More on Auto Transmission and the Transmission Governor

["Mark Hershey" <dmcinfo@xxxx>]

There is a lot of confusion about the D's auto transmission computer, in
fact the term 'computer' is quite misleading. The D's 'computer' is entirely
analog, and does not have any influence on shift quality, only on shift
points.
As Bob correctly mentions below, the electronic computer on many late model
cars controls shift points and fluid pressure, but on the D the computer
controls only shift points. Fluid pressure is controlled by an entirely
conventional mechanical vacuum modulator. The idea, as I understand it, is
to let the bands (and clutches) slip a little on a slow acceleration to make
the shift smooth. Engine manifold vacuum is used as a measure of
acceleration-- high vacuum = slow acceleration. Jackrabbit starts cause the
air intake to increase, which reduces manifold vacuum. The vacuum modulator,
mounted on the rear of the transmission, uses this vacuum to increase
transmission fluid pressure delivered to the band acuators to make the bands
grab with little slippage.

The D's vacuum modulator is adjustable, and a procedure for doing so is
included in the DeLorean service manuals. It involves a fluid pressure gauge
connected to a test point on the transmission case and the adjustment is to
rotate the outer ring of the modulator.

The D's so-called computer, being analog, has no eprom and doesn't do any
calculating. Sole function is to run the shift solenoids via a couple of
$0.23 (in 1981) comparator chips. Doesn't even have any control on reverse
valving so if anyone is having problems with Reverse, it ain't the governor.
Does play a role in backup lights and in downshifts.

As far as I know the vacuum modulator is trouble free; unaware of any
failures at all. Even need for adjustment is rare.
On modern transmission controllers the shift points can be programmed by
changing Eproms or sometimes via a serial data port. The D's shift points
can be changed by changing resistor values in the comparator circuits. Some
years back there was some reference to European governors having different
shift points; I think Swedish Autoservice specifically carried the Euro
version.

Today, one could make a programmable microprocessor-controlled circuit to
replace the '81 analog design. Take a look at a product called the Basic
Stamp (www.parallax.com, I think) to imagine the possibilities. It's a Basic
interpreter on a chip!

Tips I accidentally left out last time:

There is a kickdown microswitch that tells the Computer to downshift
immediately when you floor the accelerator. Bad switch or wiring = no
instant downshift. To troubleshoot, first make sure the microswitch is
mechanically engaged when you floor the pedal. The micro in question is one
of two small white ones that are near the throttle spool; the other one
shuts off the A/C compressor when you floor it. Try swapping the switch
wiring-- just swap the switch's connectors--to see if the problem follows
the switch (i.e., now you get a downshift but the A/C doesn't cut out. )
NOTE: If the switch fails ON, then the governor would think you have it
floored all the time and your symptom would probably be no upshift (1st gear
hold). Try just disconnecting the switch before blaming the computer- I've
had two go bad so far. Little buggers can get intermittent, to, so
disconnect the switch and drive awhile to be sure.

The D's backup lights can help troubleshoot gov. problems....Fuse #13 ( I
think, drawings aren't with me here at work)controls the governor and
supplies current to the backup lights, so if your car seems to start in
third gear (a commonly reported failure mode) then check to see if you have
backup lights. If not, the gov. is probably OK; check, clean, and reseat
the fuse. Pulling and reseating the red connector under the ignition coil
cover will often correct continuous or intermittent third gear starts if
fuse 13 checks OK.

\\ Mark


----- Original Message ----- s a measure of acceleration- high vacuum means
tFrom: Bob Brandys <oehcs@xxxx>
To: <dmcnews@xxxxxxxxxxx>
Sent: Monday, July 03, 2000 10:07 AM
Subject: Re: [DML] Re: Auto Transmission Governor Secrets Revealed...(long
post)


> The main difference between the DeLorean automatic and the later model
> transmissions is how the computer control the pressure modulating valve.
>