Tuesday, June 21, 2011


If equipped with 4WD, this adapter is located on the transfer case extension (left side). If equipped with 2WD, this adapter is located on the extension housing of the transmission (left side).

If the Check Engine Light was on, a code will be stored in the PCM. That code will confirm the VSS is the cause of the problem.

You will need a scan tool to access the codes since Chrysler does not provide a procedure for retrieving or clearing Diagnostic Trouble Codes without a scan tool.

Description and Operation

Ford Aerostar V-6 Firing Order

Vehicle Speed And Distance Sensor - PCM Input:
The 3-wire Vehicle Speed Sensor (VSS) is located on the speedometer pinion gear adapter. If equipped with 4WD, this adapter is located on the extension housing of the transfer case (drivers side). If equipped with 2WD, this adapter is located on the left side of the transmission extension housing.

The VSS is a 3-circuit (3-wire), magnetic, hall-effect sensor.

The 3 circuits are:

·A 5-volt power supply from the Powertrain Control Module (PCM).

·A ground is provided for the sensor though a low-noise sensor return circuit in the PCM.

·An input to the PCM is used to determine vehicle speed and distance traveled.

The speed sensor generates 8 pulses per sensor revolution. These signals, in conjunction with a closed throttle signal from the throttle position sensor, indicate a closed throttle deceleration to the PCM. When the vehicle is stopped at idle, a closed throttle signal is received by the PCM (but a speed sensor signal is not received).

Under deceleration conditions, the PCM adjusts the Idle Air Control (IAC) motor to maintain a desired MAP value. Under idle conditions, the PCM adjusts the IAC motor to maintain a desired engine speed.


The speedo signal is sent to the cluster by the PCM.
I'd check the speedometer gear & the speedometer gear housing's position. If the housing isn't rotated properly, the speedo gear may not be fully meshed with its pinion drive gear. And if it's not, it's possible the speedo gear teeth have become damaged. I'd especially suspect this if your speedometer gear has ever been changed to recalibrate the speedometer after installing different size tires or axle ratio.

I'm running without a speedo gear to try and not have the needle fluctuate as much (and add a ton of miles to my odometer,) but it still does it occaisonally, even without a speedo gear in the t-case, which leads me to believe that it's a problem somewhere in the wiring.


I replaced the 'new' speedo gear with the original one, and haven't had a problem since, so it might have been something down in the transfer case where it reads the speedo gear (since it was still bouncing when there wasn't a speedo gear in there at all.)

Have you done anything (removed, replaced, messed with) your speedo gear lately?

it was that speedo gear: since it didn't do anything until awhile after I had installed it, it probably wasn't lined up quite right and got stripped. There was some small damage to the plastic teeth in a few spots when I took it out, but it seemed very minor.



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When mine did that it turned out to be the printed circuit board behind the speedometer. One of the cooper traces on it had corroded in two.

before looking at your ckt board, see if you have power to the gear on the xfer case. the three pin plug on the gear should have: 1 pin=9vdc 1 pin=ground and one pin with no reading. the 9vdc comes from the speedo cluster. if all reads out, your cluster circuit is good.If not then cluster board is bad.get it repaired.

max