc I was running before. The trans has a leak at the pan though so there's probably not much if any Dex/Merc left in it.

I can force a shift if I let off the gas and cause the engine vacuum to go back up. 1-2 shifts don't require me to do that but sometimes 2-3 does.

I will pick up a can of the Sea Foam trans stuff on payday and run it through. Do I need to change the fluid after a while though?

 

No, as Bruno pointed out, the Sea Foam stuff smells like it has alcohol in it so probably evap's. If you had a really cruddy tranny then maybe you should change the fluid after SF.

 

I read the bottle when I looked at some the other day and it said it's 100% petroleum based...? I don't know, so long as it works I'm happy. lol

It couldn't hurt to do another fluid change after running SF for a while. I could drain the torque convertor and put in straight up Trick Shift, then go ahead and fix my pan leak. I read on here that the 4WD pans are a little deeper than the 2WD versions, I might look into swapping one of those onto my truck as well as adding a drain plug. New filter too, just for gits and shiggles.

Oh and it's raining today so I might end up having to wait on checking the mod valve unless I can find a dry place to do it.

 

For sure put a drain plug in.