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Ask A. Penland Your Own Question
A. Penland
A. Penland, Ford Senior Master Technician, ASE
Category: Ford
Satisfied Customers: 6603
Experience:  ASE Master Certified, ASE Advanced Gasoline Engine Performance
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Speedometer bounces and goes to zero causing erratic

Customer Question

speedometer bounces and goes to zero causing erratic shifting and acts like it goes into neutral
JA: How old is the transmission? Have you noticed any leaking?
Customer: not sure and no leaking
JA: What is the model/year of your Ford?
Customer: 1990 f150 4x4 xlt
JA: Are you fixing your F150 yourself? What have you tried so far?
Customer: put aq
JA: Anything else you want the mechanic to know before I connect you?
Customer: yes put a new vss in speedo still bounv
Submitted: 9 months ago.
Category: Ford
Expert:  A. Penland replied 9 months ago.

The problem is the speedometer in your F150. The VSS sends the signal to the speedometer and the speedometer sends the signal to the PCM (Engine Computer) which controls the transmission shifting. You can either have the speedometer replaced or repaired by a speedometer repair shop. It is actually and small module called the PSOM (Programmable Speedometer/Odometer Module).

Customer: replied 9 months ago.
I have replaced the speedometer head with a new one. This vehicle is cable driven to the speedo. Could that gear in the transfer case be bad
Expert:  A. Penland replied 9 months ago.

What else have you replaced or repaired to fix this problem so I do not duplicate anything.

Customer: replied 9 months ago.
Put in a new sensor in rear axle
Expert:  A. Penland replied 9 months ago.

If I remember correctly, where the speedometer cable connects to the transfer case, there is an electrical connector there also.

Customer: replied 9 months ago.
there is. That is where I think the problem is. What are your thoughts
Expert:  A. Penland replied 9 months ago.

That is the VSS for the transmission and the speedometer. You may also want to inspect the speedometer cable where it connects. We have seen them twist and also break back in the 1990s.

Customer: replied 9 months ago.
Ok I will check that and get back to you
Expert:  A. Penland replied 9 months ago.

Sounds good.