How can a torque converter fail and what symptoms would I notice when driving to indicate a problem with it? Maybe someone could educate me a little on what the "Stall speed" of the torque converter means. What happens when the wrong stall speed is selected?

The reason I ask is that I'm still chasing a rough idle problem under load. With no load the engine and truck run smoothly, just when in drive and not moving the engine and truck shake. I've been focusing on the engine and solved some intake leaks. I haven't had time to do a compression check yet, but the engine doesn't blow smoke - even after sitting for a month it started and nothing out the exhaust. So now I'm wondering if the problem could be in the C6 tranny rather the 300 I-6 engine (all of this is in a 1965 F100, but the engine and tranny are from a 1977 E100 according to the previous owner). I did have a trans fluid leak when the truck sat for a month (I posted on that issue as well).
Thanks for any insight.

 

I did a compression check on the engine last night and all 6 cylinders were within 10% of each other around the 105 psi mark, so I don't think there is anything wrong with the power cylinders.

Anyone have experience with faulty torque converters or trannys that could cause the drivetrain to shake at idle but only under load? (i.e. sitting at a stop light).

 

FoMoCo A/T Basics 101

Torque converters can fail in several ways, but regardless of which way they fail, they do not cure themselves and work later. If failure is catastrophic vehicle usually will not move because the T-C is primary source of motive power for an A/T.

The torque converter is a radial fluid pump affair where a rotor, which is part of the torque converter bolted to a flywheel/ flex plate turns a stator, mounted on a tranny input shaft. Stator turns input shaft which in turn runs your transmission & front ATF pump.

Stall speed has nothing to do with Idle. It is at the higher RPM scale where the rotor & stator lock up and become monolithic, or mechanically rotate as a single unit with no fluid slippage & no mechanical advantage (reduction factor), In esence when stall speed occurs a torque converter becomes a "direct drive" unit. This lock up or "Stall" takes place when a sprag unit reaches a predetermined RPM the centrifigul weights slide out locking rotor & stator together. As forward motion/ RPM fall off these weights drop out, torque converter again acts as a radial fluid pump assy.

In newer A/Ts like AOD so forth, they may use electronic stall torque converters. They are governed by the ECM and the Speed Sensor the ECM uses. . .It has nothing to do with a 70s A/T.

If you have power brakes you may have a vacuum leak in your boster. Also if you have the DeLuxe Heater/ superheater with vacuum servos you could have a leaky diaphram in a servo. You could also have a vacuum modulator leaking at tranny.

Rough Idle is "a generic" description, it can alsobe caused by too low an Idle speed, or faulty Ignition below a given RPM. . . there are a bunch of variable involved there.

I would tell you this though, If you can put it in drive, pull off at moderate to neutral pedal and upshift smoothly, on time from "lo" to intermediate around 17 mph, then upshift smoothly and on time into high or "D" at 25 - 30 mph you torque converter isn't your problem.

Low Intake Vacuum can also be caused by worn cam lobes, weak lifters or low oil pressure with hydraulic lifters, worn timing chains at low RPM and so forth. . . .

As far as a wrong stall speed, stall speed is generally calculated to coincide with cam dynamics of a given power plant. If stall occurs prematurely engine will "lug" & labor until it gets into its' "powerband" aka torque curve range. Conversely if a stall speed is too high the engine will tend to overspeed before moving the vehicle, and in extreme cases can have a "slam" start when it does finally move. . . it's why drag guys run high stall speed T/Cs.

FBp

 

When you put the transmission in gear he engine is forced to turn the hydraulics in the transmission. This puts a load on the engine and amplifies miss fires that are occuring when the engine is idling not in gear. I would look at the engine for a cause to this shaking you refer to. Check the usual suspects, vacuum leaks, sticky distributor advance (vacuum and mechanical), and ignition timing.

 

....spark plugs, wires, points, cap, rotor, etc.