I have an 85 f150 6 cyl 4wd, and when I drive the vehicle over what the speedo says is 45 mph, the entire vehicle starts vibrating, but not from the front end....and when i hit the clutch it usually goes away...I am thinking this is a drive train issue, ...what would be a good way to figure this out, and what kind of things could potentially cause this problem?
Thanks,
Mike

 

If it goes away when you hit the clutch i would say that most likely it is a U-joint. You can crawl underneath and see if any of them are worn out.

-Jon

 

I agree that it's probably the u-joints.

If you like to do your own diagnostics, here's a flowchart for you...

1) Slow Acceleration Test:
*Slowly accelerate to the speed where the vibration occurs.
*Note vehicle speed and engine rpm.

2) Heavy Acceleration Test:
*Accelerate hard from 0-40mph.
*Decelerate in a lower gear.
*The vibration is torque related if duplicated while carrying out this test.

3) Neutral Coast Down Speed Test:
*Drive at a higher rate of speed than where the vibration occurred during the Slow Acceleration Test.
*Place the tranny in neutral (do this rather than just pushing the clutch pedal in), and coast down past the speed where the vibration occurs.
*The vibration is vehicle speed related if duplicated while carrying out this test. This eliminates the engine and clutch assembly as possible sources.
*If the vibration was not duplicated while carrying out this test, carry out the Downshift Speed Test to verify if the vibration is engine speed related.

4) Downshift Speed Test:
*Shift into a lower gear than the gear used when carrying out the Slow Acceleration Test.
*Drive at the engine rpm where the vibration occurred during the Slow Acceleration Test.
*The vibration is engine speed related if duplicated while carrying out this test. This eliminates the tires, wheels, brakes, and the suspension components as possible sources.

5) Steering Input Test:
*Drive at the speed where the vibration occurs, while making sweeping turns in both directions.
*If the vibration goes away or gets worse, the wheel bearings, hubs, U-joints (contained in the axles of 4WD applications), and tire tread wear are all possible sources.

6) Brake Test:
*Warm the brakes by slowing the vehicle a few times from 50-20 mph using light braking applications. At highway speeds of 50-60 mph, apply the brake using a light pedal force.
*Accelerate to 55-60 mph.
*Lightly apply the brakes and slow the vehicle to 20 mph.
*A brake vibration noise can be felt in the steering wheel, seat or brake pedal. A brake noise can be heard upon brake application and diminish when the brake is released.

7) Neutral Engine Run-up (NERU) Test:
*Increase the engine rpm up from an idle to approximately 4000 rpm while in Neutral.
*If the vibration occurs, note the rpm.

8) Drive Engine Run-up (DERU) Load Test:
*WARNING: Block the front and rear wheels, and apply the parking brake and the service brake, or injury to personnel can result.
*CAUTION: Do not carry out the Engine Load Test for more than five seconds or damage to the transmission can result.
*Shift the transmission into first gear, and increase and decrease the engine rpm between an idle to approximately 2000 rpm.
*Repeat this test in Reverse.
*If the vibration is duplicated while carrying out this test, inspect the engine and transmission mounts.