From: Bob Hale <email@example.com>
Subject: Chevy TH-350 shift points
Date: Mon, 04 Apr 94 17:52:13 GMT
Jon Warwick wrote:
> I have a TH-350 sitting behind a Chevy 350 in my project
>truck. I have a problem in that the trans. shifts to taller gears
>way too soon (the motor dosen't get to rev very high at all in L1
I was going to reply to Jon directly but I thought that the answer
might be of general interest so I decided to post it instead.
First, I need to ask a qualifying question: has the transmission
been modified (e.g., installation of a shift kit)? If so, then
the kit may the cause of your problems. In any case, read on
for some ideas.
A few comments about how the trans works: decisions about what
road speed to shift at, and how harsh the shift is, are made by
a simple hydraulic computer. The harshness of the shift is
determined solely by engine vacuum; the vacuum modulator is used
to sense the vacuum and to change the vacuum signal into a pressure
signal inside the tranny. The higher this pressure, the more
harshly the trans shifts. This modulator pressure is also used
to decide at what road speed to shift.
The road speed is measured by a governor which is at the tail
of the tranny. It is gear driven from the tailshaft and therefore
it responds to driveshaft RPM which is proportional to road speed
(unless the tires are spinning). The governor generates a pressure
signal internal to the trans; this pressure increases with road
speed but not in a linear fashion. Since the governor uses weights
which are spun out by its rotation, the force (and therefore the
pressure) generated is proportional to the square of the RPM.
In an attempt to somewhat linearize this the designers have used
two sets of weights, one light and one heavy. The two are
separated by small springs. The heavy weights operate at lower
RPM (road speed). When a sufficiently high RPM is reached then
the weights hit stops and do not contribute any more to the
pressure signal. At this RPM the light set of weights begins to
provide the increases in the pressure signal. Higher RPM always
means a higher pressure, but not necessarily in proportion to the
The transmission decides which gear to use by comparing the pressures
provided by the modulator and by the governor. If the difference
between these two pressures exceeds an amount determined by a spring
in a shift valve assembly then that shift valve operates and selects
the next higher gear. There are two shift valve assemblies, one for
the 1-2 shift and the other for the 2-3 shift. The system works
as follows: assume that you are idling and then begin to open the
throttle. The engine vacuum at idle causes a very low modulator
pressure (nearly 0). The governor pressure is 0 because there
is no tailshaft RPM (no road speed). As you open the throttle and
begin to accelerate the engine vacuum drops. This causes the
modulator pressure to rise. Assuming that the engine vacuum remains
constant due to a steady throttle position, the modulator pressure
will remain at a steady value. As the vehicle picks up road speed the
governor pressure rises. When the governor pressure force exceeds the
modulator pressure force by a small amount then the 1-2 shift valve
operates and engages the clutch which shifts the tranny into 2nd gear.
The rate of clutch engagement is a function of the modulator pressure;
low modulator pressure causes slow engagement and a smooth shift.
High modulator pressure due to large throttle opening causes harsh shifts.
As the vehicle continues to gain road speed the governor pressure
continues to increase. Eventually the governor pressure force exceeds
the modulator pressure force by enough to operate the 2-3 shift valve.
This activates another clutch pack which shifts the transmission
into third gear.
Note that I spoke of pressure forces above. This is due to the fact
that the pressures operate on the ends of small pistons (the shift
valves). The pressure applied to the area of the piston develops
a force. Since the shift valves have two ends, and since pressure
is applied to both ends (one end from the modulator and one end
from the governor), the difference between the two forces causes
the valve to move to one end or the other of its travel. Because
the areas of the two ends of the valve do not have to be equal, the
valve will operate in accordance to the difference between some
constant times the governor pressure versus the modulator pressure.
This fact allows the designer quite a bit of latitude in determining
at what road speeds the transmisison will shift. There are also small
springs which apply force to one end of the shift valves and these
also affect the road speed at which the shifts occur.
There are other complications in the tranny. For example, it has a
means of deciding when the throttle is wide open (the detent cable)
and WOT causes the modulator pressure to be ignored and instead a
constant pressure from a detent regulator is used on the shift
valves. This causes shifts to occur at the maximum possible road
speed for each gear. There is a manual valve which is connected to
the gear selector lever and which selects reverse, neutral, and
can lock out either 3rd gear or lock out 2nd and 3rd gear. There
is a main pressure regulator which is controlled by the modulator
pressure; higher modulator pressures increase the pressure of the
main oil which supplies everything else. Most of these complications
can be ignored for the present time because Jon's interest is in the
road speeds at which the tranny shifts.
Now, to get to the question of adjusting the shift speeds. There
is no "adjustment bolt on the side of the trans". The only way to make
major changes to the shift speeds is to modify or replace either the
governor or the valve body. You can make minor changes to the
shift speeds by installing an adjustable aftermarket vacuum modulator.
The total adjusting range of these modulators is about 4 to 5 MPH
at WOT. At cruising speeds you will get about 2 MPH total adjustment
range. This probably isn't enough to suit.
Working on the governor is probably the best approach to adjusting
the shift speeds. The factory actually offers a high shift speed
governor; it was used in a few HP applications such as Camaros.
You can just swap one of these in and you will see a dramatic
increase in shift speeds. In fact, it may shift at too high a
road speed after the change. E-mail me if you are interested in
the part number; I will have to look it up and it may take a couple
Another approach to modifying the governor is to get a B&M governor
kit and to install it in a NEW factory governor. Governors are
very sensitive and delicate beasts; they "take a set" when they
have been run for a while and they _don't_ like to be messed with.
A new governor will probably save you the grief of a sticking
governor after modification. The B&M kit comes with an assortment
of small weights and several springs. You have to experiment to find
the right combination. It took me 10 or 11 tries to get the WOT
shift speeds that I wanted. The B&M instructions are downright poor;
you need to change the weights to get the 1-2 shift speed that you want,
and you need to change the springs to get the 2-3 shift speed that you
want. Unfortunately, these changes interact and you will need to
iterate to get both of them where you want them.
The governor is fairly easy to change because it is accessible
by removing a small cover on the side of the tranny at the rear.
Caution: the trans will be hot when you are down there messing with
the governor. It is easy to burn yourself. I suggest gloves.
And you will probably want to lay in a small stock of gaskets for
the governor cover.
As I mentioned above, governors are finicky. Absolute cleanliness
is a must. If ANY amount of dirt, grit, grinding compound, metal
shavings, or any other foreign material gets into the governor then
you will have all sorts of shifting problems which will likely
be intermittent and erratic and near impossible to troubleshoot.
The contamination can get washed out of the governor and end up
in the valve body where it will cause bizarre problems.
One final thought: your original question suggested that the
shift speeds are too low at all throttle openings. One possible
cause of this is a leaky governor. The leakage could be due to
scoring of the valve journals, or to a loose fit between the
valve journals and the governor shaft. Or it may be due to wear
on the journals which the governor engages in the case. If looseness
is the cause of your problem then just replacing the modulator should
fix it. If case journal wear is the problem then you can get a
kit from a tranny specialist which will allow you to put new bushings
in the journal area. This fix requires removing the tranny from the
vehicle so that you can properly clean out the metal chips after you
Bob Hale firstname.lastname@example.org
From: Bob Hale <email@example.com>
Subject: T350 governor tips
Date: Tue, 05 Apr 94 18:17:49 GMT
> have included a GIF of the governor weight, that shows how you can
>grind it a bit. This will raise WOT and part throttle shifts.
Yes, this is an inexpensive way to raise the shift points. In my
previous article I tried to avoid getting into too much detail in
order to avoid confusion but my posting has generated a lot of
interest so I'll expand upon the subject a bit.
(for the unintimidated)
The GM T350 and T400 modulators are built from only a few parts -
the shaft (an aluminum casting), the valve (a steel spool valve),
two pivot shafts, two pairs of weights, two springs, and a plastic
drive gear which is pinned to the end of the shaft. The design is
elegantly simple - oil pressure acts on one end of the valve while
force from the spinning weights ats on the other. The valve allows
oil to flow from the main line into the output passage until the
pressure in the output passage causes sufficient force to counter
the force developed by the spinning weights at which time the valve
moves to the closed position and prevents any further pressure
If you want higher shift points then you want the governor to develop
a given pressure at a higher RPM. This means that you want less force
from the weights at any given RPM. The obvious modification is to
take some of the mass off of the weights, and this works well.
However, there are some practical details to keep in mind.
As I mentioned in the previous posting, governors are finicky.
They need to be really clean to work properly. If you do any work
on one, be sure that you have cleaned out all of the grunge, chips,
and other contaminants before you resintall it. If you do any
grinding on it you might want to tape it up as much as possible so
that the grinding grit has less chance to get inside, but still do
a thorough cleaning job before reinstalling it.
Now, how to decide what to remove and where to take it off. The
large weights are of no real interest to us - they only affect
the governor output at low road speeds, probably not more than
15 MPH. Leave them as is; they work fine that way. Between the
large weights and the small weights there are a couple of small
coil springs, one on each side. The small weights are bent into
roughly a right angle shape. One end of the small weight has a
tab which presses against the end of the spool valve; this is
where the force is applied to the valve. The other end of the
small weight hangs out and is the portion which the rotation of
the governor forces outwards. The outermost end of this portion is
the place where you remove material. The middle of the small
weight is pivoted on a pin which runs through the shaft housing.
You want to remove material from the corners and, if necessary, the
edges of the outermost portion of the weights. Removing material from
the end of the weight has the greatest effect; removing material from the
sides of the weight has less effect. Don't remove any from the area
near the spring; the spring needs a place to sit! The question of how
much to remove has to be answered by trial and error. The high RPM
factory governor appears to have about 1/3 less metal in the weights
than the standard factory governor. This should give you an idea of
how much effect trimming the weights has. I suggest caution when
removing metal - it's much harder to put it back than to take it off!
Note that the factory governor is very symmetrical. There's a couple
of reasons for that - one, it's cheaper to make all the parts using
the same setup, and two, the symmetry balances the forces and minimizes
the tendency for the governor to wear and to stick. You want to try to
retain as much symmetry as you can in order to minimize these problems.
In other words, take equal amounts of material off of each side of the
weights, and take the same amount from both weights.
Lightening the weights will raise the shift points, but it won't
raise both the 1-2 and 2-3 shifts by the same amount. The other
variable which affects the shift speeds is the springs. To really
tailor both shifts you will need to change the spring force as well
as the mass of the weights. The only source that I know of for
replacement springs of different force is the B&M kit. It may
be possible to find suitable springs elsewhere, or to wind them
A few words about governor failures
There are a few standard governor problems which are worth mentioning.
The first is dirt in the valve assembly. This can cause sticking and/or
unusual wear of the valve portion. The valve is made from steel but
its housing (the shaft) is an aluminum casting. Any abrasives in the
area can cause wear which mostly happens to the housing. Wear causes
an increase in oil leakage around the valve, and gives unusually
high governor pressures. This results in a symptom of shifts occurring
at too low a road speed. If the governor valve is sticking then the
shifts occur at unpredictable road speeds, either higher or lower
Another problem is wear in the transmission housing where the
governor shaft engages the journals in the housing. This can cause
the same problems as wear in the valve area. This problem can be fixed -
transmission shops can usually sell you a kit which allows the housing
to be re-bushed. It does take a good bit of work to do this because
you must disassemble the transmission to do a proper cleaning job
Another problem is a worn or broken gear on the governor. The plastic
gear serves the function of a mechanical "fuse". If the governor binds
for some reason then the gear breaks or its teeth strip off. This
prevents major damage from happening to the remainder of the transmisison.
If the drive gear breaks then the governor doesn't turn and, if there
are no other problems, the governor thinks that the road speed is very
low and the trans won't shift out of first gear.
There is a screen in the oil supply to the governor. This is normally
in one of the governor pipes inside the pan. If the screen gets clogged
then the oil pressure supply to the governor is reduced and the
governor's output pressure will be low. The vehicle will shift at
road speeds which are higher then expected, or, if the screen is
totally blocked, the transmission will stay in first gear.
Tips on governor R&R
The governor is at the rear of the transmission and it sits above
the oil in the pan. This usually means that you can remove the
governor without much oil spillage, maybe a couple of spoonfuls.
I find it advantageous to park heading slightly downhill, or to jack
up the rear end of the vehicle, so that it tilts forward. This
gets the oil even farther away from the governor and minimizes the
amount that you lose.
When you have been driving the vehicle, the transmission gets
fairly hot. Amongst other things, the transmission fluid heat
exchanger in the radiator raises the transmission temperature
to near that of the engine, about 200 degrees Fahrenheit. If
you have been using the torque converter then the fluid will be
hotter than that. All this means that the governor and the
transmission case will be hot when you are pulling the governor.
I suggest gloves for handling the governor cover and the governor.
Bob Hale firstname.lastname@example.org