Re: Gears

From: Matt Beazer (teseract@moparhowto.com)
Date: Sat May 09 2009 - 02:32:25 EDT


Some clarifications:

* My truck is a 1995 Dakota Sport 318 automatic
* It has 31" tires on it on stock chrome rims
* Per the FSM, it is an electronically driven speedometer that runs off
the vehicle speed sensor, per section 8E page 8E-2: "The VSS is mounted
to an adapter near the transmission (two-wheel drive) or transfer case
(four-wheel drive) output shaft. The sensor is driven through the
adapter by a speedometer pinion gear. The adapter and pinion vary with
transmission, transfer case, axle ratio and tire size."

Considering the mess of wiring under the dash and the mysterious
switches I have, it wouldn't surprise me if there's a tire size
corrector somewhere in the mix, but again, I haven't been under the
truck due to back issues. I have a lot of experience in the mid
'80s/early 90's 2.2/2.5L turbo cars that used the same setup and from
the diagrams in the FSM the Dakota even uses the same sensor, though it
just had a plastic gear that ran off the transmission, according to the
FSM you can "index" the pinion adapter that bolts to the speed sensor
for different "speedometer pinions" that look to be plastic gears for
different tire sizes/gear ratios with different numbers of teeth. From
what I can glean from the FSM, there looks to be 4 different speedometer
pinions to choose from, 32 teeth, 38 teeth, 39 teeth or 45 teeth. How
that relates to specific tire sizes and rear end ratios the FSM doesn't
say. I'd guess 45 teeth would be for a tall gear ratio (spin the gear
more slowly with more teeth) and 32 teeth would be for the shorter gear
ratios with shorter tires.

Either way I guess I'll find out eventually. I'm not going to trust the
tag on the diff though, if there even is still one on it. I'll have
have to check by jacking the truck up, putting it in neutral and
counting the number of turns on the drive shaft while I spin the rear
wheels.

There's no possibility that the transmission was replaced with one from
a Ram or something that has different gear ratios, is there?

MattB

Terrible Tom wrote:
>
>
> Gen I and II's use a sensor in the tail shaft of the transmission...
> or transfercase if its a 4x4. This sensor on older Gen I and some Gen
> II models uses a cable drive for the speedo. On newer Gen II's the
> speedo is wire driven - no cable. The ability to correct for tire
> size can be done electronically and there are a number of aftermarket
> gizmos. I don't remember what year Matts truck is - but if its a 1996
> OBD II model - the tire size can be corrected for using an OBD II type
> programmer - like my Hypertech Power Programmer I use for the Ram.
> Otherwise you can hardwire in a device by Superlift, which is
> programmable and switchable bwtween tire sizes. It wires in-line
> between the sensor and the ECM.
>
>
>



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