I just spent an hour looking back through past threads regarding the same problems I am having with my 1995 F150. At Speeds above 55 MPH my Speedometer starts jumping 5-10 mph off track. It also causes the Cruise Control to "Speed-up - Slow-down" constantly. My ABS warning light is Not coming on like others mentioned. My truck is equiped with the 5.8 liter and Automatic trans. It sounds like the best place to start is with the VSS in the rear Diff. Checking for crudd and maybe pulling the rear cover and cleaning out the gear assy? If this doesn't solve the problem then I may be looking to replace the PSOM? Does this sound right or do you have other recommendations? Thanks!
hmmmmm interesting...I too have the 5 mph wander in my 94 f150, but only around 80-85 mph. haven't had time to t/s it. I doubt it's the sensor connection, why would it only deviate above 80? I suspect it's the speedometer itself wearing out. Hopefully someone else has dove into this and can offer some input.
Deherrmann, all your symptoms point to a bad PSOM. It wouldn't hurt to check the VSS first, and even pull the cover on the rear end if you haven't changed the fluid in awhile. But you'll probably end up replacing the PSOM. I have never personally seen the VSS go bad in one of these trucks.
Definetly sounds like the PSOM, I think this TSB is exactly the problem your having.
Programmable Speedometer/Odometer Module- Pointer Waver-Service Tip
Article Number 96-21-11
1992-96 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES
The speedometer needle may waver and/or a light surge may occur on some vehicles when speed control is used at highway speeds between 80-113 km/h (50-70 mph). This may be due to slight dents/chips in either the exciter ring or the Vehicle Speed Sensor (VSS) and air gaps between the VSS and the exciter ring.
Replace the Programmable Speedometer/Odometer Module (PSOM) if required. New PSOM's come with increased immunity to system variability. Refer to the following Diagnostic Procedure for details.
BRONCO/F-SERIES - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 F-Series/ Bronco Body/Chassis Service Manual, Pages 13-01-30 through 13-01-32 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-11.
ECONOLINE - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 Econoline Body/Chassis Service Manual, Pages 13-01-29 through 13-01-31 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-9.
AEROSTAR - Perform normal PSOM diagnostics per Pinpoint Test "K" of the 1994 Aerostar Body/Chassis Service Manual, Page 13-01B-31.
1. Any slight dents, chips, etc., in either the exciter ring or VSS will create needle waver. Measure air gap between the VSS and exciter ring. It should be 0.38-0.51mm (0.015-0.020").
2. Check exciter ring runout per the same ring gear backface runout procedure of the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-00. Make sure the exciter ring is mounted correctly to the ring gear. If runout is more than 0.1mm (0.004"), perform the differential runout check per the procedure in the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-02A or 05-02D, to find cause and repair as needed.
3. If all items listed above check good, replace the Instrument Cluster Assembly. Obtain the correct service part number from the Parts Catalogue and then contact the Electronic Odometer Exchange Center at (800) 259-9700 for U.S. Dealers and (800) 663-9974 for Canadian Dealers.
DEALERSHIP MUST TELL THE ODOMETER EXCHANGE CENTER THAT YOU NEED A PSOM3 LEVEL CLUSTER FOR A SPEEDOMETER NEEDLE WAVER CONCERN.
OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage
Mine has this problem at speeds above 70. I recently replaced my cluster with one out of a Lightning and it still does it. It drops 5 mph and comes back to the speed I am going. I replaced the VSS a few years ago and then again about a month ago. The first time the part did not look quite the same. The VSS I have now looks like the factory one. I think it is the tone/exictor ring in the rear end. I do think that the very slight waver is the PSOM, but the jumping up and down is the VSS or the exitor ring. I don't see what else it can be except maybe a bad connection at the connector or wiring by the rear end. The first time I replaced the VSS the jumping stoped, but the wavering was still there. What I am talking about is where at highway speeds the needle slightly moves 1-2 mph. Does anyone else have any thoughts on this?
Guys, I greatly appreciate all of your wonderful help! I've seen many folks on here whom first tried cleaning the VSS and even replaced it but ultimately had to replace the expensive PSOM instead. Whats strange is that even Ford recommends in the TSB above (thanks Alz!) to replace the PSOM when "air gaps between the VSS and the exciter ring" cause the speedometer problems.
So here is my next question.. Can't we replace the the Exciter Ring and VSS to solve the problem instead? I don't know what the Exciter Ring looks like in the rear Deff or even if it can be replaced, but I have seen no mention in the forums about replacing that. Is it possible? And how difficult is it?