At least he knew that the warning lights worked.
Dad came in the shop the other day and said, "Todd called and Tyran told him that the speedometer on the truck quit working and the transmission doesn't shift right." Todd is my brother-in-law and Tryan, or Rufus as I call him, is my nephew.The truck is a 2000 Ford Ranger and if something happens to the Vehicle Speed Sensor (VSS) signal, the Generic Electronic Module (GEM) won't send the proper signal to the speedometer and the Powertrain Control Module (PCM). Without the proper vehicle speed information, the PCM can't properly control the transmission shift solenoids, and it reverts to a fail-safe mode.
CHECKING CODES
When we got the truck in the shop it wouldn't up-shift from second gear, and, of course, the speedometer wasn't working. Rufus said that at least he knew that the warning lights worked. He had never had the experience of an antilock brake system (ABS) warning lamp or the malfunction indicator lamp (MIL) coming on while he was driving. When we connected the scan tool and checked both generic and Ford-specific code memories, we found a P0500 – Vehicle Speed Sensor malfunction – and a P1502 – vehicle speed circuit (VSC) check.
Before we checked for Technical Service Bulletins (TSB) or checked the diagnostic procedures for the codes, I wanted to check the voltage waveform of the VSS. The sensor is on the differential on this truck and is easy to access with the labscope leads.When we got the back of the truck in the air, we back-probed the sensor and put it in gear. We saw a virtually flat line. We then disconnected the sensor and connected the scope leads directly to the sensor terminals. This time when we put it in gear, there was an alternating current (AC) waveform that was about 3 volts peak to peak. Granted, the waveform wasn't the best looking pattern I'd ever seen, but it should have been sufficient for the GEM and PCM, at least if it wasn't being killed somewhere along the circuit.
There is a TSB about water intrusion into the harness weather-pack connector where the wiring harness goes through the floorboard under the driver's seat. We disconnected the connector and removed the weather-pack seal only to find that there wasn't any corrosion or moisture. The driver's seat only has four bolts holding it to the floor so we removed the seat and pulled the carpet back in order to check the top of the connector. It was also clean and dry.
IT MUST BE AN INTERMITTENT
Okay, the TSB didn't pan out so we checked our information system for the diagnostic procedure. Both P0500 and P1502 lead to test procedure DF. Step Number One tells you to connect a scan tool, gradually accelerate to 50 miles per hour and compare the scanned VSS reading with the speedometer. According to the test procedure, if the scanned VSS reading matches the speedometer the problem is intermittent. Gee, I guess the problem was intermittent because both the VSS and the speedometer values matched. They were both zero even after the transmission had shifted into second gear.Well, let's go to test E – 'Speed-ometer/Odometer Inoperative' – with rear antilock brake system (RABS). Did I mention that this truck has RABS rather than four-wheel antilock (4WAL) brakes? Test E tests the ignition switch, GEM and instru-ment cluster.
The ignition switch checked out fine. However, the GEM was in the center of the dash and even though it can be reached without removing the dash pad by removing the radio, it still isn't easy to get to. The instrument cluster can be a pain in the neck to remove, plus how many rattles will you install when you reinstall the dash? Before spending the time and money necessary to get to the GEM and instrument cluster, there were a couple of other things that I wanted to check. Something had to be killing the VSS signal.According to the wiring diagram, the two wires from the VSS – light green with a black tracer and red with a pink tracer – went to the RABS module, the GEM central timer module (CTM) and the RABS data link test connector. From the GEM-CTM, a gray wire with a black tracer went to splice number 130, or it might have been 190. These old eyes had trouble making out the number on the diagram. It was supposed to be in the dash panel-to-headlamp junction harness on the left side of the engine compartment. From there it went to the instrument cluster, cruise control and the PCM at terminal number 58.
Because the VSS is a magnetic, voltage-generating sensor, the wires are a twisted pair from the sensor to the main connector under the seat and from the connector across the floor pan under the carpet up into the dash. That made them easy to pick out of the loom. We did a quick inspection of the loom along the floor pan to see if there were any bare spots that might be causing a short to ground or a short to each other. Either one could cause a loss of signal.We didn't find any obvious bare spots in the wiring, but it then dawned on me that we hadn't checked the wiring from the sensor to the harness connector. Neither of the wires showed a short to ground according to the multimeter and there was resistance between the two wires, which indicated that they were not shorted to each other. I didn't pay too much attention to what the resistance value was because I hadn't seen a specification for the sensor resistance in any of the diagnostic procedures.
In order to be 100 percent certain that the problem wasn't in the loom, we took the sensor wires out of the connector and reconnected it so that we could run the truck. The wiring for the fuel pump runs through the connector so it had to be connected. When we started the truck and put it in gear, we had an AC waveform on the scope. That was another indicator of the integrity of the harness up to the connector.When we jumped the VSS wires back together the waveform disappeared again. We then checked the interior section of the VSS wires for a short to ground. The red wire with a pink tracer did not show a short; however, the light green wire with a black tracer showed 3.0 ohms to ground. Why in the heck didn't we check that earlier?
Like I said earlier, dash work takes time and money, and it can cause rattles no matter how careful you are. I decided to call our hotline subscription and ask if they had seen problems with any of the modules killing the VSS signal by shorting the light green wire with a black tracer to ground.
BLACK BOX TECHNOLOGY
I suppose that black box technology is a good thing, but I wish that the powers that be would clue us techs in on just exactly what happens inside one of those boxes. When Dave called back from the hotline, I asked him if they had seen any problem with any of the modules grounding the light green/black wire. He told me that the ABS module is supposed to ground the wire. I told him that we were getting a 3-volt peak to peak AC waveform from the VSS as long as the sensor was disconnected, but that as soon as the sensor was connected to the harness the signal went away.He asked me if we had checked the resistance across the sensor, and I said that we had, but that we were just checking it for an open or short circuit because we didn't have a resistance spec. He told me that the resistance should be between 900 and 2,500 ohms. I wished that I could have remembered what the resistance reading was when we checked it earlier, because this time when we checked it, it showed an open circuit.
After I ordered a new sensor I wanted to recheck for a pattern from the sensor. The main harness was still connected at the floorboard with the VSS wires out of the connector, so we connected the scope, started the truck and put it in gear. There it was: a nice 3-volt peak to peak AC waveform.
How can an open magnetic voltage generator-type sensor generate a voltage signal when it's open? That's when my memory finally kicked in. I remembered a crankshaft position sensor on a 2.5L Chevrolet that ate my lunch several years ago because of the same thing. I think we still have that ignition module stocked on the shelf. It was the same scenario: The AC waveform from the sensor went away when it was connected to the ignition module. After replacing the module to no avail and inspecting the wiring harness more times than I care to remember, I replaced the sensor and solved the no-start problem. Luckily, the sensor on that one came out relatively easily. Those through-the-block crank sensors can stick tight at times.After we replaced the VSS, the scanned VSS reading still agreed with the speedometer; however, this time they were showing the correct speed. The transmission also shifted correctly. I just had to see the waveform from the new sensor, so after the test drive we ran the truck back into the shop and back probed the sensor. At 1,500 RPM the waveform was still 3 volts peak to peak. Granted, it was a better-looking pattern than the original sensor, but it was still 3 volts. I still wish I knew how an open magnetic generator-type sensor can generate a 3-volt peak to peak signal. If anyone can explain that to me please let me know.
I guess the important thing is that the truck was fixed and Rufus will no longer have to drive around in second gear with the warning lights on. There is also an added benefit; when the police officer asks him if he knows how fast he was going, he can say "Yes officer, I do."
