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Old 01-30-2008, 01:09 AM   #1
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What is the difference between a VSS and an OSS?

What are the physical differences between the two sensors:
Vehicle Speed Sensor, Outputshaft Speed Sensor?
I'm talking about the actual sensors themselves.
Do they emit a different waveform, polarity, voltage, etc.?

The reason I am asking, is that I was wondering why a Speed-Cal, Abbott ERA or similar signal generator box cannot be used on some car models.
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Old 01-30-2008, 07:00 PM   #2
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Slithering Joe:

The VSS is a little alternating current generator (yes, AC!). I don't know about the OSS (haven't measured the output).

For Mustangs (including Cobras) the SpeedCal will work with all pre-99 models (auto or manual) and all 99-04 manual Mustangs. They are less popular in the 99-04 models because the speedo/odo can be easily corrected using a handheld flasher (bringing the benefits of a tune).

Outside of Mustangs, I can't tell you anything. DallasMustang has a forum and a FAQ on their site for SpeedCal users and I think you will find better answers (than mine) over there.

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Old 01-31-2008, 08:59 AM   #3
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both sensors operate the same(hall effect). I think it has to do with how the manufacture uses the output,how many teeth fly by the magnet,spacing of teeth etc. the oss is usually used in trans for shifting,ratio determination etc
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Old 01-31-2008, 07:33 PM   #4
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The source of vehicle speed (VSS) is model dependent. Possible sources of vehicle speed input are anti-lock brake sensor (ABS), a gear-driven vehicle speed sensor (VSS), or an output shaft speed sensor (OSS). The VSS signal is either an A/C signal whose frequency changes with speed, or an SCP data message depending on the source(ABS sensor). Some applications will have both. The vehicle speed signal is an input to various vehicle subsystems such as the powertrain control module (PCM), instrument cluster (speedometer and odometer), speed control system, etc.
Regardless of the type of vehicle speed system, the PCM always uses the OSS for transmission and engine control.
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Old 01-31-2008, 08:16 PM   #5
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chuck943:

Excellent info!

Slithering Joe: Sorry, the A/C generator is what I have on my '98. I was a little too specific!

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Old 01-31-2008, 11:08 PM   #6
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It sounds like only the automatic Mustangs have problems with the SpeedCal, ERA, type boxes. Correct me if I am wrong, but the automatic trans Mustangs are the only ones using an OSS, right? I was wondering if the OSS returned a different type of signal compared to the signal from a VSS. By "different" I mean the signal has different peak voltages, sawtooth wave, square wave, sine wave, pulsed ground, pulsed + , etc.

Please don't confuse my question is to why an auto trans won't shift correctly. That is something else. I was asking if the signals were different from an OSS compared to a VSS and that would be why the problem exists.

It sounds like the output from a VSS is the same as a OSS. Correct?

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Old 02-01-2008, 01:58 PM   #7
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Slithering Joe:

'96-98 auto Mustangs can use a SpeedCal with no problem. It taps into the VSS wires.

+'99 auto Mustangs "shouldn't" use a SpeedCal but it can be plumbed up. A handheld tuner is strongly recommended. These models don't use separate VSS/OSS; there's just one sensor. The +'99 stick cars can use a SpeedCal and it plumbs into the "speed signal wiring". The SpeedCal used is the "extended range" -- whatever that means. I can't tell you what the official name of the sensor is for the +99 model cars.

Lastly, I have no idea about the +2005 Mustangs, stick or auto. You'd be better off calling DallasMustang or checking their website to see if they have an application for these models.

Chris
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Old 02-01-2008, 04:21 PM   #8
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Thanks, but my question was about the sensor signals.
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Old 02-01-2008, 08:01 PM   #9
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I am pretty sure they are all hall effect sending unit(ac generator) the voltage range(output) of the sensors should be different because of the amount of "teeth" passing the sensor per revolution. maybe the speedcal won't work because of the voltage output. every thing else about the sensors should be same. there are a lot of ways a computer can read the signals,like in a distributer.but all hall effect sensors work the same.Are the input voltages the same on the oss and vss you are refering to?
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Old 02-01-2008, 09:38 PM   #10
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I have not worked with the speed cal but I would assume it has to do with where the vehicle gets its speed signal from. If vehicle is equiped with a VSS sensor 96-98 then it would be easy but on 99+ models most get that signal thru the SCP network from the ABS wheels speed sensors(which on earlier versions was AC signal,newer models are digital) via the ABS module.I am not 100% sure on this but I believe that once it is transmitted from the ABS module to the SCP network it is in a digital form.
The OSS although it is a HZ signal the PCM reads as an RPM.
Does this help?
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Old 02-03-2008, 12:00 PM   #11
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The VSS and OSS sensors on late model Mustangs are both "variable reluctance" style pickoffs. They both generate an AC waveform whose frequency is inversely proportional to vehicle speed. All that means is the faster you go the higher the frequency of the AC signal.

The computer measures the frequency in both of these signals with a simple zero crossing detect circuit.

Prior to 99 all Mustangs had a VSS sensor if it was a manual and a VSS and OSS sensor if it was an auto. The VSS fed the speedo and computer in parallel and was directly connected to the speedo (IE the computer did not generate and thus could not modify the speedo value). Thus the only way to change the speedo reading up to 99 on any model mustang was to modify the frequency scaling of the VSS sensor. The OSS sensor used by the autos is fed to the tranny control module and is used to directly measure tranny output shaft speed (thus the name). I can think of no reason you would ever want to modify this sensor as it will screw with you shift program.

In 99 Ford combined the two sensors into one as they essentially measure the same thing, they directly measure the rotational rate of the transmission output shaft. Also, the speedo in 99 and newer Mustangs is controlled by the computer. In combining sensors they forever combined the speedo and shift program on auto cars. This is why electronic recalibrators should never be used on a 99 or newer automatic Mustang. Plus, the unit is redundant anyway in these applications. You're gonna want to screw with the shift program if you change the gears so rescale the computer while you are at it.

For 99 and newer manual tranny Mustangs the electronic calibrators will still work for the speedo since there is no auto shift program to muck up. However, if anybody is going to do a chip they are redundant in these applications also.

Hope this cleared things up.

Last edited by TriBlkCobra; 02-03-2008 at 12:02 PM.
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Old 02-03-2008, 06:19 PM   #12
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TriBlkCobra to the rescue!

Thanks, Willie!

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Old 02-03-2008, 10:51 PM   #13
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Thanks TriBlkCobra, this is the kind of answer I was looking for.


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Originally Posted by TriBlkCobra View Post
The VSS and OSS sensors on late model Mustangs are both "variable reluctance" style pickoffs. They both generate an AC waveform whose frequency is inversely proportional to vehicle speed. All that means is the faster you go the higher the frequency of the AC signal...
That said, wouldn't it be speed is proportional to frequency not inversely proportional ?
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Old 02-03-2008, 11:04 PM   #14
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Yeah I stated it wrong. The computer measures time between zero crossing edge events ( I said frequency in the previous post, but the initial measurement done is time between edges). So as you go faster the time between edges goes down. Frequency is 1/time so yes speed and frequency are proportional. Good catch, thanks for getting that to keep it clear. I get to typing a lot of times and dont keep my units straight sorry.

Its also worth noting that the computer is pretty tolerant of the input signal type. I think the basic signal coming out of the Abbott is a ttl type of signal, it might have an AC coupling network, I dont remember right now. But it is quite low voltage.

With the speedcal I have tried all three output types. TTL signal levels (basically 0V to 5V logic levels), AC coupled +/-10V signals with low drive capability, and DC coupled +/-10V signals with a few milliamps of drive. The input to the computer is quite high impedence and tolerant of very high voltages. On the 99 cars I've measured I think in excess of 50V peak with the car at highway speed.

Neither the VSS or OSS on the Mustangs are hall effect either. From what I've seen most of the North American designs use the basic variable reluctance type of sensor. The Focus which is European in its roots does indeed use a Hall Effect style of speed sensor however.

Last edited by TriBlkCobra; 02-03-2008 at 11:07 PM.
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Old 02-03-2008, 11:06 PM   #15
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Good info TriBlkCobra!
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Old 02-04-2008, 07:45 PM   #16
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I looked at the speedo information on mustangs all the way to 2008 to see if the newer models changed and all of the mustangs get the speedo signal from the PCM(via the OSS as triblkcobra stated).
"Instrument Cluster (IC) Gateway Function
The instrument cluster (IC) acts as a gateway module by receiving information in one format and transmitting it to other modules using another format. For example, the instrument cluster (IC) receives the vehicle speed data from the PCM over the high speed controller area network (HS-CAN), converts the data into a medium speed controller area network (MS-CAN) message and sends (gateways) the message to other network modules such as the heating ventilation air conditioning (HVAC) module, the audio control module (ACM), and the SJB. This enables network communication between modules that do not communicate using the same network (HS-CAN or the MS-CAN)."

Where as some of the other vehicles example Lincoln LS get the signal from ABS, "Speedometer Gauge
The instrument cluster uses a vehicle speed signal from the ABS/TC/IVD system to control the movement of the speedometer pointer. Vehicle speed information is sent to the ABS/TC/IVD module from all four wheel speed sensors, which is then sent to the instrument cluster by the ABS/TC/IVD module over the SCP communication network."
Makes it kinda confusing to keep up with.

On Mustangs though the ABS wheel speed sensors are used by the PCM as a secondary signal only in case of loss of OSS signal.

Last edited by chuck943; 02-04-2008 at 07:58 PM.
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