I'm hoping to get some help troubleshooting my 1999 F-250 Superduty 4x4 XLT Lariat with the V-10 and automatic transmission. The issue I'm having is that it will sometimes shift very hard and at higher speeds than normal and the cruise control doesn't keep the speed steady, it erratically speeds up and slows down the vehicle, sometimes even causing the transmission to shift up and down. This will typically last for an entire trip, but on the next trip it may drive perfectly fine. The speedometer reads fine. I've been able to fix the issue simply by shutting of the ignition and restarting the truck.

I believe the issue is due to noise in the VSS signal. I think the noise confuses the computers and they don't know how fast the truck is moving. Hopefully someone will recognize this issue and tell me what's causing the noise and an easy way to fix it. I've attached a pictures of the VSS signals recorded on my oscilloscope. IMAG002 is what I see when the vehicle is stopped, but my scanner says it's moving. The noise is about 1.4V peak-peak and about 180hz. It varies with engine RPM. When I'm not seeing the problems, the noise is only about 0.5V peak-peak (IMAG005). IMAG006 shows the signal with the vehicle moving (at a different scale). I tried disconnecting the cruise control module and the overhead display that shows mpg and miles remaining. I also turned off the fan and the radio, but this did not make the issue go away. It only went away when I shut off the truck and started it again.

Some more information from my research and troubleshooting, if you're interested: On this vehicle, the VSS signal comes from the ABS control module, which receives a speed signal from the Differential Speed Sensor (DSS) located in the rear differential. The ABS control module also reads signals from the front wheel speed sensors, but I don't think the ABS control module uses the front wheel speed to determine vehicle speed. I did some testing with the rear axle on jackstands and with the rear wheels moving and the front wheels stopped, I still got a speed reading that looked accurate. The front wheel and differential speed sensors are the passive magnetic type, which output an AC signal. I didn't test my front wheel sensors, but the signal from the DSS looked good when tested at the ABS module. I had read that the ABS module would output a DC square wave VSS signal, but that is not what I observed with my vehicle. What I'm seeing is more of a pulsing AC, with positive and negative pulses and a 4VDC bias. This VSS signal is sent to the PCM, the GEM, the cruise control servo, the overhead console, and the instrument cluster (speedometer).

My theory is that sometimes the noise is strong enough to make the PCM and cruise control servo think the vehicle is going faster than it is, but it isn't strong enough to confuse the speedometer, which always reads correctly. At other times, the noise is low enough that everything works fine.

One more detail, I have had some weird electrical issues in the past that I think were caused by the GEM getting moisture in it. It didn't get flooded or anything, but I used to have a small leak in the windshield, I found some condensation inside the cab, and there was power coming out of the GEM when it shouldn't have been. It has been dry lately, but I'm a little concerned that might be related to my current issue. Is it possible to run the truck with the GEM disconnected?

Thanks for any help you can provide!