1999 Ford F-150 Errartic Speedometer

Hi Ron

Thanks for the information

They are connected. The code means:

P0720 (Manural Trans.) Insufficient input from Output Shaft Speed sensor

As for troubleshooting you need a specific scan tool to test.

TEST TJ: OUTPUT SHAFT SPEED (OSS) SENSOR
NOTE: After each service or repair procedure has been completed, reconnect all components. Clear DTCs and repeat QUICK TEST procedures to ensure all EEC-V systems are working properly and DTCs are no longer present.
Diagnostic Aids
Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the following:
"Â OSS sensor.
"Â Wiring harness circuits (OSS and SIG RTN).
"Â Powertrain Control Module (PCM).
1. 1) DTC P0720, P0721, P0722, P0723 & P1900: Verify Drive Cycle

Using scan tool, select OSS PID from PID/DATA monitor menu. Road test vehicle and observe PID value. Shift transmission through all gear rangers, shift up and down. If PID value increases and decreases with engine and vehicle speed, go to next step. If PID value does not increase and decrease with engine and vehicle speed, go to step 3.
2. 2) Visual Inspection

Turn ignition off. Disconnect OSS sensor connector located on side of transmission. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. If connector is okay, fault is intermittent. Go to TEST Z and check for intermittent faults.
3. 3) Check For Short To Power

Turn ignition on. Measure voltage between OSS terminal at OSS sensor wiring harness connector and chassis ground. See Fig. 199. If voltage is less than one volt, go to next step. If voltage is one volt or more, repair short to power in OSS circuit.
Fig. 199: Identifying Output Shaft Speed (OSS) Sensor Circuits & Connector Terminals
Courtesy of FORD MOTOR CO.
4. 4) Check For Open Circuit

Turn ignition off. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Measure resistance of OSS circuit between PCM connector pin No. 84 and OSS terminal at OSS sensor wiring harness connector. Also measure resistance of SIG RTN circuit between PCM connector pin No. 91 and SIG RTN terminal at OSS sensor wiring harness connector. If both resistance readings are less than 5 ohms, go to next step. If any resistance reading is 5 ohms or more, repair open in appropriate circuit.
5. 5) Check For Short To Ground

Measure resistance between OSS sensor wiring harness connector terminals. Also measure resistance between OSS terminal at OSS sensor wiring harness connector and chassis ground. If both resistance readings are more than 10,000 ohms, go to next step. If any resistance reading is 10,000 ohms or less, repair short circuit.
6. 6) Check OSS Sensor Resistance

Measure resistance between OSS sensor terminals. If resistance is 450-750 ohms, replace PCM. If resistance is not 450-750 ohms, replace OSS sensor.

My suggestion is it is located on the left side of the transmission. I would do the step 6 and check the resistance. They are about $20 dollars at Auto Zone. . .. . .

Look at the wiring really good. . ..broken, heated, corroded etc. If all good then ohm it then replace it and see if you still have the same problem. Make sure you check the connector wires really good. . .they may have pulled out. . ..but most likely it is the sensor.

Hello -

Also, it looks like you may have some recalls on your vehicle. The dealer may fix these for free. Please contact the dealer service department, give them the VIN number of your car and have them check on these to see if they apply to you.