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Discussion Starter · #1 ·
Hi everyone. Recently, my '97 GL/3.0 V6 flashed the SES light and the transmission wouldn't shift out of first gear until 30MPH, and the speedometer stopped working. Checking the VSS, I noticed the two wires for the sensor were melted together. I replaced the sensor, and using a factory diagram, both wires (PNK/ORG and GRY/BLK) from the PCM connector to the sensor.

The tramsmission now shifts fine, and the SES light is off, but my speedometer still doesn't work. Thinking the melted wires may have fried the speedo internals, I picked up a used cluster from a scrapyard, but it still doesn't work. All the other gauges (tach, temp, fuel) work fine as do all the indicator lights. All fuses are good and everything else on the car works.

My question is this: What should the GRY/BLK wire going to the speedo read (when measured from the cluster connector) with the key on? My meter shows .66 of a volt.

The ground to the speedo is good, so I think it may not be getting the proper signal, voltage or whatever on the GRY/BLK wire. Could it also be possible the speed control module is bad?

Any and all help is much appreciated.
 

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It is highly unlikely shorting of sensor wires (low voltage and current) can damage a circuit. The sensor is a more probable culprit than the circuit (cluster). Sorry I don't have 97 but will look up 05 wiring diagram later today.
 

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Discussion Starter · #3 ·
Thanks paker,

I solved the problem. One of my wire connections (on the GRY/BLK wire)wasn't making any connection with the speedo, and that's whay it wasn't working.

I still wonder though, what kind of signal is sent to the speedo through the VSS, and how is it measured (voltage, ohms, etc.).

Anyhoo, my speedo works, the trans shifts like new, and all is well. Again, thanks for the suggestion.
 

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The VSS/OSS is a small AC electric generator. It puts out a few volts (higher voltage at higher speed), but the output used by the speedo/ PCM is the frequency output. Ford VSS/OSS put out 8 "pulses" per revolution, and with the industry standard for older mechanical speedometers (1000 revolutions/mile) that is still used today. So, the VSS produces 8000 pulses per mile. This translates into 2.22 Hz/MPH. In other words, if the car is moving at 100 MPH, the VSS puts out a frequency of 222 Hz, at 50 MPH, it puts out a frequency of 111 Hz.
 

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Vss

The VSS/OSS is a small AC electric generator. It puts out a few volts (higher voltage at higher speed), but the output used by the speedo/ PCM is the frequency output. Ford VSS/OSS put out 8 "pulses" per revolution, and with the industry standard for older mechanical speedometers (1000 revolutions/mile) that is still used today. So, the VSS produces 8000 pulses per mile. This translates into 2.22 Hz/MPH. In other words, if the car is moving at 100 MPH, the VSS puts out a frequency of 222 Hz, at 50 MPH, it puts out a frequency of 111 Hz.
Still wonder. Does ABS use wheel pickup for speedo readout. They still have a VSS in the trans. My Lincoln had a loose front wheel speed sprocket and messed with the cruise and speedo and ABS. Tells me those use ABS pickups for those things. Then wonder what the VSS does for the system. Is it a default is the ABS pickup fails?

-chart-
 

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Some Fords do use ABS sensor signals for speed info, but the non SHO Taurus / Sable do not. VSS is used for speedo and shift stratagy. I think many cars that came standard with ABS use ABS signals for speedo/tranny, but on cars with optional ABS, like Taurus/Sable, VSS is used.
 

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Vss

Some Fords do use ABS sensor signals for speed info, but the non SHO Taurus / Sable do not. VSS is used for speedo and shift stratagy. I think many cars that came standard with ABS use ABS signals for speedo/tranny, but on cars with optional ABS, like Taurus/Sable, VSS is used.
Thanks, good info.

Lincoln all ABS and 4 wheel disc. So they can use wheel speed for cruise and speedo as there is no option for ABS on those cars.

Makes sense optionl vehicles would use VSS and let ABS stand alone with wheel sensors.

The Lin still has a VSS so it may be backup, may be some other plan like trans shift. There has to some backup so that when a ABS sensor fails, the car can still be driven.

So like in my case with the Lin, In the Taurus/Sable, bad wheel speed sense would only effect ABS, and traction control, nothing else.

-chart-
 

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Thanks paker,

I solved the problem. One of my wire connections (on the GRY/BLK wire)wasn't making any connection with the speedo, and that's whay it wasn't working.

I still wonder though, what kind of signal is sent to the speedo through the VSS, and how is it measured (voltage, ohms, etc.).

Anyhoo, my speedo works, the trans shifts like new, and all is well. Again, thanks for the suggestion.
DID you find more information about the VSS speedo measuring?
 

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The VSS/OSS is a small AC electric generator. It puts out a few volts (higher voltage at higher speed), but the output used by the speedo/ PCM is the frequency output. Ford VSS/OSS put out 8 "pulses" per revolution, and with the industry standard for older mechanical speedometers (1000 revolutions/mile) that is still used today. So, the VSS produces 8000 pulses per mile. This translates into 2.22 Hz/MPH. In other words, if the car is moving at 100 MPH, the VSS puts out a frequency of 222 Hz, at 50 MPH, it puts out a frequency of 111 Hz.
Hey Jeff..
I am bench testing my OSS sensor with a 8 pulse reluctor.

I spin the reluctor with my drill at 1500 RPM.

The sensor generates 0-2.00 volts AC.

Per the wiring diagram. This is feed into the PCM.

The PCM then outputs over the Grey/blk wire to the Speed-o, obd2 port and Cruise control module.

My question to you is :
1. What is the PCM output.. AC, DC??

2. While testing the Cruise control Module by running the Cruise test procedure. KOff, hold cruise OFF, Kon, sequence buttons and watch for actuator movement for system test..
1. The OSS can be completely removed from the system and the Cruise test will run with no error.
Meaning the PCM does NOT perform any Diagnostics of the OSS sensor during the test.
2. IF the Gry/blk wire is cut from the PCM to the Cruise module, the cruise test fails the Pull test and flashes 4 times.
So this means the Cruise test is looking for A signal from the PCM - not based on ANY OSS output..
WHAT is the PCM sending to the Cruise Module to allow the cruise test to complete?
It cant be dc volts??? that would interfere the OSS pass thru signal to the Cruise.

3. I want to figure out how to Bench test the cruise module without the presence of the PCM..

4. IF I can figure out what the PCM is communicating over the gray/blk wire, I could possibly removed the complete Cruise unit, emulate the required speed in put and use the Cruise unit of another project..

Your time and knowledge is GREATLY appreciated.
Kenneth Winter
 

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Hey Jeff..
I am bench testing my OSS sensor with a 8 pulse reluctor.

I spin the reluctor with my drill at 1500 RPM.

The sensor generates 0-2.00 volts AC.

Per the wiring diagram. This is feed into the PCM.

The PCM then outputs over the Grey/blk wire to the Speed-o, obd2 port and Cruise control module.

My question to you is :
1. What is the PCM output.. AC, DC??

2. While testing the Cruise control Module by running the Cruise test procedure. KOff, hold cruise OFF, Kon, sequence buttons and watch for actuator movement for system test..
1. The OSS can be completely removed from the system and the Cruise test will run with no error.
Meaning the PCM does NOT perform any Diagnostics of the OSS sensor during the test.
2. IF the Gry/blk wire is cut from the PCM to the Cruise module, the cruise test fails the Pull test and flashes 4 times.
So this means the Cruise test is looking for A signal from the PCM - not based on ANY OSS output..
WHAT is the PCM sending to the Cruise Module to allow the cruise test to complete?
It cant be dc volts??? that would interfere the OSS pass thru signal to the Cruise.

3. I want to figure out how to Bench test the cruise module without the presence of the PCM..

4. IF I can figure out what the PCM is communicating over the gray/blk wire, I could possibly removed the complete Cruise unit, emulate the required speed in put and use the Cruise unit of another project..

Your time and knowledge is GREATLY appreciated.
Kenneth Winter
PCM outputs are all DC to my knowledge. As for the second question, I have some guesses. For one, it could be that it's looking for the brake switch signal to know when to disengage when you hit the brake. It could also be looking for throttle position %.
 

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Thanks for taking the time to respond.
PCM out put is tested to be 12 Volts at Key on
PCM out put decreases 12 volts dc down to 4.4 volt as the OSS rpm increases up to 1.800 volts AC.
The PCM seems to convert the output to DC and reverse the output to a decreasing output?????

Brake out puts: The Brake light signal and brake failure signal go straight to the Cruise control module.
I have those outputs identified.

I just can not figure out the PCM output is..
With the PCM out put (gry/blk) wire cut to the speedo, cruise control modue, obd2 port, the Cruise control test procedure fails.
 
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