For a couple of months now, I've been having a problem with the speedometer on my 1997 F-150. After a few minutes (or miles) of driving at speeds ranging from 50 to 70 mph… the speedometer begins to move back and forth. The speedometer appears to move uniformly and smoothly while accelerating to 50 or 70 mph… but the needle begins to move after a few minutes of driving at a steady speed.

I know that the powertrain control module (PCM) needs to monitor vehicle speed to properly control transmission, fuel, and emission systems… and while traveling at (or around) 55 mph… the speedometer needle movement is usually slight… and rarely drops down enough to cause any other noticeable problems. However… shortly after accelerating to 65 or 70 mph… the needle begins to move back and forth enough to cause the transmission and/or the fuel system to act up. At the higher speeds… the needle moves back and forth erratically and unpredictably (down to 40 and back to 60… down to 30 and back to 70… and so on). In the some cases the speedometer needle will drop to 0… and try to stay there for a while before moving back up.

When the problem first began… I installed a new vehicle speed sensor (VSS) and a new plastic gear… but it did not correct the problem. Before I installed the new VSS… I reached into the hole with my finger… and the drive gear on the transmission shaft appeared to be OK (secure and not loose, if that means OK). I thought that the VSS gear may be bouncing around on the drive gear at high speeds (which may be happening)… but it appears that everything is OK in this area.

Also when the problem first began… I borrowed a scan tool from a local garage… expecting to find a P0500 code (VSS fault)… when actually there was none. Using a digital volt-meter connected to a convenient harness located under a removable panel on the passenger side of the dashboard… I've been able to monitor the AC signal generated by the VSS while driving the truck at various speeds.

I've observed while accelerating from 0 mph to around 60± mph that the AC voltage increases proportionately from 0.00 to around 6.XX±… as it is supposed to do depending on actual speed. The voltage fluctuates very little (if any) for a while… but when the speedometer needle begins to drop… the digital readout on the volt meter begins to drop/fluctuate accordingly. There is no distinct pattern to the voltage fluctuations… but readings ranging from 1 to 6 volts occur… sometimes actually dropping to near 0 volts at times.

The questions are: is the VSS gear bouncing around on the output gear… is the AC pulse being interfered with by DC voltage bleeding from other circuits into the VSS circuit… or is there some other cause? I could go with the gear idea if the signal didn't drop down so low at times… because it seems that the signal would not go to 0 volts at anytime unless the VSS gear actually quit spinning completely for brief periods… before making contact with the drive gear again. As mentioned previously… the drive gear appeared to be OK (just by finger feeling) before I installed a new VSS.

I've traced the VSS circuit (grey/black signal wire and pink/orange ground wire) as best I can… and it appears that the grey/black wire goes directly from the VSS sensor to the PCM (pin 58) and speed (cruise) control servo… with a parallel feed going to the interior fuse panel (junction box) which further distributes the signal to the instrument cluster (speedo head) and to the test harness under the removable panel on the passenger side of the dashboard. I also traced the pink/orange wire to the PCM (pin 33) and the dashboard harness.

In situations with the ignition switch off… the ignition switch on (engine not running)… and the engine running… I've yet to find any stray DC voltage in the VSS circuit while the truck is sitting still. And I really can't tell what's happening (as far as stray DC voltage or other interference with the AC signal generated by the VSS… if any) while I'm driving and the speedometer needle begins to move erratically.

I removed the fuse panel (junction box) and the generic electronic module (GEM) that piggybacks onto the back of the fuse box… and I didn't see any signs of corrosion on (or inside) the fuse box… and I couldn't detect any physical damage to the GEM. I'm considering replacing both of these components… but they are somewhat expensive… as far as trial-and-error work goes. If I knew that either (or both) are causing the problem… I would purchase and install them in a heartbeat. I forgot to mention that I also installed a new PCM about the time I installed the new VSS (and I found out that PCM's are not cheap either).

I bought the truck used (96,000 miles) last December for my son… from an individual who bought the truck when it was new. I've talked to the previous owner about the problem I'm having… but he said that he never had a problem with the speedometer. As a matter of fact… the speedometer worked fine for us until a couple of months ago.

I know that this is a lot of information… but I'm kind of desperate and at a dead-end now… and I could really use a tip or two… if anyone has something to suggest. Any information or comments provided would be greatly appreciated.

Thanks,

Fred Hendrix

 

 

Just a thought, have you changed tire sizes to something larger? It would require recalibrating the speedo, there are instructions for that on the site. I went to 31x10.5x15 with only a 2 MPH error at 75 MPH so it would have to be a large change. I'm troubleshooting my 96 Bronco, the speedo keeps "dropping out", Then returns after awhile, seems to have the same setup, I monitored the voltage at the input to the speedo, it goes to about 4 volts from just above a few MPH, a logic probe shows it to be a square wave, of a varying frequency, I'm sure, as it reaches about 4 volts at low MPH and stays at that level from then on, no matter what the speed. My VSS is in the tailstock of the transfer case, have to drop the driveshaft and remove the tail end of the transfer case so will monitor the output at that point, hope the computer isn't the fault. The problem is a little puzzling as at one time I cleaned the connector on the rear of the speedo and drove over 3300 miles with no problem, then it returned, really irritating!
You might want to go to the AutoZone web site, you can register and then go to a site that lists all the TSB's from Ford, there is one named Programmable Speedometer/Odometer Module Pointer waiver (SIC?) TSB#96211, dated 10/07/96, that may have some info you can use? It does cost to sign up but guess you've spent a few bucks already? Popular Mechanics also lists TSB's. I'll get back to you if I find out anything else.
DRC@pmt.org

 

Did you ever figure this out? I am having a similar problem, except that my Speedometer doesn't read at slow speeds. I have just started diagnostics. I've changed the VSS, but it didn't make any differnce.