Auto Transmission Governor Secrets Revealed...(long post)

["Mark Hershey" <dmcinfo@xxxx>]

OK, maybe not everything we'd like to know. But as promised awhile back,
here's enough to explain
some strange auto transmission symptoms and get started on the story in
detail.

First, I'd like to thank Dave Santos and others who have contributed bits
and pieces of knowledge, experiences, and spare parts to my What Makes the
Governor Computer Tick and Why Most Eventually Fail project.

Over the years I have experienced multiple failures of the GC and have
gained considerably more experience with them than I intended. Been
able to repair my own, but experiences reported by other owners revealed a
variety of failures, only some of which I saw on mine.

To determine what was going on I reverse engineered the mis-named "computer"
and made my D into a test fixture for several possible circuit changes. I'm
working on a comprehensive document but for now here's some of the
results. I'll also report perodically on how well some changes work after
they
have more time and miles on them.

Here is some key info for the technically inclined among us. I'll try to get
schematics and pictures up on
a forthcoming Web page after I get 'em cleaned up.


Missing parts:
Good Engineering practice dictates some preventatve measures that simply are
not included in the original design. I'll describe a few examples, but first
it may help to understand a bit about how the GC works.

The GC works by comparing a variable voltage derived from a small alternator
attached to the transmission (and reduced by advancing the throttle
position) to a constant 7-volt source derived from the battery with a
voltage regulator device called a Zener diode. When the variable
speed/throttle position voltage equals the 7 volt constant voltage, the
first transmission control solenoid valve drops out and the transmission
shifts from 1st to 2nd. Similarly, when the variable voltage continues to
climb to a specific point (about 9.5 volts), the second solenoid drops out
and the transmission shifts to 3rd. All this depends on keeping the 7-volt
reference at 7 volts no matter what-- and in a hostile electrical
environment, that can be difficult. The cheap Zener diode circuit used in
the GC is a typical design, and it appears adequate so far. However, Zeners
can be forced out of their Zener regulation "knee" by large spikes and
transients. Though they rarely fail with permanent damage the circuit can
behave erratically. Expected symptoms:if your transmission goes flakey
(downshifts erraticlly, refuses to upshift) BUT you can immediately stop,
shut off the ignition, restart the car, and everything is OK for days and
weeks at a time this circuit is a likely culprit. I'm interested in feedback
if that happens to any of you; if so I'll test a design with a better
regulator. For now I'm leaving that as is.

The main "computer" devices are two integrated circuits technically
referrred to as op amps configured as voltage comparators. These particular
Siemens Corp. chips are long unavailable but rarely fail. The circuit they
are used in is extremely sensitive; even condensation on the board will
cause the circuit to function improperly. Holding your thumb on the board
while driving around (don't try that one yourself)will even cause it to
fail! This was the core of my research since the most common GC failure
symptoms are caused by this circuit, that being the car randomly downshifts
from third to second with no cause/effect pattern other than perhaps
relative humidity. Reason for such instability: two things...First, there
are two electrolytic capacitors ("caps") on the GC board that, when new,
help filter out transient variations (electrical noise)from the car battery.
These caps nearly always fail, just time and transmission heat will kill
'em. Every single GC I've looked at (5 years or older)had bad caps. That in
itself would be OK, except the comparators depend on a good, clean reference
voltage and without these caps they are less likely to get it. The fix:
replace the caps with more rugged parts (I used tantalum caps seven years
ago, still OK)AND add .1 microfarad bypass caps across pins 1 and 4 (supply
and ground)of each of the comparator chips. Yes, all you incredulous
engineers out there, the Renault designers actually left out the IC bypass
caps!Amazing...Anyway I added these last week so time will tell if they
reduce instability long term. I can, at least, handle the board live on
rainy days without downshifting :-)

Also for you engineers, they left out antispiking diodes in the solenoid
driver circuit-- an invitation to intermittent / erratic operation. Add
reverse biased 1n4007's (radio shack)across the transistor output leads.

The number one problem I saw on examination was poor solder joints! All
seven units I have examined (two mine, one Dave's, and various Dallas
friends over the years)had really bad solder job all over the board. Worst
problem is where the 8-wire cable attached to the two boards and where the
six-wire inter-board jumper cable solders on to both boards.Can't stress
enough how removing the red conformal coating (I used lacquer thinner),
generously fluxing the board, and resoldering everything can help. Certain
it causes many of the intermittent problems.

Drive Transistors:
The two transistors that drive the solenoids are RCA units long out of
production. Although they only fail ocassionally, they can be replaced with
a more rugged TIP42 or TIP42C part from Motorola. TIP42Cs can still be found
in some Radio Shack stores and are available from Radio Shack's online
store. Symptoms:car starts out in second gear but shifts OK to third, or car
starts out in 3rd and stays. Note that a blown A/T fuse can also cause it
start and stay in 3rd. Generally these transistors don't fail
intermittently, so if your car sometimes shifts OK but gets screwy on
occasion, this ain't it.

Multiple switch
The multiple or "combination" switch controls backup lights and 1st gear
hold when you move the lever to "1". The switch rarely fails, and isn't
repairable in any case. Just make sure the retainer screw that holds it to
the transmission case is firmly in place. I'll offer more detail on this
switch's complete function later.

Well, enough for now...I'll try to write up a more coherent description with
pictures for download. Perhaps Mr. Zilla could be persuaded to offer
TransZillas with various design improvements. I'd like to see someone
replace the whole circuit board set with an A/D converter driving a PIC
microprocessor and let you adjust shift performance curve via a Laptop
serial port connection. Wouldn't be all that hard to do...

Bye!
\\ Mark Hershey
Vin 2790,now full-time automatic (so far)