Bob Tom <tigers@bserv.com> wrote:
: I don't have my FSM right now so I was wondering if anyone
: has knowledge readily at hand on how the OD button actually works.
I have done some research in this area, but its been a while so
I'm a bit rusty. Someone please correct me if I say something stupid
and/or wrong. :-) If I remember correctly, the "OD OFF" button
is just a momentary switch which grounds one of the PCM inputs. Its
a momentary SPST switch, one lead of which is connected to ground, the
other is connected to one of the PCM pins (via wires of course) ;-)
Anyway, pushing the switch merely temporarily grounds that input
which is normally "floating" or "open"; when the PCM sees the ground
on that wire, it does whatever it has to do to keep the tranny out
of OD. (It also activates a PCM output whose sole purpose is to
light up the OD OFF lamp.) As soon as it sees ground again on the
input from the OD OFF button, it turns off the light and allows the
tranny to go into overdrive again.
So, the PCM itself keeps track of the current "state" of the OD OFF
feature, and activates the light and controls the actual ability of
the tranny to go into OD, based merely on watching that input and
flip flopping the overdrive state every time it sees ground.
: For the races at the track, I push the button to take OD OFF (OD dash
: lamp comes on) while I'm in the staging lanes just before heading onto
: the track. The last couple of times at the track, on about 30-40% of
: my runs, while I'm wot in 3rd (manually shifted) and after the 1000' marker,
: OD will come ON by itself (dash lamp goes off). Naturally, my rpms
: drops from the 5000+ range to 4000.
: Anyone have an ideas at what areas I should look at to investigate
: and find a solution to this intermittent problem? Thanks.
Assuming that switching the OD on isn't some sort of "feature"
within the PCM itself, I would suspect that in order for the OD
to go on, that PCM pin is getting grounded somehow. I think the OD
OFF button is in the gear selector on the newer Daks - if that's the
case, maybe you are accidentally touching the button while shifting,
or maybe the action of moving the handle is grounding the wire via
frayed wires, a defective switch, etc. Something you might want
to try, if you are comfortable cutting up your wiring harness, is to
find the OD OFF input wire and snip it. If the problem goes away,
then somewhere between your cut point and the OD OFF button, that
wire is getting grounded. The trick then is to figure out where,
and why. (Of course, if you do this, in order to "activate" OD OFF,
you will need to use an alligator clip or something which is connected
to ground and briefly touch the end of the wire which runs into the
PCM. This would need to be done every time you start the truck.
A temporary switch inside the cab would make this a bit less of
a hassle.) Another thought might be to take a look at where
the OD OFF button itself is connected to ground, and disconnect it.
If the OD OFF button isn't connected to ground, then it can't
function. (Same as above, you then need an alternative method of
grounding that PCM input in order to turn OD off.) If you remove
ground from the OD OFF button and the problem goes away, I would
highly suspect that there is a problem within the OD OFF switch
itself - it must be allowing the contacts to come together even
though the button isn't being pressed.
I hope this helps! Should give you some ideas to start with anyway!
---Jon-
.---- Jon Steiger ------ jon@dakota-truck.net or jon@jonsteiger.com ------. | I'm the: AOPA, DoD, EAA, NMA, NRA, SPA, USUA. Rec & UL Pilot - SEL | | 70 Cuda, 90 Dak 'vert, 92 Ram 4x4, 96 Dak, 96 Intruder 1400, 96 FireFly | `------------------------------------------ http://www.jonsteiger.com ----'
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