jon@dakota-truck.net wrote:
> "Zito, James A \(GE Infra, Energy\)" <james.zito@ge.com> wrote:
>> Ok folks odd one on the Dak, I keep getting the upshift light when I'm in top gear......
>> Thoughts??
> Other than installing a 6 speed transmission, you mean? :-)
Ok, I finally had a chance to look at an '03 PCM, so I figured I'd
take a stab at a more serious answer to your question. :-)
Unfortunately, the FSM doesn't seem to say exactly how the PCM decides
when an upshift is required, but it does say that the PCM is the one
that makes the decisions regarding when the upshift indicator is
turned on and off. (The indicator is controlled by the instrument
cluster logic circuit, but it is the PCM which tells the instrument
cluster to turn the light on and off.) It would appear that the PCM
uses engine RPM and load (MAP?) to determine when an upshift should
be performed. I do not believe that the Dak has any switches which
would indicate gear selection to the PCM, with the exception of
reverse, so it would seem the only thing which normally keeps the
indicator light off in 5th gear is simply the tall overdrive ratio
which keeps the engine RPM low and the engine load high.
Does the upshift indicator appear to function properly in other
gears? What is the nature of the indicator coming on when in 5th - is
it random, or does it seem to happen more when you are at a certain
RPM range, accellerating, at a constant speed, etc? The PCM will
normally send a signal to turn off the upshift indicator 3 to 5
seconds after it has turned it on if you do not shift, so if the
indicator comes on and you either stay constant on the throttle or
accellerate for 5-10 seconds or so, does the light turn back off? If
so, that would seem to indicate that this portion of the logic is
operating properly. If you have a vacuum leak somewhere, this might
throw off the PCM, and a flaky tach signal might also, though both of
those problems would probably manifest themselves in other ways as
well, and you would probably notice that the vehicle was not running
the way it should.
Unfortunately, to do any diagnostics of the PCM sending the upshift
signal to the instrument cluster requires a DRBIII, which would mean a
trip to the dealer and so probably not worth the expense for a
relatively benign problem like this.
The FSM does mention one other possibility which would be worth
a try. The instrument clusters are all the same and when they are
attached to a particular truck, they basically adapt themselves to
that truck (i.e. determine wether they are in an automatic or a
manual, 2wd or 4wd, wether cruise control or a security system is
present, etc.) and they enable and disable certain features to match
the equipment installed in the truck - pretty slick, actually. The
FSM says that you can reset this logic by disconnecting the negative
battery cable and waiting 5 minutes, then when you reconnect the
battery, the instrument cluster will re-learn the truck. If the
problem is in the instrument cluster itself as opposed to the logic
within the PCM, this may solve the problem, but its hard to tell given
the info you gave about the problem. It's certainly easy (and free)
to do, so its worth a try. (The FSM basically gives a list of certain
items, one of which is the upshift indicator light, and says that if
the problem involves any one of those items not illuminating properly,
the battery disconnect procedure should be done as a matter of
course.) This battery disconnect procedure has been used by DMLers
for years to reset the PCM, and doing it as mentioned in the FSM will
work, but if you are the impatient type, you can use the Bernd method
of disconnecting the negative battery cable and then turning the key
to start. (This drains any juice which might be left in the system
right away so that you don't need to wait the 5 minutes.)
I hope that helps, good luck with it!
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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