Transmission speed sensors - LH cars.

Based on posts to this group, I've noticed that the magnetic
speeds sensors seem to be the weakest link of the 42LE
transmission used in second generation LH cars, but that it
is apparently not too hard to replace the sensors. I'm
looking at the factory service manual, but it is unclear
from the diagrams & directions if the sensors are accessible
from above, or if the vehicle must be raised. (If the
transmission is stuck in 2nd gear limp mode, I can imagine
that the transmission may not like pushing a car up onto the
lift). The step by step directions in this manual usually
include the step raise vehicle when necessary, but they
don't in this case.

Couple of questions:

Will autostick still be available to shift manually if the
sensor(s) fail and the transmission goes into Limp Mode?
This would be an ideal use for the autostick, and I can't
imagine driving in 2nd gear for long to get to a place where
I can repair.

For the sensors that failed, was it due to something simple
like dirty sensors, or was it something internal to the
sensor?

Looking at the parts manuals, I noticed that 1998 model
year LH cars use Speed sensor (input/output) part numbers
4412878 / 4412879 while 1999 model year and later LH cars
use Speed sensor (input/output) part numbers 4800878 /
4800879. Does the change in part numbers represent a more
reliable sensor for later vehicles?

Any hints on finding the sensor?

Thanks!
(want to file this away in case or until I have this problem

 

Greg - You may find this thread on the 300M ezBoard helpful (diagram of
location from the FSM shown):
http://pub88.ezboard.com/f300menthusiastsclubfrm23.showMessage?topicID=46.topic.

You cannot access the sensors from above. Jack the left front of the
car up and put on jack stands to get to them. Takes 1" socket to R&R
the sensors. Some standard length sockets may not quite be long enough
on one of the sensors - could possibly need a deep well. The bodies are
plastic, so there's not a lot of torque involved.

My understanding is that, although external conditions can affect the
sensor (gunk around the sensors and the target splines), the failure is
generally due to the sensor itself weakening (my guess is that the
magnet weakens so that the signal amplitude falls below a threshhold
that the computer has to see to count the pulses).

Some report that the connectors themselves can be the problem and that
simply demating, cleaning the terminals and re-mating has fixed it
sometimes, but I think usually that is wishful thinking. Sensors are
around $20, which you probably already know since you've been reading up
on it. Some owners are known to keep spares in the glove copmpartment
(probably an over-reaction, but not unreasonable).

Don't know about the reason for the new part numbers. You'd like to
think it is a new and improved part (i.e., lasts longer), but not sure.
Could be simply an alternate source/mfgr.

Autostick function is locked out when a faulty speed sensor is
detected. Keep in mind that the computer does not know what the problem
is - it thinks that the tranny itself could be failing and is trying to
protect it from further damage. It simply goes into a limp module of
the firmware and stays there (or goes in and out if the sensor is
intermitent or marginal - which does happen).

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

You *might* be able to get to the input sensor from above on the
driver's side, but I doubt it. And the output sensor would be hopeless.
Better to raise it and get at both.

(If the

Second gear should be no problem for driving up a lift, might be a bit
tricky driving up do-it-yourself ramps but probably not bad.

No! Autostick control is all routed through the computer, and the
computer still controls the exact timing, rate, and firmness of your
"manual" shifts. With a dead sensor, the computer cannot do any of that
so it reverts to second-gear-only mode, which is the default if you
remove all electrical power to the transmission and the only control is
the manual lever.
?

Driver's side, above the pan rail line. A single wire bundle drapes down
the side of the transmission and splits to two plugs on the side- those
are your sensors. The crank positioin sensor is on the passenger's side
and can't be confused with the input/output speed sensors.