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Marc, Technician
Category: Car
Satisfied Customers: 1888
Experience:  33 years exp. automotive repair NYS emisisions repair cert.
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Ford econoline: The speedometer seems to be..shifter..speedo..vacuum

Resolved Question:

I have a 1996 e250 ford econoline van. The speedometer seems to be jumpy, and the overdrive light on the end of the shifter blinks constantly, as well as the transmission seemingly being sort of jerky too. I understand there is no speedo cable, it's electronic, and vacuum related operation. Where do I begin looking first?

Optional Information: 1996 Ford E250 van 4.9L - 6 cyl.
Already Tried:
Checked fuses, and vacuum lines
Submitted: 11 years ago.
Category: Car
Expert:  Marc replied 11 years ago.
The flashing OD light indicates there is a problem in the transmission control system. When the Transmission Control Unit detects a malfunction it will flash the OD light and store a Diagnostic Trouble Code (DTC) in the computer. You will need to have a Ford Dealer pull the codes and determine exactly what is wrong. It is possible the fluctuating speedo is setting the light. But getting the codes read will help you to dianose the problem with that as well.

I wish I could be more specific, but there are about 120 different reasons for the flashing OD light.

Also no Vacuum lines on that transmission.

Customer: replied 11 years ago.
Reply to Marc's Post: Hi Marc,

Thanks for your advice, I will have the codes pulled soon as I can. I do have one other question. Is the speedo attached to the the transmission. Is it electronic, or vacuum controlled, or what? It does seem to me that the error is, or could be caused from the inoperable speedo. I am curious as to just what or how the speedo operates. As I has stated before, it seems to bounce all over the place up to about 45 MPH, afetr 45, it seems to level, and work fine. I did have advice given from another, to disconnect the battery (to clear the code(s) ), and then drive it, and see what "new" codes it comes up with. Start at that point was the recommendation. What do you think of that ?

Thanks again,

Expert:  Marc replied 11 years ago.

Vehicle speed sensor is located in the rear axle (TOP)

To test for a faulty programmable speedometer/odometer module (PSOM), unplug the vehicle speed sensor located in the rear axle and recheck for the driveability symptom to be present when moving away from a stop. If the symptom is no longer present, the PSOM located in the cluster assembly is the cause of the condition. The PSOM converts the analog vehicle speed sensor signal into a digital signal and sends it on to the powertrain control module (PCM) for a vehicle speed input. When the PCM receives this erratic indication from the PSOM, it generally calculates vehicle speed above 100 MPH and cuts out an injector bank and locks the converter. This causes the stall or surge type condition to avoid vehicle overspeed. If the vehicle speed sensor (VSS) in the rear axle were causing an erratic input, the vehicle operator would notice the ABS or RABS warning light would also illuminate as the PSOM and the ABS module are fed the VSS signal in parallel with the PCM. The PSOM assembly contains the speedometer and odometer.

It doesn't matter if you clear the codes if there is a problem the codes will return in a short amount of time .

Try above test may help you narrow down your problem or even nail it.

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Customer: replied 11 years ago.

Thanks so much for that GREAT advice. I forgot to mention that the ABS light was on as well. You merely confirmed the diagnosis by stating that the ABS light should be on as well if the VSS is faulty. I think I have it now.

Thanks again,