Truck transmission wizards need some advice

   / Truck transmission wizards need some advice #1  

super55

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Jan 27, 2012
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Great North of Michigan
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Oliver Super55, John Deere 4310, John Deere 4400, Kubota L2500 (had),
I have a '67 ford F100 that has a transplant late 70's ford 351m with a C6 transmission in it. Problem I am having is the the truck is acting like a manual when you don't push in the clutch when stopped. It just flat out stalls. Put it in neutral starts right up put it in Drive and everything works fine. No slipping, no clunking and just runs smooth.
I plan on doing a transmission flush through the cooler lines and filter change. I peeked at the torque converter through the inspection cover and it does have a drain plug so I can get that as well. I'm not sure if that will be successful but since I bought it used and don't know the history I figured it would be worth a shot.

I'm thinking I might have a bad torque converter but I can't find any info of this being a typical symptom. I would hate to dig that deep in especially if it would be something simple like a vacuum modulator valve causing the problem.
 
   / Truck transmission wizards need some advice
  • Thread Starter
#2  
I'm rather rusty on this, but you might have a locking torque converter in the C6. It is staying electrically locked and the clutch switch is allowing it to unlock when the clutch is pushed in.

It's just a plain jane column shift auto trans so the only clutches are inside the transmission. Even when I turn down the idle to as low as possible like 600 rpm the torque from the engine still wants the car to creep which is typical but as soon as I press the brake to hold it in place it will stall out. Does the same thing if I have the idle turned up to around 900 rpm put in Drive press the brakes it will stall. Put it in neutral or park and start it up it will idle all day long without issue.
 
   / Truck transmission wizards need some advice #3  
sounds like a problem in the valve body to me, sticky check ball or something or shift valve. I'm no expert, I just have a friend that does that stuff and have seen a bunch of transmissions apart...
 
   / Truck transmission wizards need some advice #4  
If pressing the brake causes a stall be sure your brake booster and hoses to it are intact.

A bad diaphragm can cause a vacuum leak that leads to a lean-condition stall-out.

Also a vacuum advance distributor won't pull off (******) at idle w/o sufficient manifold vacuum.

IMO These could both come into play, and there's no vacuum circuit or portion of under the hood with the capacity of the brakes. (to cause a sudden huge drop in vacuum.)

An auto parts store may have a $20-30 vacuum gauge that can be dash mounted ... or kept in the tool box. :)

Do the brakes seem to work ok? (full assist under load or quick stop)
 
   / Truck transmission wizards need some advice #5  
I second the vacuum concern possibly stalling the engine. Old days there was a vacuum accumulator under hood to store vacuum. Might look like coffee can. Make sure it's not rusted through on bottom. Pinch off hose to power brakes booster. If engine doesn't die booster is leaking vacuum internally?
 
   / Truck transmission wizards need some advice #6  
Accumulators were typically on smaller lines and to quicken response of the heater controls. IIRC only Ford used a metal vac accumulator, about the size of a large vegetable can. Such gadgets were larger in the days of vacuum-operated wipers.

None of this means it' causing the OP's problem, but replacing a brake booster is a DIY bit and ~ the cost of a transmission flush, say for a guy who wouldn't DIY that.

(I'm going on hope here too. :p)
 
   / Truck transmission wizards need some advice
  • Thread Starter
#7  
Found a service manual for the C6 trans online luckily. Unfortunately it doesn't really describe my issues. Could be Vac issues but if I keep it in park or N at idle it runs fine. Put it in D and it just wants to take right off even at 700 rpm like if you had a manual trans and popped the clutch to take off. No power brakes or vac cannister. Vac line to trans is drawing from the intake manifold. That's not to say it didn't have that stuff originally. This is 78-79 so vac lines were everywhere for emissions at that time but as for now they aren't there. Literally 78 powertrain/drivetrain dropped into a 67 vehicle.

Valve body might be something to consider. Also wondering if I throw in a high stall torque converter 1800-2000 rpm instead of the stock 1500-1600 rpm if that might help the issue. If that doesn't help then move to the valve body since that can be accessed without dropping the trans. I guess on a positive note at least it isn't too difficult to drop the trans. You can actually get to all the bolts on the bell housing without 3' of extensions.
 
   / Truck transmission wizards need some advice #8  
It really sounds like a hydraulic problem like the shift shaft isn't moving releasing things in the valve body, not saying it couldn't be the converter but it sounds more like a valve body problem, might be able to pull the valve body and clean it up with the trans in, I don't remember...
 
   / Truck transmission wizards need some advice #9  
Check the timing on that engine with the vacuum hose removed and plugged from the distributor.
You may find that it looses a lot of rpm and almost stalls just removing the vacuum hose from the distributor.

And these are good distributor for your truck;
Distributor
 
   / Truck transmission wizards need some advice #10  
So , we have a 1200+ rpm-stall converter not stalling(?) and we think below 1000 it's something in the transmission causing the engine to stall instead?

I've driven hot rods with Vega torque converters that wouldn't budge much below 2500 rpm ... because the converter slips below stall speed. There's not much 'hookup' there.

If someone tried to get the newer model lock-up to work they may have done a poor job of connecting bits. Was this truck just bought this way or was it drivng well for months or years?

Occam's razor isn't always your friend if sticking to a first conclusion denies testing-out other ideas. All we know about this F-150 is year(s), make, and model.
 
 
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