From davek@net.express.net Wed Jun 30 10:45:51 1999
From: Dave 

Eric said:

>I began having an intermittent sudden downshifting of my A/T the other day.
Seems to happen once or twice a day.  It first happened at about 45 mph ...
then the next day at 35 mph ... and so on.  It shifts itself back up to the
proper gear and goes away.  I saw the discussion of the TCU and am wondering
if the TCU may be culprit.
------------

Could be, but the TCU is pretty hard to damage... I'll bet it's something
simpler, and less expensive.  Let's do classic troubleshooting and eliminate
a few possibilities.

First of all, be aware that there's two types of 'shifting' going on under
you-  one is gear selection, the other is torque converter lockup.  The
former is kinda soft and mushy, while the latter is quite firm.

When the torque converter is unlocked, a slight modulation of the throttle
will cause several hundred RPM of engine variation, without an immediate
change of road speed.

When the torque converter is locked, a slight modulation of the throttle
will not cause change in engine RPM without change in road speed.  Engine is
physically locked to wheels.

This 'downshift' that you're feeling could be an errant change of ratio (a
true downshift) or an unexpected unlocking of the torque converter.

Before we start chasing this gremlin, let's find out what's really
happening.  Next time it downshifts, see if you can modulate the throttle a
bit and see if the RPMs fluctuate without changes in road speed.  When it
seems to 'upshift', do same.  Determine wether you're actually getting a
ratio change, or an unlock-relock condition.  After you've had a chance to
noodle with the throttle a bit, you'll be able to tell wether the converter
is locked or unlocked.

The TCU has two defaults programmed in which force TC lock operations-
first is mandatory unlock-  which occurs when you release the throttle (to
idle position).  This prevents you from coasting to a stop with the
converter locked.

The second is mandatory lock, which occurs when ground speed is pretty high
(somewhere above 50mph?), and the throttle is forced to full-open.  This
allows you to make most efficient use of the engine's top-end RPM.

The TCU determines thresholds for these two mandatory unlock and manditory
lock conditions based primarily upon throttle position.  What you could be
experiencing is a situation where the TCU is getting a poor or errant TPS
signal, which the TCU might be construing as an idle, or WOT state.  You
could be getting an errant signal from a failed TPS, it could also be due to
an improperly adjusted TPS, or, since the '93-on models 'shared' the TPS
signal between EFI ECU and TCU modules, an impedance mismatch in the EFI ECU
or circuitry could be fouling the TPS signal enough to make it 'think' the
throttle is elsewhere.  If the situation is REALLY bad (electrically), then
the TCU might actually be thinking that you're demanding LOTS of
accellerator pedal, and forcing a ratio change.

Or, your problem could be much simpler-  a bad connector, shorted wire, or
something similar (wires to SOL1, SOL2, SOL3, TCU power, a bad ground wire?
Poor connection on SOL3 will cause the TC to unlock unexpectedly...  oh-
Check that big strap that goes from rear of cyl head to firewall... it needs
to be clean, tight, clean, and tight.

THEN there are the mechanical possibilites.  The tranny's Servos need
pressure to operate.  If, for some strange reason, you've got improper
pressures (low or high fluid? too hot?) the valving may not be able to hold
the shift position, and momentarily drop out to the next lower ratio... or
unlock the converter.

Find out what that pig is really doing, then drop me a note with your report.  

DK  :-)
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Censorship reflects a society's lack of confidence in itself.
             --Supreme Court Justice Potter Stewart