Re: Fuel Starvation & Transmission Shifting??

OldsFan4Ever wrote:

> A well known professional transmission rebuilder has informed me that
> an engine that is starved for fuel will cause late and hard
> transmission shifts.
>
> Here's what he said exactly:
>
> "One thing however, now that you mention this...  suppose that the
> transmission shifts late when it shifts hard (when it appears to get
> hot)...
> This would probably mean that you have an ENGINE problem...  When the
> engine is not running correctly, it may demand more throttle to make
> the car go,
> which would pull the cable more, causing late shifts, AND, drop the
> vacuum,
> causing harsher shifts.  We see this when the vehicle has a plugged
> fuel
> filter, etc...  The more starved the vehicle gets for fuel, the more
> the
> driver steps on the throttle, which delays upshifts and harshens the
> shift,
> as well."
>
> I don't quite understand or "get" how exactly his makes any sense!? 
> Late shifts are SUPPOSED to be hard, right?  I just don't see how a
> non-electronically shifted transmission can somehow KNOW that you're
> stepping on the throttle more than normal because the engine isn't
> getting enough fuel??
>
> Anyone?

Non-electronic automatics most certainly DO "know" how hard you're 
pressing the throttle. There are two methods used:

1) Chrysler Torqueflite- a mechanical linkage from the carburetor to the 
transmission moves a slug in the transmission valvebody, which in turn 
modulates a hydraulic pressure (referred to as "throttle pressure") 
within the transmission. That "throttle pressure" is compared to 
"governor" pressure (which increases with road speed) and the ratio of 
those two pressures determines the shift point and shift quality 
(firmness). In addition, since the Torqueflite directly senses throttle 
position, this same linkage operates "kickdown" to downshift 1 gear 
(below a set road speed) when the throttle reaches wide-open. Some 
models of the transmission will downshift at less than full throttle 
under certain conditions also.

2) Ford and GM Hydramatic: engine manifold vacuum moves a slug in the 
valvebody (via the modulator) and that modulates a hydraulic pressure in 
the transmission.  The ratio of vacuum modulator pressure to governor 
pressure performs the exact same functions as throttle pressure/governor 
pressure does in a Chrysler transmission. Since manifold vacuum is 
rather non-linear in its relation to throttle position, these 
transmissions need a secondary mechanism to actuate "kickdown" to a 
lower gear when you hit wide-open throttle. This is accomplished with a 
cable tied to the throttle linkage, which activates a valve in the 
transmission when the throttle reaches full travel.


Any time the engine is producing less than normal power for ANY reason, 
a non-electronic transmission will seem to shift "harder" and "later" 
than it normally does because the ratio of throttle pressure to governor 
pressure (or modulator to governor pressure) is skewed.