Vehicle Speed Sensor Malfunction P0500

FlipIt · Post by **FlipIt** » 21 Jun 2011, 12:48

My 1996 850 turbo wagon has a persistent Check Engine light due to a vehicle speed sensor malfunction (P0500). The speedometer works fine which I believe uses the anti-lock braking system for vehicle speed information. Since my wiring harnesses were in bad shape I traced the wires for continuity from the sensor to a multipin connector located under the ECM/TCM main connectors. I was surprised the signal did not go to either the ECM or the TCM. It ran along the inside fender into the firewall. I thought maybe the broken trip and total mileage odometers might be causing the fault but after replacing the odometer drive gear I still have the problem. I removed the sensor and measured it's resistance (540 ohms) and passed a piece of steel by it and noticed a very small spike in current on my 60ma scale. Even though it has no moving parts and appeared to be functional I ordered a replacement (PN W0133-1841585 by Scan-Tech from PartsTrain). The new one reacted the same as the old one to the piece of steel but its resistance was about 500 ohms so I installed it. I cleared the OBD fault and the Check Engine light but 0.7 miles from my house the Check Engine light came on and the forward arrow started flashing. I read the code and it was the familiar P0500.

I don't know if the vehicle operation is affected when the fault occurs because I'm afraid to venture far from the house. I've read on the internet that the VSS fault can affect torque converter lockup but don't know if that applies to my model. I'm beginning to suspect that the only use of the VSS output is for radio automatic volume control. However, if that were true then I don't understand how the ECM determines the output has malfunctioned.

Can anyone provide the wiring path from the VSS for a 1996 turbo 850 wagon?

Is anyone familiar with the VSS fault for a 1996 turbo 850 wagon?

BigRed · Post by **BigRed** » 21 Jun 2011, 13:03

FlipIt wrote:Is anyone familiar with the VSS fault for a 1996 turbo 850 wagon?

Yes, I have had that code persistently on a '96 855t for 2 years+. With the code present and the VSS on the top of the transmission connected the vehicle would hang on to 1st for far to long, bang into 2nd then hunt back and forth for awhile before deciding on 3rd. Crusing in 4th it would some times through a P0734 and drop into limp mode. More common would be fine at highway speeds, but after comping to a stop, it would then throw P0734 and be stuck in limp mode.

I recently got motivated to dig into it more. I disconnected the VSS and tested the resistance, measured infinity. I decided to leave it un-plugged pending a replacement sensor. Driving with it disconnected, obviously the code has stayed non-stop, but it shifts as good as it ever has! Has not entered limp mode since! So you may be on to something in regards to it not being responsible for much.

I did get what I believed was the right sensor from the dealer to replace it, but the wiring harness is different. I'll post pictures when I get back home.

FlipIt · Post by **FlipIt** » 21 Jun 2011, 15:18

Here's what my original and the replacement VSS looks like:

: W01331841585STP.jpg (5.74 KiB) Viewed 10719 times

Did you have a flashing up arrow when the Check Engine light illuminated?

I also drove the vehicle with the VSS disconnected and it didn't seem to make any difference. I placed the front end on jack stands and "drove" at low speeds for almost 5 miles. I speeded up to 25 mph briefly. The Check Engine light never came on. Since I don't have limited slip only the right front wheel was rotating and the ABS light came on.

I've read that on the newer models the VSS has been eliminated. Some members have run into the problem when swapping transmissions. I guess I won't make any progress resolving the issue until I get the wiring paths. It's possible the signal is routed to the instrument cluster and the microprocessor in the cluster sends the data to the ECM.

BigRed · Post by **BigRed** » 21 Jun 2011, 16:07

That looks like the one on the vehicle, do you have the part number? It's the one located on top of the transmission, right?

Both the Flashing Arrow and Engine light were on together, whole time (still are).

I think I read that the VSS signal is fed to the ABS Module and that, along with Wheel Sensor info goes to TCU. So my guess was that it uses all available info to make decisions, and my VSS was sending bogus information screwing it up. Remove it from the mix, and it makes decision based on what's coming from the wheel sensors. Pure conjecture, but seems plausible.

FlipIt · Post by **FlipIt** » 21 Jun 2011, 20:09

The Scan-Tech PN is W0133-1841585. It may be the same as the OEM PN which I couldn't find on the unit.
I downloaded a 1996 850 wiring diagram and have been searching thru it for the past couple of hours. So far I have not found the VSS on any of the pages. The ABS generates a speed signal that goes to the instrument cluster. The wiring diagram shows the ABS getting data from the wheel sensors but not the VSS. The transmission control module (TCM) also generates a km/h signal that goes to the speedometer. I can't find the source for the TCM to generate the km/h signal. The forward arrow is controlled by the TCM. The radio gets a speed signal from the instrument cluster. It is probably an analog signal generated by the microprocessor in the instrument cluster. I'll compare the 1995 and 1996 wiring diagrams to see if they match. I don't understand why I can't find the VSS on the 1996 diagram. I may try to find a 1997 wiring diagram.

When I had the wiring harnesses apart I checked for continuity between any pin of the engine control module (ECM) and the TCM and either pin of the VSS. I repeated the process several times and found no continuity. As I stated earlier the VSS lines went to a multipin connector below the ECM and TCM main connectors and then headed into the firewall.

FlipIt · Post by **FlipIt** » 22 Jun 2011, 12:45

I was wrong about the part number. I checked the number on the box and its 9168039, AT Speed Sensor, made in Italy. It looks identical to the one that's used for the engine speed sensor further forward and outboard on the transaxle.

I believe I have identified the VSS on the 1996 wiring diagrams. It is the item labeled Speedometer on the diagram and goes to pins 11 and 12 of the TCM. The wire colors are yellow/brown and green/yellow which matches what I found when I took apart the wiring harness. Unfortunately, as I mentioned in a previous post, there is no continuity between either wire and any of the pins on the TCM main connector. The wires go to another connector under the TCM main connector base. Maybe my vehicle was a late 1996 and caught in the transition to eliminating the VSS. Further wiring research is needed.

FlipIt · Post by **FlipIt** » 23 Jun 2011, 19:30

The 1995 wiring diagrams show the VSS outputs going to the instrument cluster pin A2 (green/yellow wire) and pin A3 (yellow/brown wire) with no vehicle speed output from the ABS. The 1996 wiring diagrams show the ABS vehicle speed output (another yellow/brown wire) going to the instrument cluster pin A3. I'm wondering if I have an early 1996 with 1995 wiring for the VSS and the later ABS with the vehicle speed output. I seem to recall that the speedometer worked even with the VSS disconnected but have to confirm that. I doubt that both vehicle speed sources would be connected to the same pin on the instrument cluster. I'll continue tracing the wiring from the VSS to see where it goes. I wish that I hadn't installed the upper dash after fixing the odometer drive gear so I could check the connections to the instrument cluster. But I wanted to test the odometer and connecting the battery with the airbag disconnected results in a fault that requires special equipment to clear. Maybe I'll try to determine if I have the 1995 or 1996 version of the ABS.

FlipIt · Post by **FlipIt** » 25 Jun 2011, 18:54

Well, I wasted my money and time purchasing and installing a replacement VSS for my 1996. Since I could not find any continuity between the wires coming from the VSS and the bundle of wires under the airbag I again removed the TCM and ECM and housing. I found that the yellow/brown wire goes to pin 5 of the multipin connector that was under the TCM and ECM main connectors but is not connected to anything on the other side of the connector.

: YelBrn.jpg (180.36 KiB) Viewed 10653 times

The green/yellow wire goes to pin 6 of the connector and also never comes out.

: GrnYel.jpg (199.78 KiB) Viewed 10653 times

There's a note on the 1995 wiring diagrams that for 1996 the Yel/Brn goes to TCM Pin A11 and the Grn/Yel goes to TCM Pin A12 so that's why the wires are routed there. Apparantly my vehicle was built before the full implementation. I assume that my vehicle speed signal comes from the ABS and goes to the instrument cluster. Since the speedometer and odometer work that signal and path must be functional. According to the 1995 diagrams a vehicle speed output signal is generated by the microprocessor in the instrument cluster and routed to the speed warning relay, the cruise control module and the ECM. There should also be a signal routed to the radio but my wiring diagrams don't include the radio. There are probably similar paths for the 1996 but I still have to determine them from the wiring diagrams. Then I will attempt to verify the paths on the vehicle. I suspect the VSS fault is due to either a bad speed signal from the instrument cluster or it is not reaching one of the intended destinations.

When I discovered that the VSS output is not used I was aggravated at myself for buying one before actually determining it was bad. But then I was thankful that the fault was not due to a broken trigger tooth inside the transaxle. I believe VADIS identifies the same part number for the VSS and the rpm sensor so now I have a spare.

FlipIt · Post by **FlipIt** » 28 Jun 2011, 15:39

I have confirmed that my wiring is basically the same as a 1995 model with minor exceptions (i.e. ABS). The ECM connector is the two part (A & B) used with the Motronic 4.3 multiport fuel injection. I haven't verified that the vehicle speed is generated by the ABS but I'm confident that it is since the odometer and the speedometer work with the VSS disconnected. The path should be ABS pin 6 to the VDO Instrument cluster pin A3 via a Yel/Brn wire. The microprocessor in the instrument cluster generates another vehicle speed signal output. It goes from pin A7 via a Yel/Blk wire to speed warning relay pin 6, cruise control module pin 8S, ECM pin B18 and somewhere at the radio. So far I have verifed continuity between the cruise control module and the ECM. I have also determined that path is not shorted to ground.

My current plan is to disconnect the speed warning relay and the radio and verify the speed signal continuity. I'll leave the cruise control module disconnected. That will reduce the load on the instrument cluster ouput to just the ECM which I assume is the current fault being detected. Then I will perform another test drive.

FlipIt · Post by **FlipIt** » 30 Jun 2011, 18:24

I have to admit that this VSS fault is starting to irritate me. I removed the radio and verified continuity of the vehicle speed signal path between the radio connector and the cruise control module connector. Then I slid under the dash to remove the speed warning relay. I had to remove another relay to get access to the #207 position in the relay module and was surprised that no relay was plugged into the connector. Either the function was never implemented in my vehicle or someone had previously disabled it. Because it was so awkward under the dash I didn't bother to verify the continuity of the signal path. I reassembled the wire bundles, ECM/TCM box and plugged in the ECM and TCM. Even with only the ECM as a load on the instrument cluster generated vehicle speed signal the forward arrow began to flash and the Check Engine illuminated in less than one mile of driving. The code was the usual P0500.

I've decided that either the instrument cluster output is bad or the ECM input is not being read. I plan to reinstall the cruise control module and see if it works. I'll also connect a multimeter to the speed signal at the radio connector and check for a reading while driving. Unfortunately, I have no way to monitor the waveform. If the cruise control works then the problem is probably with the ECM.