I have a 2000 Excursion Limited, with the V10, which I bought the first weekend of November. I knew it had some problems when I bought it, but I enjoy a project, and I figured I could probably handle this one. Fast forward a few weeks, and it started getting cold outside, but the heater in the truck really wasn't cutting it. I plugged in my bluetooth OBDII device, and fired up FORScan Lite on my phone. Sure enough, the coolant temp wasn't getting above 150F. Okay, easy fix, right? Replaced the thermostat with a brand new one (old one that came out said Motorcraft and 185F) at 195F. Suddenly, the heater works like a champ, and I even picked up a half MPG on my last tank (half of which was used with the old thermostat). Tonight I drove it about an hour to meet up with a friend, and on the way back, the overdrive light started flashing. The transmission was shifting normally the entire time. I also got 5 sets of 5 beeps, and ultimately the check engine light came on as well. Around the same time, the speedometer started going crazy (going down the highway about 75MPH, so I know the tach should be steady around 2200, which it was). I've pulled codes, and saw that the ABS sensors for the rear axle and front left wheel were both setting codes. I've been aware of the ABS sensors since the day I bought it. I have purchased a brand new rear axle sensor, and front hub, but they haven't arrived yet. Now here's the real curve ball: the windshield leaks, and I'm nearly 100% certain that the GEM is not working correctly, if at all. I've purchased a replacement GEM with an exactly matching part number, but haven't yet installed it, because I still need to fix the leaky windshield.

Enough rambling. Here's my questions:

1. Could the bad ABS sensors or the GEM be causing the overdrive light to flash?
2. Is my crazy speedometer related?
3. I've driven the vehicle much more than an hour at a time, before I replaced the thermostat, and never had the flashing OD light. Could the low temperature brought on by the old thermostat have been masking the problem? I.e. the computer doesn't bother to report transmission failures if it hasn't reached operating temperature?
4. Googling the 5 sets of 5 beeps suggested that it's a notification that the airbag light is not working, but the airbag light illuminates normally when I turn the key on.

 

Yes, they can. The one on the rear axle is used to read vehicle speed for the speedometer and the PCM.
Yes.
No. It will report transmission problems at any temperature.

Read the transmission codes so that you don't have to guess.