2006-11-02 06:08:25 · 7 answers · asked by gmv20002000 1 in Cars & Transportation ➔ Car Makes ➔ Ford
the mod valve or throttle diaphram valve is a vacuum opperated valve that allows the trans to shift into appropriate gear depending on speed. the ford basic part# is 7a377
it is used on auto trans only)
2006-11-02 06:26:41 · answer #1 · answered by bonsai bobby 7 · 0⤊ 0⤋
"vacuum modulator" controls main line pressure and the modulator valve that controls boost pressure.
It's calibrated to sense engine vacuum,,,
which is in turn an "interpretation" of engine torque.
High Vacuum=small throttle opening=LOW torque/Low RPM
Low Vac= Large Throttle opening = Hi Torque/HI RPM
The BOOST valve controls apply/release Pressures on Clutches,Bands,etc
By sensing Engine Vacuum,,the Vac Modulator tells the trans' hydraulic control whether the TRANS is under Heavy Load or Light Load.
An apparatus called a "Governor" is another hydraulic valve which senses Shaft Speeds and controls pressures accordingly.
Interaction of Boost Pressures and Governor pressures control Shift points by applying/releasing appropriate clutch/band setups which control the planetary gear system.
a '66 C4 auto was a common box in mustangs.
They have the old-style Push-In Vac Modulators.
Fault symptoms are simple.
If Vacuum source is a problem,,ie broken hose,,or even a "slipped" engine camshaft timing chain>.dropping vacuum severely,,,
The symptoms are LATE,Hard Shifts,,,due to BOOST PRESSURE being inactive.
The Shift Point is left entirely to Governor RPM regulated Pressures
Low/Vacuum,,,Broken hose,,,,the Trans "Thinks" the Throttle is Wide Open cuz of No Vacuum.
So it "fails" to FULL THROTTLE/MAX RPM Upshift.
LATE/HARD shifts,,with FULL apply pressure on bands/clutchs.
Second symptom and rather common one is a ruptured Diaphragm in the vacuum modulator.
You get "all the above" symptoms ,,for same reasons.
Late ,Hard,Shifts due to No Boost Pressure cuz Vac Modulator is "broken".
But You ALSO get,,SMOKE.
LOTS of Blue/Gray Smoke from engine exhaust.
The Engine Vacuum sucks Trans Fluid straight out of the trans,,,thru the ruptured diaphragm and into the engine to be burned along with it's normal fuel/air charge
.................................
Some modulators are "Adjustable".
There is a screw which adjusts the Pre-Load on the Spring which the Diaphragm acts against.
The adjustment Raises/Lowers the Vacuum required ON the Diaphragm to Overcome the spring pressure
Altering the Required Vaccum to move the Modulator>Boost Valve to a given position,,,,
Makes the TRANS "think" the engine is at a Different RPM/LOAD mode than it actually IS according to shaft speeds.
The effect of that on the Boost Valve/Governor pressure balance ratio is that it Changes Shift Points
GOVERNOR controls Shift Points.
Boost Valve buffers the Governor and alters it's FIXED SETTING Shift points.
Vac Modulator controls Boost Valve to control Shift Quality and Timing.
Anything that affects Engine Vacuum will affect trans shift points & quality.
Bad Valves,Bad Spark Plugs,Low Compression,collapsed exhaust,,Ignition Timing..,,Valve/Cam timing....Anything
...............
Later model cars no longer use Vacuum Modulators,,,
they use electro-mechanical devices controlled by the car's computer system
When "all was well" with a "OLD CAR",,the vac modulated controls worked very well---basically as good as "Latest Greatest" electronic controls for average,everyday use.
Biggest gains from Electronic Controls came from SEPARATING Trans functions Totally from Engine/Car/Atmospheric Conditions.
A Trashed Engine running on 1/2 it's cylinders NOWDAYS will have it's Transmission function Correctly from sea-level to Highest Altitudes.
Before electronics,,,An engine fault would Directly affect Trans operation.
More or less,,something along those lines,,in general terms
Hope any of that helps
2006-11-02 15:14:24 · answer #2 · answered by TXm42 7 · 0⤊ 0⤋
THE MODULATOR ON YOUR C4 TRANSMISSION IS USED TO UPSHIFT YOUR TRANSMISSION USING VACUUM THAT THE ENGINE CREATES .
2006-11-02 14:20:33 · answer #3 · answered by GENE B 1 · 0⤊ 0⤋
Bonsi is on the ball with this one.Mick
2006-11-02 14:41:15 · answer #4 · answered by mick 6 · 0⤊ 0⤋
MODULATER VALVE WORKS ON VACUUM AND ASSISTS TRANS UPSHIFT.
2006-11-02 14:16:22 · answer #5 · answered by midnighttoker 3 · 0⤊ 0⤋
That is what tell you tranny to shift gears.
2006-11-04 03:29:41 · answer #6 · answered by George K 6 · 0⤊ 0⤋
tells trany when to shift.
2006-11-02 14:12:40 · answer #7 · answered by Anonymous · 0⤊ 0⤋