Win 95 3.8 Speed signal to PCM missing

[famzapata]

I have P0500 code in my WIn 95 3.8. I´ve changed tha VSS Sensor Clear code and returns the engine light and the same code, with scanner freeze frame utility I can detect that RPM =1140 but the VSS = 0 when the P0500 occurs, SO I don´t have VSS signal to PCM ??? What could I Check ???

[wiswind]

To avoid confusion, the Vehicle Speed Sensor (VSS) is located on the back side of the transmission, above a catalytic converter......tough to get at.
NOT to be confused with the Turbine Speed Sensor (TSS) which is located on the side of the transmission......easy to get at.
2 Completely different sensors......the TSS senses the speed of the shaft coming out of the torque converter.....before the different gearing.

Do you get a speedometer reading?, the shaft turns both the speedometer cable and the electrical sensor for the PCM.
Looking on Rockauto, there is 1 version for electronic speedometer and another version for the mechanical speedometer.

If the speedometer is working correctly, then you have the gear/shaft correctly working.
Then the problem is with the electrical portion......and the new unit should give you SOMETHING, so.....that leades us to verify the ellectrical connection between the 2 terminals inside the connector on the speed sensor and the connection to the PCM.

I just had this part replaced on my '96 3.8L, and RockAuto lists the same part for 95/96 windstar.
I was far from home with a group......so I had the job done at a shop.
The guy had to do the job by feel......cannot see in there.
I would verify that the ELECTRICAL cable is properly seated onto the connector....and feel along the cable for any signs that it has become damaged.
You are right above a HOT catalytic converter.....hot motor and exhaust parts.....so the cable MUST be routed such that it is not in contact with any of these hot parts.....or any places that it could rub through the insulation over time.

Looking this up on the electrical diagrams on alldatadiy, it is a simple "magnet signal generator"
1 pin goes directly to GROUND, Ground # G200
The other pin goes directly to pin #58 on the PCM through a Dark Green with White stripe wire.
It does NOT show any splices or other connectors other than the connectors to the speed sensor, PCM and ground. So that limits what could be unpluged. The PCM connector is 1 BIG connector with lots of wires.....so if you are driving the vehicle, one can assume that it is connected......although there is the outside chance of a problem wiht corrosion on 1 pin (not likely).

[famzapata]

I´ve checked with my mechanic and the connector and the cable seems OK, Could be PCM??

[Selectron]

I had a look at my diagrams for the '95 3.8 just out of curiosity, and they're showing a different configuration, Wiswind, so I wonder whose are correct. Mine show the VSS as connecting to a few other items in addition to the PCM, so if these are correct, famzapata, then you can't yet assume that the PCM is faulty, especially not if the vehicle is running well apart from the code. There are links to the four diagrams for the '95 engine controls here.

The VSS is at the top left of diagram 4, and you'll see that the signal side of the VSS connects to:

• Speed Control Servo (pin 3)
• Generic Electronic Module (GEM, pin 14)
• PCM (pin 58)
• Instrument Cluster (pin 12)

... and the ground side of the VSS connects to:

• PCM (pin 33)
• Ground G201 (analog cluster) or G134 (digital cluster)

Additionally, there are three splice points along the signal path, and two along the ground path.

I don't know that it will help us any, but does your speedometer work, is your instrument cluster analog or digital, and, when using your scanner, can you see if the VSS signal is always missing, or is it intermittent?

[famzapata]

The Speedometer is analog and it´s working fine, the VSS signal is ok a few minutes then go to 0 in the scanner, We have check it with the sensor out and turn clockwise and the scanner detects de Signal of the VSS, but when you put it in his place and run the windstar a few minutes ago the signal just disapear.

[wiswind]

I wonder if the problem is heat related?

[Selectron]

This is an awkward fault, and I don't think I can help any. I would agree there's a good chance it could be heat-related, but it's going to be difficult to find the location of the problem.

There's not much point in checking the wiring for continuity because it would almost certainly show as being good, because the VSS signal is present when you spin the sensor by hand, and when the engine is first started.

It might be worth checking to see if the van maintains a steady speed when the cruise control is set, although you'd probably have to get up to speed quickly, and get the cruise control set quickly before the fault appears. If the code then appears but the van maintains a steady speed, it would suggest that the ground is ok, that the VSS signal is reaching the Speed Control Servo, but might possibly indicate that the signal isn't reaching the PCM, or is being lost within the PCM. That's only a guess though.

Obviously it would be worth locating the three modules - PCM, Speed Control Servo, and Generic Electronic Module, and ensuring that the connectors are firmly plugged in and that the wiring isn't being stretched. If you want to unplug the connectors and reseat them then disconnect the battery negative cable first before doing that, to avoid damage to the PCM.

An oscilloscope would enable you to trace the signal, or a good quality multimeter with a sensitive AC voltage range, but of course you can't do that on a moving vehicle. With the van stationary, and spinning the sensor by hand, you could trace the signal, but of course you have already proved with your scanner that the signal is present at that time. This could be a difficult one to solve.

Edit: I was reading in this thread that the VSS has a heat shield - is the shield in position?

[tomj76]

I have a suggestion for remotely measuring the voltages on wires inside the engine compartment with the vehicle in operation.

Using a straight pin (the kind used for sewing), pierce the wire that you'd like to monitor. Clip to the straight using a long insulated wire with allegator clips at each end. Route the wire from the engine compartment, over the cowl, through the passenger door. Clip the other end of the wire to the meter. Ground can be obtained at the power connector on dash board.

There should be two people in the vehicle, one to drive and one to monitor the meter. Multiple points can be checked by adding more straight pins and wires. If this is done, a piece of tape can be wrapped on each wire to attach a label.

Choose the test points in order to isolate where the signal is lost. Keep in mind that a fault in the ground wire connection could also be the cause of this problem.