I have a problem with one of the computers on a 1998 Ford Ranger. One of the computer’s functions is to reduce a variable frequency analog input to a lower frequency digital output. In my case the analog signal is being digitized, but the output frequency is not being reduced.
Specifically: the Vehicle Speed Sensor (VSS), a variable reluctance sensor, generates an AC signal and sends it to the Generic Electronic Module (GEM) via a twisted pair. The GEM is the computer that processes this signal by reducing its frequency and changing it to a digital 5V DC signal. This signal is sent from the GEM to the speedometer and the Powertrain Control Module (PCM), the vehicle’s main computer. The PCM controls ignition timing, fuel injection, transmission shifting and emissions control devices.
The problem is that the GEM is not reducing the frequency of the VSS signal. The un-modulated signal sent to the PCM indicates a vehicle speed much greater than its actual speed. The PCM is programmed to protect the engine by shutting off the fuel injectors at approximately 95mph, equal to approximately 212Hz. At a creep, perhaps 15mph, the VSS output is near 1KHz and it increases proportional to vehicle speed. This is normal, but because the GEM is sending its signal to the PCM at the same frequency, the PCM shuts the engine off. So something in the GEM has failed. But . . .
I have replaced the GEM and the VSS, with original Ford components. I have isolated the circuit by running dedicated wires from the VSS to the GEM, with the same results.
I’ve examined the reluctor for the VSS, which is inside the differential and is in perfect shape
With the speedometer/PCM wire disconnected the vehicle runs reasonably well, but definitely not right, and there is no speedometer reading.
The same symptoms occur on some Ford F-150 trucks when a faulty windshield seal allows water to leak into the GEM, which is near the left windshield pillar. This GEM is under the dash and dry.
On some Ford vehicles the GEM needs to be programmed to match the PCM. This is definitely not one of them.
The GEM also controls things such as the door locks, power windows, dash lighting, etc.
There are no more GEMs in the country for this vehicle. GEMS are very vehicle-specific, so nothing else will work.
I am the vehicle’s second owner and I know for a fact it has never been modified in any way.
So: 1) What’s killing the GEM?, and 2) How do I fix it? With no speed signal, the Check Engine Light comes on.
The truck will not pass a smog test with the light on. When it fails a smog test it can’t be registered for its annual new license plates.
In order to pass its previous smog test I built a Rube Goldberg device that convinced the PCM it was receiving a valid signal, which extinguished the light.
I can post photos and circuit diagrams should anyone desire.
Thanks,
Bob
Specifically: the Vehicle Speed Sensor (VSS), a variable reluctance sensor, generates an AC signal and sends it to the Generic Electronic Module (GEM) via a twisted pair. The GEM is the computer that processes this signal by reducing its frequency and changing it to a digital 5V DC signal. This signal is sent from the GEM to the speedometer and the Powertrain Control Module (PCM), the vehicle’s main computer. The PCM controls ignition timing, fuel injection, transmission shifting and emissions control devices.
The problem is that the GEM is not reducing the frequency of the VSS signal. The un-modulated signal sent to the PCM indicates a vehicle speed much greater than its actual speed. The PCM is programmed to protect the engine by shutting off the fuel injectors at approximately 95mph, equal to approximately 212Hz. At a creep, perhaps 15mph, the VSS output is near 1KHz and it increases proportional to vehicle speed. This is normal, but because the GEM is sending its signal to the PCM at the same frequency, the PCM shuts the engine off. So something in the GEM has failed. But . . .
I have replaced the GEM and the VSS, with original Ford components. I have isolated the circuit by running dedicated wires from the VSS to the GEM, with the same results.
I’ve examined the reluctor for the VSS, which is inside the differential and is in perfect shape
With the speedometer/PCM wire disconnected the vehicle runs reasonably well, but definitely not right, and there is no speedometer reading.
The same symptoms occur on some Ford F-150 trucks when a faulty windshield seal allows water to leak into the GEM, which is near the left windshield pillar. This GEM is under the dash and dry.
On some Ford vehicles the GEM needs to be programmed to match the PCM. This is definitely not one of them.
The GEM also controls things such as the door locks, power windows, dash lighting, etc.
There are no more GEMs in the country for this vehicle. GEMS are very vehicle-specific, so nothing else will work.
I am the vehicle’s second owner and I know for a fact it has never been modified in any way.
So: 1) What’s killing the GEM?, and 2) How do I fix it? With no speed signal, the Check Engine Light comes on.
The truck will not pass a smog test with the light on. When it fails a smog test it can’t be registered for its annual new license plates.
In order to pass its previous smog test I built a Rube Goldberg device that convinced the PCM it was receiving a valid signal, which extinguished the light.
I can post photos and circuit diagrams should anyone desire.
Thanks,
Bob