Speedometer Troubleshooting : Diagnosing and Testing

Diagnosing Speedometer Issues
The easiest way to test the speedometer system is to simulate turning the wheel by disconnecting one of the speedometer cables and chucking it into a power drill. You need to set the drill to run in reverse to turn the speedometer the correct direction.
Checking the Dust Cap
The dust cap is mounted just behind the front left tire. “Dust cap” is a misnomer, as it is pivotal to the operation of the speedometer system.
Note that when removing the dust cap, a broken piece of the angle drive spindle may come out as well. This is a sign that the angle drive has seized and caused the spindle to snap, and you need a new angle drive. Be sure to check the hole in the dust cap and make sure it is still squared. If it is rounded, you will likely need a new dust cap as well.
Jack up the car and remove the front left tire.
Remove the dust cap. This is the yellow plastic disc in the center of the hub. Simply pull it off straight.
Make sure the hole in the center of the dust cap is squared. If it is rounded, it may not be engaging the angle drive spindle properly.
Test the angle drive via the instructions below.
If the angle drive works, the problem is with the dust cap. There are two common failures:
The dust cap may simply be slipping against the hub due to resistance from the angle drive. Use some tape, silicone sealant or other mild adhesive to stick the cap to the hub, then put the wheel back on the car and see if this helps.
The dust cap’s square hole has become rounded to the point that it can no longer properly engaged and turn the angle drive’s spindle. This may happen due to general wear, but may also be due to previously seized angle drive. Replace the dust cap with a new one from a DeLorean vendor.
See the Angle Drive article for general instructions on removing and replacing the dust cap.
Testing the Angle Drive The easiest way to test the angle drive without removing it from the car is to simulating a turning dust cap. This can be done by chucking its spindle into a power drill.
If the spindle is broken, the angle drive is likely seized and needs to be replaced. Be sure to check that the hole in the dust cap is still square and not rounded, as you may also need to replace that as well.
Jack up the car and remove the front left tire.
Remove the dust cap, as described above.
Chuck a power drill with the angle drive spindle that passes through the center of the hub.
Switch the power drill to reserve and run it.
Lean into the car and read the speedometer, or have an assistant read it for you. An 800 RPM power drill at full power will run the speedometer at about 40 MPH.
If the speedometer turns, the problem may be with the dust cap. See the instructions above for checking the dust cap.
If the speedometer does not turn, test the cable from the angle drive to the Lambda unit as described [#Testing the Cable from the Angle Drive to the Speedometer | below]. If the cable is OK, replace the angle drive. See the Angle Drive article removal and installation instructions.
Testing the Lower Speedometer Cable
Speedometer cables are tested without removing them from the car by using a power drill.
Turn the steering wheel all the way to the right. This will provide easier access to the cable. You may find it easier to do this after removing the tire.
Jack up the car and remove the front left tire.
Reach behind the wheel hub. The angle drive is mounted on a large nut on the back of the hub. The speedometer cable extends from the angle drive towards the back of the car and through the firewall into the cabin.
Twist the speedometer cable’s connector counter-clockwise to unscrew it from the angle drive. Pliers may be of assistance here.
Pull the end of the cable out and chuck it into a power drill.
Run the power drill in reverse.
Lean into the car and read the speedometer, or have an assistant read it for you. An 800 RPM power drill at full power will run the speedometer at about 40 MPH.
If the speedometer turns, the problem may be with the angle drive or the dust cap. See the instructions above for information on testing them.
If the speedometer cable does not work, test the Lambda unit as described Testing the Lambda Counter below. If the Lambda counter is working, replace the speedometer cable with a new one from a DeLorean vendor.
To further test the cable, you may want to remove it from the car completely and test it as per the instructions in the speedometer cables article. If it kinks or bends, replace it with a new cable from a DeLorean vendor.
Testing the Upper Speedometer Cable
The upper speedometer cable extents from the Lambda counter to the speedometer.
To test just the upper speedometer cable, you will need to remove it from the car. This involves disconnecting the cable from the Lambda Counter and the speedometer by removing the binnacle.
Once removed, the cable can be tested in the same way as the lower speedometer cable, as described in the speedometer cables article. If it kinks or has other issues, replace it with a new cable from a DeLorean vendor.
Testing the Speedometer
The speedometer is easiest to test when it is connected to the upper speedometer cable, thus allowing it to be chucked into a power drill and spun to simulate a turning wheel. See the instructions on testing the upper speedometer cable above for how to do this.