2002 Ford F-150 Speedometer - Transmission problem

Tiny
DBLD123
  • MEMBER
  • 2002 FORD F-150
  • V8
  • 2WD
  • AUTOMATIC
  • 118,000 MILES
While driving 70 mph, the speedometer dropped to 45 (it bounced to 70 several times). I tried my cruise control but it didn’t hold speed at 70 mph (it may have held speed at 45 mph?). When I turned the cruise off, the OD light began blinking. I tried the OD button and it worked fine. I exited the freeway and stopped. When I took off, the light stopped blinking and everything seemed normal. I went to the Ford dealer after work and had the codes checked. It came up with P0500 (VSS malfunction), P0721 (OSS range/performance) & P1728 (Transmission slip error). They recommended changing both sensors. They ordered the parts and I returned a week later to have them replaced. During that week the speedometer worked fine when I was on the freeway, but I noticed the speedometer would sometimes drop from 20 mph to 0 when slowing or stopping. If I had to accelerate when the speedometer was at 0, the transmission would only be in first gear. I also noticed that if I was stopping real slowly, the speedometer would drop at 20 mph (3-2 shift) and then come back up at 10 mph (2-1 shift). The day I went in to have the sensors changed, when leaving for work and after work, the speedometer did not work from a dead stop and the transmission was only in first gear until the speedometer started working (within a minute). Changing both OSS and VSS sensors and clearing the codes did not correct the problem. I went back to the dealer a week later to have it re-diagnosed. The technician was not sure what was causing the problem. His laptop did indicate something happening with the 3-2 downshift. I was told it could possibly be one of four things: GEM module, wiring, transmission problem or something wrong with the ignition coils. I believe he told me it was showing the P0500 code. I am waiting for them to investigate the problem. The cruise control works fine and I’ve experienced more problems from a dead stop (usually after starting the vehicle). Any suggestions would be greatly appreciated!
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Saturday, October 10th, 2009 AT 2:11 PM

5 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Start with the GEM module. The GEM is bolted to the back of the fuse box. It is very common for them to get corroded due to leaky windshields and cause all kinds of crazy electrical problems. As far as the other suggestions, the coils have nothing to do with it, wiring is unlikely, and with only 118K on the truck, chances are the trans is fine.

Let me know what you find.

Joe
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Sunday, October 11th, 2009 AT 1:03 AM
Tiny
DBLD123
  • MEMBER
The only thing Ford came up with is they want to pull the instrument cluster and check the speedometer. I have been paying attention to see if the problem is worse on rainy days. I can't really say for sure (we haven't had any heavy rain days). I looked under the dash and didn't see any signs of water leaks (everything looks very clean). For the past week and a half, I always go from Park to Drive. If I need to back up, I still go from Park to Drive first, then into Reverse. I have not had one incident of the speedometer not working when I take off or fluctuating while driving, or dropping to zero when stopping. The transmission and speedometer work fine when I use this technique. Do you still think it's the GEM module?
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Monday, November 16th, 2009 AT 8:09 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
THat is a good question at this point. If you go straight to reverse, do you have the problem?
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Monday, November 16th, 2009 AT 8:59 PM
Tiny
DBLD123
  • MEMBER
If I start up cold and go to Reverse to backup, then go into Drive, I do have a problem. Sometimes the speedometer does not work and it's stays in first gear until the speedo starts working (maybe a minute). Even if I don't have that problem, the speedometer sometimes will drop to zero when coming to a stop. If I am slowing to make a turn, and drop below 20mph and the speedometer drops to zero, I will be in first until the speedometer starts working. If I come to a complete stop, it usually takes off just fine. Crazy problem! I still want to see if I have any problem on a rainy day with going from Park to Drive.
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Tuesday, November 17th, 2009 AT 6:59 PM
Tiny
DBLD123
  • MEMBER
I have driven the truck in two days of rain. All my driving has been with shifting from Park to Drive when I have to start up. I have not had any problems at all. I don't want to shift from Park to Reverse and have this frustrating problem reoccur. If the GEM module processes signals thru a different path (Park to Drive vs. Park to Reverse), then it would make sense that the problem lies in the GEM module. I'm not familiar with how the electronically controlled valve body works. I plan on driving with the same technique so I don't have a problem. If it's the GEM module, I'll wait till next summer and try to replace it myself (the dealer wants $250 labor).
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Saturday, December 5th, 2009 AT 8:25 AM

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